Re: [Qemu-devel] [PATCH v25 5/7] target/avr: Add limited support for USART and 16 bit timer peripherals

[Philippe Mathieu-Daudé]

Hi Michael, Sarah.

Sorry for reviewing this series that late.

On 7/9/19 11:36 PM, Michael Rolnik wrote:
> From: Sarah Harris <address@hidden>
> 
> These were designed to facilitate testing but should provide enough function 
> to be useful in other contexts.
> Only a subset of the functions of each peripheral is implemented, mainly due 
> to the lack of a standard way to handle electrical connections (like GPIO 
> pins).
> 
> Signed-off-by: Michael Rolnik <address@hidden>
> ---
>  hw/char/Kconfig                |   3 +
>  hw/char/Makefile.objs          |   1 +
>  hw/char/avr_usart.c            | 316 ++++++++++++++++++
>  hw/timer/Kconfig               |   3 +
>  hw/timer/Makefile.objs         |   1 +
>  hw/timer/avr_timer16.c         | 587 +++++++++++++++++++++++++++++++++
>  include/hw/char/avr_usart.h    |  99 ++++++
>  include/hw/timer/avr_timer16.h |  99 ++++++
>  8 files changed, 1109 insertions(+)
>  create mode 100644 hw/char/avr_usart.c
>  create mode 100644 hw/timer/avr_timer16.c
>  create mode 100644 include/hw/char/avr_usart.h
>  create mode 100644 include/hw/timer/avr_timer16.h
> 
> diff --git a/hw/char/Kconfig b/hw/char/Kconfig
> index 40e7a8b8bb..331b20983f 100644
> --- a/hw/char/Kconfig
> +++ b/hw/char/Kconfig
> @@ -46,3 +46,6 @@ config SCLPCONSOLE
>  
>  config TERMINAL3270
>      bool
> +
> +config AVR_USART
> +    bool
> diff --git a/hw/char/Makefile.objs b/hw/char/Makefile.objs
> index 02d8a66925..09ed50f1d0 100644
> --- a/hw/char/Makefile.objs
> +++ b/hw/char/Makefile.objs
> @@ -21,6 +21,7 @@ obj-$(CONFIG_PSERIES) += spapr_vty.o
>  obj-$(CONFIG_DIGIC) += digic-uart.o
>  obj-$(CONFIG_STM32F2XX_USART) += stm32f2xx_usart.o
>  obj-$(CONFIG_RASPI) += bcm2835_aux.o
> +obj-$(CONFIG_AVR_USART) += avr_usart.o
>  
>  common-obj-$(CONFIG_CMSDK_APB_UART) += cmsdk-apb-uart.o
>  common-obj-$(CONFIG_ETRAXFS) += etraxfs_ser.o
> diff --git a/hw/char/avr_usart.c b/hw/char/avr_usart.c
> new file mode 100644
> index 0000000000..26c711336b
> --- /dev/null
> +++ b/hw/char/avr_usart.c
> @@ -0,0 +1,316 @@
> +/*
> + * AVR USART
> + *
> + * Copyright (c) 2018 University of Kent
> + * Author: Sarah Harris
> + *
> + * Permission is hereby granted, free of charge, to any person obtaining a 
> copy
> + * of this software and associated documentation files (the "Software"), to 
> deal
> + * in the Software without restriction, including without limitation the 
> rights
> + * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
> + * copies of the Software, and to permit persons to whom the Software is
> + * furnished to do so, subject to the following conditions:
> + *
> + * The above copyright notice and this permission notice shall be included in
> + * all copies or substantial portions of the Software.
> + *
> + * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
> + * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
> + * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
> + * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
> + * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING 
> FROM,
> + * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
> + * THE SOFTWARE.
> + */
> +
> +#include "qemu/osdep.h"
> +#include "hw/char/avr_usart.h"
> +#include "qemu/log.h"
> +
> +static int avr_usart_can_receive(void *opaque)
> +{
> +    AVRUsartState *usart = opaque;
> +
> +    if (usart->data_valid || !(usart->csrb & USART_CSRB_RXEN)) {
> +        return 0;
> +    }
> +    return 1;
> +}
> +
> +static void avr_usart_receive(void *opaque, const uint8_t *buffer, int size)
> +{
> +    AVRUsartState *usart = opaque;
> +    assert(size == 1);
> +    assert(!usart->data_valid);
> +    usart->data = buffer[0];
> +    usart->data_valid = true;
> +    usart->csra |= USART_CSRA_RXC;
> +    if (usart->csrb & USART_CSRB_RXCIE) {
> +        qemu_set_irq(usart->rxc_irq, 1);
> +    }
> +}
> +
> +static void update_char_mask(AVRUsartState *usart)
> +{
> +    uint8_t mode = ((usart->csrc & USART_CSRC_CSZ0) ? 1 : 0) |
> +        ((usart->csrc & USART_CSRC_CSZ1) ? 2 : 0) |
> +        ((usart->csrb & USART_CSRB_CSZ2) ? 4 : 0);
> +    switch (mode) {
> +    case 0:
> +        usart->char_mask = 0b11111;
> +        break;
> +    case 1:
> +        usart->char_mask = 0b111111;
> +        break;
> +    case 2:
> +        usart->char_mask = 0b1111111;
> +        break;
> +    case 3:
> +        usart->char_mask = 0b11111111;
> +        break;
> +    case 4:
> +        /* Fallthrough. */
> +    case 5:
> +        /* Fallthrough. */
> +    case 6:
> +        qemu_log_mask(
> +            LOG_GUEST_ERROR,
> +            "%s: Reserved character size 0x%x\n",
> +            __func__,
> +            mode);
> +        break;
> +    case 7:
> +        qemu_log_mask(
> +            LOG_GUEST_ERROR,
> +            "%s: Nine bit character size not supported (forcing eight)\n",
> +            __func__);
> +        usart->char_mask = 0b11111111;
> +        break;
> +    default:
> +        assert(0);
> +    }
> +}
> +
> +static void avr_usart_reset(DeviceState *dev)
> +{
> +    AVRUsartState *usart = AVR_USART(dev);
> +    usart->data_valid = false;
> +    usart->csra = 0b00100000;
> +    usart->csrb = 0b00000000;
> +    usart->csrc = 0b00000110;
> +    usart->brrl = 0;
> +    usart->brrh = 0;
> +    update_char_mask(usart);
> +    qemu_set_irq(usart->rxc_irq, 0);
> +    qemu_set_irq(usart->txc_irq, 0);
> +    qemu_set_irq(usart->dre_irq, 0);
> +}
> +
> +static uint64_t avr_usart_read(void *opaque, hwaddr addr, unsigned int size)
> +{
> +    AVRUsartState *usart = opaque;
> +    uint8_t prr;
> +    uint8_t data;
> +    assert(size == 1);
> +
> +    cpu_physical_memory_read(usart->prr_address, &prr, 1);

