[PATCH 12/12] hw/char/pl011: Implement TX FIFO

[Philippe Mathieu-Daudé]

If the UART back-end chardev doesn't drain data as fast as stdout
does or blocks, buffer in the TX FIFO to try again later.

This avoids having the IO-thread busy waiting on chardev back-ends,
reported recently when testing the Trusted Reference Stack and
using the socket backend:
https://linaro.atlassian.net/browse/TRS-149?focusedCommentId=149574

Implement registering a front-end 'watch' callback on back-end
events, so we can resume transmitting when the back-end is writable
again, not blocking the main loop.

Similarly to the RX FIFO path, FIFO level selection is not
implemented (interrupt is triggered when a single byte is available
in the FIFO).

Due to the addition of the TX FIFO in the instance state, increase
the migration stream version.

Reported-by: Mikko Rapeli <mikko.rapeli@linaro.org>
Suggested-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
---
RFC because I'm pretty sure I got the migration code wrong.

After writing this I noticed the hw/char/cmsdk-apb-uart.c model
is much more complete. Instead of copy/pasting its code, I'd rather
try to extract some generic/bstract "FIFO based chardev" QOM class;
but this is beyond the scope of this series.
---
 include/hw/char/pl011.h |   2 +
 hw/char/pl011.c         | 105 +++++++++++++++++++++++++++++++++++++---
 hw/char/trace-events    |   4 ++
 3 files changed, 105 insertions(+), 6 deletions(-)

diff --git a/include/hw/char/pl011.h b/include/hw/char/pl011.h
index d853802132..20898f43a6 100644
--- a/include/hw/char/pl011.h
+++ b/include/hw/char/pl011.h
@@ -18,6 +18,7 @@
 #include "hw/sysbus.h"
 #include "chardev/char-fe.h"
 #include "qom/object.h"
+#include "qemu/fifo8.h"
 
 #define TYPE_PL011 "pl011"
 OBJECT_DECLARE_SIMPLE_TYPE(PL011State, PL011)
@@ -53,6 +54,7 @@ struct PL011State {
     Clock *clk;
     bool migrate_clk;
     const unsigned char *id;
+    Fifo8 xmit_fifo;
 };
 
 DeviceState *pl011_create(hwaddr addr, qemu_irq irq, Chardev *chr);
diff --git a/hw/char/pl011.c b/hw/char/pl011.c
index 03c006199e..a957138405 100644
--- a/hw/char/pl011.c
+++ b/hw/char/pl011.c
@@ -57,6 +57,9 @@ DeviceState *pl011_create(hwaddr addr, qemu_irq irq, Chardev 
*chr)
 /* Data Register, UARTDR */
 #define DR_BE   (1 << 10)
 
+/* Receive Status Register/Error Clear Register, UARTRSR/UARTECR */
+#define RSR_OE  (1 << 3)
+
 /* Interrupt status bits in UARTRIS, UARTMIS, UARTIMSC */
 #define INT_OE (1 << 10)
 #define INT_BE (1 << 9)
@@ -152,19 +155,94 @@ static inline void pl011_reset_tx_fifo(PL011State *s)
     /* Reset FIFO flags */
     s->flags &= ~PL011_FLAG_TXFF;
     s->flags |= PL011_FLAG_TXFE;
+
+    fifo8_reset(&s->xmit_fifo);
+}
+
+static gboolean pl011_drain_tx(PL011State *s)
+{
+    trace_pl011_fifo_tx_drain(fifo8_num_used(&s->xmit_fifo));
+    pl011_reset_tx_fifo(s);
+    s->rsr &= ~RSR_OE;
+    return FALSE;
+}
+
+static gboolean pl011_xmit(void *do_not_use, GIOCondition cond, void *opaque)
+{
+    PL011State *s = opaque;
+    int ret;
+    const uint8_t *buf;
+    uint32_t buflen;
+    uint32_t count;
+
+    if (!qemu_chr_fe_backend_connected(&s->chr)) {
+        /* Instant drain the fifo when there's no back-end */
+        return pl011_drain_tx(s);
+    }
+
+    count = fifo8_num_used(&s->xmit_fifo);
+    if (!count) {
+        return FALSE;
+    }
+    if  (!(s->cr & CR_UARTEN)) {
+        /* Allow completing the current FIFO character before stopping. */
+        count = 1;
+    }
+
+    /* Transmit as much data as we can */
+    buf = fifo8_peek_buf(&s->xmit_fifo, count, &buflen);
+    ret = qemu_chr_fe_write(&s->chr, buf, buflen);
+    if (ret >= 0) {
+        /* Pop the data we could transmit */
+        trace_pl011_fifo_tx_xmit(ret);
+        fifo8_pop_buf(&s->xmit_fifo, ret, NULL);
+        s->int_level |= INT_TX;
+    }
+
+    if ((s->cr & CR_UARTEN) && !fifo8_is_empty(&s->xmit_fifo)) {
+        /* Reschedule another transmission if we couldn't transmit all */
+        guint r = qemu_chr_fe_add_watch(&s->chr, G_IO_OUT | G_IO_HUP,
+                                        pl011_xmit, s);
+        if (!r) {
+            return pl011_drain_tx(s);
+        }
+    }
+
+    pl011_update(s);
+
+    return FALSE;
 }
 
 static void pl011_write_tx(PL011State *s, const uint8_t *buf, int length)
 {
     if (!(s->cr & (CR_UARTEN | CR_TXE))) {
+        if (!fifo8_is_empty(&s->xmit_fifo)) {
+            /*
+             * If the UART is disabled in the middle of transmission
+             * or reception, it completes the current character before
+             * stopping.
+             */
+            pl011_xmit(NULL, G_IO_OUT, s);
+        }
         return;
     }
 