This looks wrong...

You should implement the PowerManagement block as a device, map it, link
it to this device, then access directly usart->prr_link->regs[PRR0].

> +    if (prr & usart->prr_mask) {
> +        /* USART disabled, ignore. */
> +        avr_usart_reset(DEVICE(usart));
> +        return 0;
> +    }
> +
> +    switch (addr) {
> +    case USART_DR:
> +        if (!(usart->csrb & USART_CSRB_RXEN)) {
> +            /* Receiver disabled, ignore. */
> +            return 0;
> +        }
> +        if (usart->data_valid) {
> +            data = usart->data & usart->char_mask;
> +            usart->data_valid = false;
> +        } else {
> +            data = 0;
> +        }
> +        usart->csra &= 0xff ^ USART_CSRA_RXC;
> +        qemu_set_irq(usart->rxc_irq, 0);
> +        qemu_chr_fe_accept_input(&usart->chr);
> +        return data;
> +    case USART_CSRA:
> +        return usart->csra;
> +    case USART_CSRB:
> +        return usart->csrb;
> +    case USART_CSRC:
> +        return usart->csrc;
> +    case USART_BRRL:
> +        return usart->brrl;
> +    case USART_BRRH:
> +        return usart->brrh;
> +    default:
> +        qemu_log_mask(
> +            LOG_GUEST_ERROR,
> +            "%s: Bad offset 0x%"HWADDR_PRIx"\n",
> +            __func__,
> +            addr);
> +    }
> +    return 0;
> +}
> +
> +static void avr_usart_write(void *opaque, hwaddr addr, uint64_t value,
> +                                unsigned int size)
> +{
> +    AVRUsartState *usart = opaque;
> +    uint8_t mask;
> +    uint8_t data;
> +    assert((value & 0xff) == value);
> +    assert(size == 1);
> +
> +    uint8_t prr;
> +    cpu_physical_memory_read(usart->prr_address, &prr, 1);
> +    if (prr & usart->prr_mask) {
> +        /* USART disabled, ignore. */
> +        avr_usart_reset(DEVICE(usart));
> +        return;
> +    }
> +
> +    switch (addr) {
> +    case USART_DR:
> +        if (!(usart->csrb & USART_CSRB_TXEN)) {
> +            /* Transmitter disabled, ignore. */
> +            return;
> +        }
> +        usart->csra |= USART_CSRA_TXC;
> +        usart->csra |= USART_CSRA_DRE;
> +        if (usart->csrb & USART_CSRB_TXCIE) {
> +            qemu_set_irq(usart->txc_irq, 1);
> +            usart->csra &= 0xff ^ USART_CSRA_TXC;
> +        }
> +        if (usart->csrb & USART_CSRB_DREIE) {
> +            qemu_set_irq(usart->dre_irq, 1);
> +        }
> +        data = value;
> +        qemu_chr_fe_write_all(&usart->chr, &data, 1);
> +        break;
> +    case USART_CSRA:
> +        mask = 0b01000011;
> +        /* Mask read-only bits. */
> +        value = (value & mask) | (usart->csra & (0xff ^ mask));
> +        usart->csra = value;
> +        if (value & USART_CSRA_TXC) {
> +            usart->csra ^= USART_CSRA_TXC;
> +            qemu_set_irq(usart->txc_irq, 0);
> +        }
> +        if (value & USART_CSRA_MPCM) {
> +            qemu_log_mask(
> +                LOG_GUEST_ERROR,
> +                "%s: MPCM not supported by USART\n",
> +                __func__);
> +        }
> +        break;
> +    case USART_CSRB:
> +        mask = 0b11111101;
> +        /* Mask read-only bits. */
> +        value = (value & mask) | (usart->csrb & (0xff ^ mask));
> +        usart->csrb = value;
> +        if (!(value & USART_CSRB_RXEN)) {
> +            /* Receiver disabled, flush input buffer. */
> +            usart->data_valid = false;
> +        }
> +        qemu_set_irq(usart->rxc_irq,
> +            ((value & USART_CSRB_RXCIE) &&
> +            (usart->csra & USART_CSRA_RXC)) ? 1 : 0);
> +        qemu_set_irq(usart->txc_irq,
> +            ((value & USART_CSRB_TXCIE) &&
> +            (usart->csra & USART_CSRA_TXC)) ? 1 : 0);
> +        qemu_set_irq(usart->dre_irq,
> +            ((value & USART_CSRB_DREIE) &&
> +            (usart->csra & USART_CSRA_DRE)) ? 1 : 0);
> +        update_char_mask(usart);
> +        break;
> +    case USART_CSRC:
> +        usart->csrc = value;
> +        if ((value & USART_CSRC_MSEL1) && (value & USART_CSRC_MSEL0)) {
> +            qemu_log_mask(
> +                LOG_GUEST_ERROR,
> +                "%s: SPI mode not supported by USART\n",
> +                __func__);
> +        }
> +        if ((value & USART_CSRC_MSEL1) && !(value & USART_CSRC_MSEL0)) {
> +            qemu_log_mask(LOG_GUEST_ERROR, "%s: Bad USART mode\n", __func__);
> +        }
> +        if (!(value & USART_CSRC_PM1) && (value & USART_CSRC_PM0)) {
> +            qemu_log_mask(
> +                LOG_GUEST_ERROR,
> +                "%s: Bad USART parity mode\n",
> +                __func__);
> +        }
> +        update_char_mask(usart);
> +        break;
> +    case USART_BRRL:
> +        usart->brrl = value;
> +        break;
> +    case USART_BRRH:
> +        usart->brrh = value & 0b00001111;
> +        break;
> +    default:
> +        qemu_log_mask(
> +            LOG_GUEST_ERROR,
> +            "%s: Bad offset 0x%"HWADDR_PRIx"\n",
> +            __func__,
> +            addr);
> +    }
> +}
> +
> +static const MemoryRegionOps avr_usart_ops = {
> +    .read = avr_usart_read,
> +    .write = avr_usart_write,

This misses:

       .impl.max = 1,

> +    .endianness = DEVICE_NATIVE_ENDIAN,
> +};
> +
> +static Property avr_usart_properties[] = {
> +    DEFINE_PROP_CHR("chardev", AVRUsartState, chr),
> +    DEFINE_PROP_END_OF_LIST(),
> +};
> +
> +static void avr_usart_init(Object *obj)
> +{
> +    AVRUsartState *s = AVR_USART(obj);
> +    sysbus_init_irq(SYS_BUS_DEVICE(obj), &s->rxc_irq);
> +    sysbus_init_irq(SYS_BUS_DEVICE(obj), &s->dre_irq);
> +    sysbus_init_irq(SYS_BUS_DEVICE(obj), &s->txc_irq);
> +    memory_region_init_io(&s->mmio, obj, &avr_usart_ops, s, TYPE_AVR_USART, 
> 7);

USART is 8 byte wide.

> +    sysbus_init_mmio(SYS_BUS_DEVICE(obj), &s->mmio);
> +}
> +
> +static void avr_usart_realize(DeviceState *dev, Error **errp)
> +{
> +    AVRUsartState *s = AVR_USART(dev);
> +    qemu_chr_fe_set_handlers(&s->chr, avr_usart_can_receive,
> +                             avr_usart_receive, NULL, NULL,
> +                             s, NULL, true);
> +    avr_usart_reset(dev);
> +}
> +
> +static void avr_usart_class_init(ObjectClass *klass, void *data)
> +{
> +    DeviceClass *dc = DEVICE_CLASS(klass);
> +
> +    dc->reset = avr_usart_reset;
> +    dc->props = avr_usart_properties;
> +    dc->realize = avr_usart_realize;
> +}
> +
> +static const TypeInfo avr_usart_info = {
> +    .name          = TYPE_AVR_USART,
> +    .parent        = TYPE_SYS_BUS_DEVICE,
> +    .instance_size = sizeof(AVRUsartState),
> +    .instance_init = avr_usart_init,
> +    .class_init    = avr_usart_class_init,
> +};
> +
> +static void avr_usart_register_types(void)
> +{
> +    type_register_static(&avr_usart_info);
> +}
> +
> +type_init(avr_usart_register_types)
> diff --git a/hw/timer/Kconfig b/hw/timer/Kconfig
> index 51921eb63f..ad754df750 100644
> --- a/hw/timer/Kconfig
> +++ b/hw/timer/Kconfig
> @@ -61,3 +61,6 @@ config CMSDK_APB_TIMER
>  config CMSDK_APB_DUALTIMER
>      bool
>      select PTIMER
> +
> +config AVR_TIMER16
> +    bool
> diff --git a/hw/timer/Makefile.objs b/hw/timer/Makefile.objs
> index 123d92c969..4f853e265d 100644
> --- a/hw/timer/Makefile.objs
> +++ b/hw/timer/Makefile.objs
> @@ -35,6 +35,7 @@ obj-$(CONFIG_PXA2XX) += pxa2xx_timer.o
>  obj-$(CONFIG_SH4) += sh_timer.o
>  obj-$(CONFIG_DIGIC) += digic-timer.o
>  obj-$(CONFIG_MIPS_CPS) += mips_gictimer.o
> +obj-$(CONFIG_AVR_TIMER16) += avr_timer16.o
>  
>  obj-$(CONFIG_MC146818RTC) += mc146818rtc.o
>  
> diff --git a/hw/timer/avr_timer16.c b/hw/timer/avr_timer16.c
> new file mode 100644
> index 0000000000..89f8f55eaa
> --- /dev/null
> +++ b/hw/timer/avr_timer16.c
> @@ -0,0 +1,587 @@
> +/*
> + * AVR 16 bit timer
> + *
> + * Copyright (c) 2018 University of Kent
> + * Author: Ed Robbins
> + *
> + * Permission is hereby granted, free of charge, to any person obtaining a 
> copy
> + * of this software and associated documentation files (the "Software"), to 
> deal
> + * in the Software without restriction, including without limitation the 
> rights
> + * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
> + * copies of the Software, and to permit persons to whom the Software is
> + * furnished to do so, subject to the following conditions:
> + *
> + * The above copyright notice and this permission notice shall be included in
> + * all copies or substantial portions of the Software.
> + *
> + * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
> + * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
> + * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
> + * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
> + * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING 
> FROM,
> + * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
> + * THE SOFTWARE.
> + */
> +
> +/*
> + * Driver for 16 bit timers on 8 bit AVR devices.
> + * Note:
> + * ATmega640/V-1280/V-1281/V-2560/V-2561/V timers 1, 3, 4 and 5 are 16 bit
> + */
> +
> +/*
> + * XXX TODO: Power Reduction Register support
> + *           prescaler pause support
> + *           PWM modes, GPIO, output capture pins, input compare pin
> + */
> +
> +#include "qemu/osdep.h"
> +#include "hw/timer/avr_timer16.h"
> +#include "qemu/log.h"
> +
> +/* Register offsets */
> +#define T16_CRA     0x0
> +#define T16_CRB     0x1
> +#define T16_CRC     0x2
> +#define T16_CNTL    0x4
> +#define T16_CNTH    0x5
> +#define T16_ICRL    0x6
> +#define T16_ICRH    0x7
> +#define T16_OCRAL   0x8
> +#define T16_OCRAH   0x9
> +#define T16_OCRBL   0xa
> +#define T16_OCRBH   0xb
> +#define T16_OCRCL   0xc
> +#define T16_OCRCH   0xd
> +
> +/* Field masks */
> +#define T16_CRA_WGM01   0x3
> +#define T16_CRA_COMC    0xc
> +#define T16_CRA_COMB    0x30
> +#define T16_CRA_COMA    0xc0
> +#define T16_CRA_OC_CONF \
> +    (T16_CRA_COMA | T16_CRA_COMB | T16_CRA_COMC)
> +
> +#define T16_CRB_CS      0x7
> +#define T16_CRB_WGM23   0x18
> +#define T16_CRB_ICES    0x40
> +#define T16_CRB_ICNC    0x80
> +
> +#define T16_CRC_FOCC    0x20
> +#define T16_CRC_FOCB    0x40
> +#define T16_CRC_FOCA    0x80
> +
> +/* Fields masks both TIMSK and TIFR (interrupt mask/flag registers) */
> +#define T16_INT_TOV    0x1 /* Timer overflow */
> +#define T16_INT_OCA    0x2 /* Output compare A */
> +#define T16_INT_OCB    0x4 /* Output compare B */
> +#define T16_INT_OCC    0x8 /* Output compare C */
> +#define T16_INT_IC     0x20 /* Input capture */
> +
> +/* Clock source values */
> +#define T16_CLKSRC_STOPPED     0
> +#define T16_CLKSRC_DIV1        1
> +#define T16_CLKSRC_DIV8        2
> +#define T16_CLKSRC_DIV64       3
> +#define T16_CLKSRC_DIV256      4
> +#define T16_CLKSRC_DIV1024     5
> +#define T16_CLKSRC_EXT_FALLING 6
> +#define T16_CLKSRC_EXT_RISING  7
> +
> +/* Timer mode values (not including PWM modes) */
> +#define T16_MODE_NORMAL     0
> +#define T16_MODE_CTC_OCRA   4
> +#define T16_MODE_CTC_ICR    12
> +
> +/* Accessors */
> +#define CLKSRC(t16) (t16->crb & T16_CRB_CS)
> +#define MODE(t16)   (((t16->crb & T16_CRB_WGM23) >> 1) | \
> +                     (t16->cra & T16_CRA_WGM01))
> +#define CNT(t16)    VAL16(t16->cntl, t16->cnth)
> +#define OCRA(t16)   VAL16(t16->ocral, t16->ocrah)
> +#define OCRB(t16)   VAL16(t16->ocrbl, t16->ocrbh)
> +#define OCRC(t16)   VAL16(t16->ocrcl, t16->ocrch)
> +#define ICR(t16)    VAL16(t16->icrl, t16->icrh)
> +
> +/* Helper macros */
> +#define VAL16(l, h) ((h << 8) | l)
> +#define ERROR(fmt, args...) \
> +    qemu_log_mask(LOG_GUEST_ERROR, "%s: " fmt "\n", __func__, ## args)
> +#define DB_PRINT(fmt, args...) /* Nothing */
> +/*#define DB_PRINT(fmt, args...) printf("%s: " fmt "\n", __func__, ## args)*/
> +
> +static inline int64_t avr_timer16_ns_to_ticks(AVRTimer16State *t16, int64_t 
> t)
> +{
> +    if (t16->period_ns == 0) {
> +        return 0;
> +    }
> +    return t / t16->period_ns;
> +}
> +
> +static void avr_timer16_update_cnt(AVRTimer16State *t16)
> +{
> +    uint16_t cnt;
> +    cnt = avr_timer16_ns_to_ticks(t16, qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) 
> -
> +                                       t16->reset_time_ns);
> +    t16->cntl = (uint8_t)(cnt & 0xff);
> +    t16->cnth = (uint8_t)((cnt & 0xff00) >> 8);
> +}
> +
> +static inline void avr_timer16_recalc_reset_time(AVRTimer16State *t16)
> +{
> +    t16->reset_time_ns = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) -
> +                         CNT(t16) * t16->period_ns;
> +}
> +
> +static void avr_timer16_clock_reset(AVRTimer16State *t16)
> +{
> +    t16->cntl = 0;
> +    t16->cnth = 0;
> +    t16->reset_time_ns = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
> +}
> +
> +static void avr_timer16_clksrc_update(AVRTimer16State *t16)
> +{
> +    uint16_t divider = 0;
> +    switch (CLKSRC(t16)) {
> +    case T16_CLKSRC_EXT_FALLING:
> +    case T16_CLKSRC_EXT_RISING:
> +        ERROR("external clock source unsupported");
> +        goto end;
> +    case T16_CLKSRC_STOPPED:
> +        goto end;
> +    case T16_CLKSRC_DIV1:
> +        divider = 1;
> +        break;
> +    case T16_CLKSRC_DIV8:
> +        divider = 8;
> +        break;
> +    case T16_CLKSRC_DIV64:
> +        divider = 64;
> +        break;
> +    case T16_CLKSRC_DIV256:
> +        divider = 256;
> +        break;
> +    case T16_CLKSRC_DIV1024:
> +        divider = 1024;
> +        break;
> +    default:
> +        goto end;
> +    }
> +    t16->freq_hz = t16->cpu_freq_hz / divider;
> +    t16->period_ns = 1000000000ULL / t16->freq_hz;
> +    DB_PRINT("Timer frequency %" PRIu64 " hz, period %" PRIu64 " ns (%f s)",
> +             t16->freq_hz, t16->period_ns, 1 / (double)t16->freq_hz);
> +end:
> +    return;
> +}
> +
> +static void avr_timer16_set_alarm(AVRTimer16State *t16)
> +{
> +    if (CLKSRC(t16) == T16_CLKSRC_EXT_FALLING ||
> +        CLKSRC(t16) == T16_CLKSRC_EXT_RISING ||
> +        CLKSRC(t16) == T16_CLKSRC_STOPPED) {
> +        /* Timer is disabled or set to external clock source (unsupported) */
> +        goto end;
> +    }
> +
> +    uint64_t alarm_offset = 0xffff;
> +    enum NextInterrupt next_interrupt = OVERFLOW;
> +
> +    switch (MODE(t16)) {
> +    case T16_MODE_NORMAL:
> +        /* Normal mode */
> +        if (OCRA(t16) < alarm_offset && OCRA(t16) > CNT(t16) &&
> +            (t16->imsk & T16_INT_OCA)) {
> +            alarm_offset = OCRA(t16);
> +            next_interrupt = COMPA;
> +        }
> +        break;
> +    case T16_MODE_CTC_OCRA:
> +        /* CTC mode, top = ocra */
> +        if (OCRA(t16) < alarm_offset && OCRA(t16) > CNT(t16)) {
> +            alarm_offset = OCRA(t16);
> +            next_interrupt = COMPA;
> +        }
> +       break;
> +    case T16_MODE_CTC_ICR:
> +        /* CTC mode, top = icr */
> +        if (ICR(t16) < alarm_offset && ICR(t16) > CNT(t16)) {
> +            alarm_offset = ICR(t16);
> +            next_interrupt = CAPT;
> +        }
> +        if (OCRA(t16) < alarm_offset && OCRA(t16) > CNT(t16) &&
> +            (t16->imsk & T16_INT_OCA)) {
> +            alarm_offset = OCRA(t16);
> +            next_interrupt = COMPA;
> +        }
> +        break;
> +    default:
> +        ERROR("pwm modes are unsupported");
> +        goto end;
> +    }
> +    if (OCRB(t16) < alarm_offset && OCRB(t16) > CNT(t16) &&
> +        (t16->imsk & T16_INT_OCB)) {
> +        alarm_offset = OCRB(t16);
> +        next_interrupt = COMPB;
> +    }
> +    if (OCRC(t16) < alarm_offset && OCRB(t16) > CNT(t16) &&
> +        (t16->imsk & T16_INT_OCC)) {
> +        alarm_offset = OCRB(t16);
> +        next_interrupt = COMPC;
> +    }
> +    alarm_offset -= CNT(t16);
> +
> +    t16->next_interrupt = next_interrupt;
> +    uint64_t alarm_ns =
> +        t16->reset_time_ns + ((CNT(t16) + alarm_offset) * t16->period_ns);
> +    timer_mod(t16->timer, alarm_ns);
> +
> +    DB_PRINT("next alarm %" PRIu64 " ns from now",
> +        alarm_offset * t16->period_ns);
> +
> +end:
> +    return;
> +}
> +
> +static void avr_timer16_interrupt(void *opaque)
> +{
> +    AVRTimer16State *t16 = opaque;
> +    uint8_t mode = MODE(t16);
> +
> +    avr_timer16_update_cnt(t16);
> +
> +    if (CLKSRC(t16) == T16_CLKSRC_EXT_FALLING ||
> +        CLKSRC(t16) == T16_CLKSRC_EXT_RISING ||
> +        CLKSRC(t16) == T16_CLKSRC_STOPPED) {
> +        /* Timer is disabled or set to external clock source (unsupported) */
> +        return;
> +    }
> +
> +    DB_PRINT("interrupt, cnt = %d", CNT(t16));
> +
> +    /* Counter overflow */
> +    if (t16->next_interrupt == OVERFLOW) {
> +        DB_PRINT("0xffff overflow");
> +        avr_timer16_clock_reset(t16);
> +        if (t16->imsk & T16_INT_TOV) {
> +            t16->ifr |= T16_INT_TOV;
> +            qemu_set_irq(t16->ovf_irq, 1);
> +        }
> +    }
> +    /* Check for ocra overflow in CTC mode */
> +    if (mode == T16_MODE_CTC_OCRA && t16->next_interrupt == COMPA) {
> +        DB_PRINT("CTC OCRA overflow");
> +        avr_timer16_clock_reset(t16);
> +    }
> +    /* Check for icr overflow in CTC mode */
> +    if (mode == T16_MODE_CTC_ICR && t16->next_interrupt == CAPT) {
> +        DB_PRINT("CTC ICR overflow");
> +        avr_timer16_clock_reset(t16);
> +        if (t16->imsk & T16_INT_IC) {
> +            t16->ifr |= T16_INT_IC;
> +            qemu_set_irq(t16->capt_irq, 1);
> +        }
> +    }
> +    /* Check for output compare interrupts */
> +    if (t16->imsk & T16_INT_OCA && t16->next_interrupt == COMPA) {
> +        t16->ifr |= T16_INT_OCA;
> +        qemu_set_irq(t16->compa_irq, 1);
> +    }
> +    if (t16->imsk & T16_INT_OCB && t16->next_interrupt == COMPB) {
> +        t16->ifr |= T16_INT_OCB;
> +        qemu_set_irq(t16->compb_irq, 1);
> +    }
> +    if (t16->imsk & T16_INT_OCC && t16->next_interrupt == COMPC) {
> +        t16->ifr |= T16_INT_OCC;
> +        qemu_set_irq(t16->compc_irq, 1);
> +    }
> +    avr_timer16_set_alarm(t16);
> +}
> +
> +static void avr_timer16_reset(DeviceState *dev)
> +{
> +    AVRTimer16State *t16 = AVR_TIMER16(dev);
> +
> +    avr_timer16_clock_reset(t16);
> +    avr_timer16_clksrc_update(t16);
> +    avr_timer16_set_alarm(t16);
> +
> +    qemu_set_irq(t16->capt_irq, 0);
> +    qemu_set_irq(t16->compa_irq, 0);
> +    qemu_set_irq(t16->compb_irq, 0);
> +    qemu_set_irq(t16->compc_irq, 0);
> +    qemu_set_irq(t16->ovf_irq, 0);
> +}
> +
> +static uint64_t avr_timer16_read(void *opaque, hwaddr offset, unsigned size)
> +{
> +    assert(size == 1);
> +    AVRTimer16State *t16 = opaque;
> +    uint8_t retval = 0;
> +
> +    switch (offset) {
> +    case T16_CRA:
> +        retval = t16->cra;
> +        break;
> +    case T16_CRB:
> +        retval = t16->crb;
> +        break;
> +    case T16_CRC:
> +        retval = t16->crc;
> +        break;
> +    case T16_CNTL:
> +        avr_timer16_update_cnt(t16);
> +        t16->rtmp = t16->cnth;
> +        retval = t16->cntl;
> +        break;
> +    case T16_CNTH:
> +        retval = t16->rtmp;
> +        break;
> +    case T16_ICRL:
> +        /*
> +         * The timer copies cnt to icr when the input capture pin changes
> +         * state or when the analog comparator has a match. We don't
> +         * emulate this behaviour. We do support it's use for defining a
> +         * TOP value in T16_MODE_CTC_ICR
> +         */
> +        t16->rtmp = t16->icrh;
> +        retval = t16->icrl;
> +        break;
> +    case T16_ICRH:
> +        retval = t16->rtmp;
> +        break;
> +    case T16_OCRAL:
> +        retval = t16->ocral;
> +        break;
> +    case T16_OCRAH:
> +        retval = t16->ocrah;
> +        break;
> +    case T16_OCRBL:
> +        retval = t16->ocrbl;
> +        break;
> +    case T16_OCRBH:
> +        retval = t16->ocrbh;
> +        break;
> +    case T16_OCRCL:
> +        retval = t16->ocrcl;
> +        break;
> +    case T16_OCRCH:
> +        retval = t16->ocrch;
> +        break;
> +    default:
> +        break;
> +    }
> +    return (uint64_t)retval;
> +}
> +
> +static void avr_timer16_write(void *opaque, hwaddr offset,
> +                              uint64_t val64, unsigned size)
> +{
> +    assert(size == 1);
> +    AVRTimer16State *t16 = opaque;
> +    uint8_t val8 = (uint8_t)val64;
> +    uint8_t prev_clk_src = CLKSRC(t16);
> +
> +    DB_PRINT("write %d to offset %d", val8, (uint8_t)offset);
> +
> +    switch (offset) {
> +    case T16_CRA:
> +        t16->cra = val8;
> +        if (t16->cra & T16_CRA_OC_CONF) {
> +            ERROR("output compare pins unsupported");
> +        }
> +        break;
> +    case T16_CRB:
> +        t16->crb = val8;
> +        if (t16->crb & T16_CRB_ICNC) {
> +            ERROR("input capture noise canceller unsupported");
> +        }
> +        if (t16->crb & T16_CRB_ICES) {
> +            ERROR("input capture unsupported");
> +        }
> +        if (CLKSRC(t16) != prev_clk_src) {
> +            avr_timer16_clksrc_update(t16);
> +            if (prev_clk_src == T16_CLKSRC_STOPPED) {
> +                t16->reset_time_ns = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
> +            }
> +        }
> +        break;
> +    case T16_CRC:
> +        t16->crc = val8;
> +        ERROR("output compare pins unsupported");
> +        break;
> +    case T16_CNTL:
> +        /*
> +         * CNT is the 16-bit counter value, it must be read/written via
> +         * a temporary register (rtmp) to make the read/write atomic.
> +         */
> +        /* ICR also has this behaviour, and shares rtmp */
> +        /*
> +         * Writing CNT blocks compare matches for one clock cycle.
> +         * Writing CNT to TOP or to an OCR value (if in use) will
> +         * skip the relevant interrupt
> +         */
> +        t16->cntl = val8;
> +        t16->cnth = t16->rtmp;
> +        avr_timer16_recalc_reset_time(t16);
> +        break;
> +    case T16_CNTH:
> +        t16->rtmp = val8;
> +        break;
> +    case T16_ICRL:
> +        /* ICR can only be written in mode T16_MODE_CTC_ICR */
> +        if (MODE(t16) == T16_MODE_CTC_ICR) {
> +            t16->icrl = val8;
> +            t16->icrh = t16->rtmp;
> +        }
> +        break;
> +    case T16_ICRH:
> +        if (MODE(t16) == T16_MODE_CTC_ICR) {
> +            t16->rtmp = val8;
> +        }
> +        break;
> +    case T16_OCRAL:
> +        /*
> +         * OCRn cause the relevant output compare flag to be raised, and
> +         * trigger an interrupt, when CNT is equal to the value here
> +         */
> +        t16->ocral = val8;
> +        break;
> +    case T16_OCRAH:
> +        t16->ocrah = val8;
> +        break;
> +    case T16_OCRBL:
> +        t16->ocrbl = val8;
> +        break;
> +    case T16_OCRBH:
> +        t16->ocrbh = val8;
> +        break;
> +    case T16_OCRCL:
> +        t16->ocrcl = val8;
> +        break;
> +    case T16_OCRCH:
> +        t16->ocrch = val8;
> +        break;
> +    default:
> +        break;
> +    }
> +    avr_timer16_set_alarm(t16);
> +}
> +
> +static uint64_t avr_timer16_imsk_read(void *opaque,
> +                                      hwaddr offset,
> +                                      unsigned size)
> +{
> +    assert(size == 1);
> +    AVRTimer16State *t16 = opaque;
> +    if (offset != 0) {
> +        return 0;
> +    }
> +    return t16->imsk;
> +}
> +
> +static void avr_timer16_imsk_write(void *opaque, hwaddr offset,
> +                                   uint64_t val64, unsigned size)
> +{
> +    assert(size == 1);
> +    AVRTimer16State *t16 = opaque;
> +    if (offset != 0) {
> +        return;
> +    }
> +    t16->imsk = (uint8_t)val64;
> +}
> +
> +static uint64_t avr_timer16_ifr_read(void *opaque,
> +                                     hwaddr offset,
> +                                     unsigned size)
> +{
> +    assert(size == 1);
> +    AVRTimer16State *t16 = opaque;
> +    if (offset != 0) {
> +        return 0;
> +    }
> +    return t16->ifr;
> +}
> +
> +static void avr_timer16_ifr_write(void *opaque, hwaddr offset,
> +                                  uint64_t val64, unsigned size)
> +{
> +    assert(size == 1);
> +    AVRTimer16State *t16 = opaque;
> +    if (offset != 0) {
> +        return;
> +    }
> +    t16->ifr = (uint8_t)val64;
> +}
> +
> +static const MemoryRegionOps avr_timer16_ops = {
> +    .read = avr_timer16_read,
> +    .write = avr_timer16_write
if you use .impl { .min = 2 } you simplify your design.

> +    .endianness = DEVICE_NATIVE_ENDIAN
> +};
> +
> +static const MemoryRegionOps avr_timer16_imsk_ops = {
> +    .read = avr_timer16_imsk_read,
> +    .write = avr_timer16_imsk_write,
> +    .endianness = DEVICE_NATIVE_ENDIAN
> +};
> +
> +static const MemoryRegionOps avr_timer16_ifr_ops = {
> +    .read = avr_timer16_ifr_read,
> +    .write = avr_timer16_ifr_write,
> +    .endianness = DEVICE_NATIVE_ENDIAN
> +};
> +
> +static Property avr_timer16_properties[] = {
> +    DEFINE_PROP_UINT64("cpu-frequency-hz", struct AVRTimer16State,
> +                       cpu_freq_hz, 20000000),
> +    DEFINE_PROP_END_OF_LIST(),
> +};
> +
> +static void avr_timer16_init(Object *obj)
> +{
> +    AVRTimer16State *s = AVR_TIMER16(obj);
> +
> +    sysbus_init_irq(SYS_BUS_DEVICE(obj), &s->capt_irq);
> +    sysbus_init_irq(SYS_BUS_DEVICE(obj), &s->compa_irq);
> +    sysbus_init_irq(SYS_BUS_DEVICE(obj), &s->compb_irq);
> +    sysbus_init_irq(SYS_BUS_DEVICE(obj), &s->compc_irq);
> +    sysbus_init_irq(SYS_BUS_DEVICE(obj), &s->ovf_irq);
> +
> +    memory_region_init_io(&s->iomem, obj, &avr_timer16_ops,
> +                          s, TYPE_AVR_TIMER16, 0xe);

A timer region is 0x10 wide, Ok.

> +    memory_region_init_io(&s->imsk_iomem, obj, &avr_timer16_imsk_ops,
> +                          s, TYPE_AVR_TIMER16, 0x1);
> +    memory_region_init_io(&s->ifr_iomem, obj, &avr_timer16_ifr_ops,
> +                          s, TYPE_AVR_TIMER16, 0x1);

These 2 registers seem part of the Interrupt Controller, and don't
belong here. You might want to use a LINK_PROPERTY to link a timer
device to the INTC.

> +
> +    sysbus_init_mmio(SYS_BUS_DEVICE(obj), &s->iomem);
> +    sysbus_init_mmio(SYS_BUS_DEVICE(obj), &s->imsk_iomem);
> +    sysbus_init_mmio(SYS_BUS_DEVICE(obj), &s->ifr_iomem);
> +
> +    s->timer = timer_new_ns(QEMU_CLOCK_VIRTUAL, avr_timer16_interrupt, s);
> +}
> +
> +static void avr_timer16_class_init(ObjectClass *klass, void *data)
> +{
> +    DeviceClass *dc = DEVICE_CLASS(klass);
> +
> +    dc->reset = avr_timer16_reset;
> +    dc->props = avr_timer16_properties;
> +}
> +
> +static const TypeInfo avr_timer16_info = {
> +    .name          = TYPE_AVR_TIMER16,
> +    .parent        = TYPE_SYS_BUS_DEVICE,
> +    .instance_size = sizeof(AVRTimer16State),
> +    .instance_init = avr_timer16_init,
> +    .class_init    = avr_timer16_class_init,
> +};
> +
> +static void avr_timer16_register_types(void)
> +{
> +    type_register_static(&avr_timer16_info);
> +}
> +
> +type_init(avr_timer16_register_types)
> diff --git a/include/hw/char/avr_usart.h b/include/hw/char/avr_usart.h
> new file mode 100644
> index 0000000000..ba28fbe5d1
> --- /dev/null
> +++ b/include/hw/char/avr_usart.h
> @@ -0,0 +1,99 @@
> +/*
> + * AVR USART
> + *
> + * Copyright (c) 2018 University of Kent
> + * Author: Sarah Harris
> + *
> + * Permission is hereby granted, free of charge, to any person obtaining a 
> copy
> + * of this software and associated documentation files (the "Software"), to 
> deal
> + * in the Software without restriction, including without limitation the 
> rights
> + * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
> + * copies of the Software, and to permit persons to whom the Software is
> + * furnished to do so, subject to the following conditions:
> + *
> + * The above copyright notice and this permission notice shall be included in
> + * all copies or substantial portions of the Software.
> + *
> + * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
> + * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
> + * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
> + * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
> + * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING 
> FROM,
> + * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
> + * THE SOFTWARE.
> + */
> +
> +#ifndef HW_AVR_USART_H
> +#define HW_AVR_USART_H
> +
> +#include "hw/sysbus.h"
> +#include "chardev/char-fe.h"
> +#include "hw/hw.h"
> +
> +/* Offsets of registers. */
> +#define USART_DR   0x06
> +#define USART_CSRA  0x00
> +#define USART_CSRB  0x01
> +#define USART_CSRC  0x02
> +#define USART_BRRH 0x05
> +#define USART_BRRL 0x04
> +
> +/* Relevant bits in regiters. */
> +#define USART_CSRA_RXC    (1 << 7)
> +#define USART_CSRA_TXC    (1 << 6)
> +#define USART_CSRA_DRE    (1 << 5)
> +#define USART_CSRA_MPCM   (1 << 0)
> +
> +#define USART_CSRB_RXCIE  (1 << 7)
> +#define USART_CSRB_TXCIE  (1 << 6)
> +#define USART_CSRB_DREIE  (1 << 5)
> +#define USART_CSRB_RXEN   (1 << 4)
> +#define USART_CSRB_TXEN   (1 << 3)
> +#define USART_CSRB_CSZ2   (1 << 2)
> +#define USART_CSRB_RXB8   (1 << 1)
> +#define USART_CSRB_TXB8   (1 << 0)
> +
> +#define USART_CSRC_MSEL1  (1 << 7)
> +#define USART_CSRC_MSEL0  (1 << 6)
> +#define USART_CSRC_PM1    (1 << 5)
> +#define USART_CSRC_PM0    (1 << 4)
> +#define USART_CSRC_CSZ1   (1 << 2)
> +#define USART_CSRC_CSZ0   (1 << 1)
> +
> +#define TYPE_AVR_USART "avr-usart"
> +#define AVR_USART(obj) \
> +    OBJECT_CHECK(AVRUsartState, (obj), TYPE_AVR_USART)
> +
> +typedef struct {
> +    /* <private> */
> +    SysBusDevice parent_obj;
> +
> +    /* <public> */
> +    MemoryRegion mmio;
> +
> +    CharBackend chr;
> +
> +    /* Address of Power Reduction Register and bit that controls this UART */
> +    hwaddr prr_address;
> +    uint8_t prr_mask;

So ideally you'd use a link to the PowerManagement block here.

> +
> +    uint8_t data;
> +    bool data_valid;
> +    uint8_t char_mask;
> +    /* Control and Status Registers */
> +    uint8_t csra;
> +    uint8_t csrb;
> +    uint8_t csrc;
> +    /* Baud Rate Registers (low/high byte) */
> +    uint8_t brrh;
> +    uint8_t brrl;
> +
> +    /* Receive Complete */
> +    qemu_irq rxc_irq;
> +    /* Transmit Complete */
> +    qemu_irq txc_irq;
> +    /* Data Register Empty */
> +    qemu_irq dre_irq;
> +} AVRUsartState;
> +
> +#endif /* HW_AVR_USART_H */
> diff --git a/include/hw/timer/avr_timer16.h b/include/hw/timer/avr_timer16.h
> new file mode 100644
> index 0000000000..301e36a154
> --- /dev/null
> +++ b/include/hw/timer/avr_timer16.h
> @@ -0,0 +1,99 @@
> +/*
> + * AVR 16 bit timer
> + *
> + * Copyright (c) 2018 University of Kent
> + * Author: Ed Robbins
> + *
> + * Permission is hereby granted, free of charge, to any person obtaining a 
> copy
> + * of this software and associated documentation files (the "Software"), to 
> deal
> + * in the Software without restriction, including without limitation the 
> rights
> + * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
> + * copies of the Software, and to permit persons to whom the Software is
> + * furnished to do so, subject to the following conditions:
> + *
> + * The above copyright notice and this permission notice shall be included in
> + * all copies or substantial portions of the Software.
> + *
> + * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
> + * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
> + * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
> + * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
> + * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING 
> FROM,
> + * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
> + * THE SOFTWARE.
> + */
> +
> +/*
> + * Driver for 16 bit timers on 8 bit AVR devices.
> + * Note:
> + * On ATmega640/V-1280/V-1281/V-2560/V-2561/V timers 1, 3, 4 and 5 are 16 bit
> + */
> +
> +#ifndef AVR_TIMER16_H
> +#define AVR_TIMER16_H
> +
> +#include "hw/sysbus.h"
> +#include "qemu/timer.h"
> +#include "hw/hw.h"
> +
> +enum NextInterrupt {
> +    OVERFLOW,
> +    COMPA,
> +    COMPB,
> +    COMPC,
> +    CAPT
> +};
> +
> +#define TYPE_AVR_TIMER16 "avr-timer16"
> +#define AVR_TIMER16(obj) \
> +    OBJECT_CHECK(AVRTimer16State, (obj), TYPE_AVR_TIMER16)
> +
> +typedef struct AVRTimer16State {
> +    /* <private> */
> +    SysBusDevice parent_obj;
> +
> +    /* <public> */
> +    MemoryRegion iomem;
> +    MemoryRegion imsk_iomem;
> +    MemoryRegion ifr_iomem;
> +    QEMUTimer *timer;
> +    qemu_irq capt_irq;
> +    qemu_irq compa_irq;
> +    qemu_irq compb_irq;
> +    qemu_irq compc_irq;
> +    qemu_irq ovf_irq;
> +
> +    /* Address of Power Reduction Register and bit that controls this timer 
> */
> +    hwaddr prr_address;
> +    uint8_t prr_mask;

Again, you should use a link to the PowerManagement block here.

> +
> +    /* registers */
> +    uint8_t cra;
> +    uint8_t crb;
> +    uint8_t crc;

Using 16-bit for following...

> +    uint8_t cntl;
> +    uint8_t cnth;
> +    uint8_t icrl;
> +    uint8_t icrh;
> +    uint8_t ocral;
> +    uint8_t ocrah;
> +    uint8_t ocrbl;
> +    uint8_t ocrbh;
> +    uint8_t ocrcl;
> +    uint8_t ocrch;

... until here would simplify your model.

> +    /*
> +     * Reads and writes to CNT and ICR utilise a bizarre temporary
> +     * register, which we emulate
> +     */
> +    uint8_t rtmp;
> +    uint8_t imsk;
> +    uint8_t ifr;
> +
> +    uint64_t cpu_freq_hz;
> +    uint64_t freq_hz;
> +    uint64_t period_ns;
> +    uint64_t reset_time_ns;
> +    enum NextInterrupt next_interrupt;
> +} AVRTimer16State;
> +
> +#endif /* AVR_TIMER16_H */
> 

If the maintainer taking your series don't think my comments are
blockers, you might consider them as later cleanup.

Regards,

Phil.