-    /* XXX this blocks entire thread. Rewrite to use
-     * qemu_chr_fe_write and background I/O callbacks */
-    qemu_chr_fe_write_all(&s->chr, buf, 1);
-    s->int_level |= INT_TX;
-    pl011_update(s);
+    if (length > fifo8_num_free(&s->xmit_fifo)) {
+        /*
+         * The FIFO contents remain valid because no more data is
+         * written when the FIFO is full, only the contents of the
+         * shift register are overwritten. The CPU must now read
+         * the data, to empty the FIFO.
+         */
+        trace_pl011_fifo_tx_overrun();
+        s->rsr |= RSR_OE;
+        return;
+    }
+
+    trace_pl011_fifo_tx_put(length);
+    fifo8_push_all(&s->xmit_fifo, buf, length);
+
+    pl011_xmit(NULL, G_IO_OUT, s);
 }
 
 static uint64_t pl011_read(void *opaque, hwaddr offset,
@@ -444,12 +522,17 @@ static int pl011_post_load(void *opaque, int version_id)
         s->read_pos = 0;
     }
 
+    if (version_id >= 3 && !fifo8_is_empty(&s->xmit_fifo)) {
+        /* Reschedule another transmission */
+        qemu_chr_fe_add_watch(&s->chr, G_IO_OUT | G_IO_HUP, pl011_xmit, s);
+    }
+
     return 0;
 }
 
 static const VMStateDescription vmstate_pl011 = {
     .name = "pl011",
-    .version_id = 2,
+    .version_id = 3,
     .minimum_version_id = 2,
     .post_load = pl011_post_load,
     .fields = (VMStateField[]) {
@@ -462,6 +545,7 @@ static const VMStateDescription vmstate_pl011 = {
         VMSTATE_UINT32(int_enabled, PL011State),
         VMSTATE_UINT32(int_level, PL011State),
         VMSTATE_UINT32_ARRAY(read_fifo, PL011State, PL011_FIFO_DEPTH),
+        VMSTATE_FIFO8(xmit_fifo, PL011State),
         VMSTATE_UINT32(ilpr, PL011State),
         VMSTATE_UINT32(ibrd, PL011State),
         VMSTATE_UINT32(fbrd, PL011State),
@@ -505,10 +589,18 @@ static void pl011_realize(DeviceState *dev, Error **errp)
 {
     PL011State *s = PL011(dev);
 
+    fifo8_create(&s->xmit_fifo, PL011_FIFO_DEPTH);
     qemu_chr_fe_set_handlers(&s->chr, pl011_can_receive, pl011_receive,
                              pl011_event, NULL, s, NULL, true);
 }
 
+static void pl011_unrealize(DeviceState *dev)
+{
+    PL011State *s = PL011(dev);
+
+    fifo8_destroy(&s->xmit_fifo);
+}
+
 static void pl011_reset(DeviceState *dev)
 {
     PL011State *s = PL011(dev);
@@ -534,6 +626,7 @@ static void pl011_class_init(ObjectClass *oc, void *data)
     DeviceClass *dc = DEVICE_CLASS(oc);
 
     dc->realize = pl011_realize;
+    dc->unrealize = pl011_unrealize;
     dc->reset = pl011_reset;
     dc->vmsd = &vmstate_pl011;
     device_class_set_props(dc, pl011_properties);
diff --git a/hw/char/trace-events b/hw/char/trace-events
index 9fd40e3aae..4c25564066 100644
--- a/hw/char/trace-events
+++ b/hw/char/trace-events
@@ -60,6 +60,10 @@ pl011_write(uint32_t addr, uint32_t value, const char 
*regname) "addr 0x%03x val
 pl011_can_receive(uint32_t lcr, int read_count, int r) "LCR 0x%08x read_count 
%d returning %d"
 pl011_fifo_rx_put(uint32_t c, int read_count) "new char 0x%02x read_count now 
%d"
 pl011_fifo_rx_full(void) "RX FIFO now full, RXFF set"
+pl011_fifo_tx_put(int count) "TX FIFO push %d"
+pl011_fifo_tx_xmit(int count) "TX FIFO pop %d"
+pl011_fifo_tx_overrun(void) "TX FIFO overrun"
+pl011_fifo_tx_drain(unsigned drained) "TX FIFO draining %u"
 pl011_baudrate_change(unsigned int baudrate, uint64_t clock, uint32_t ibrd, 
uint32_t fbrd) "new baudrate %u (clk: %" PRIu64 "hz, ibrd: %" PRIu32 ", fbrd: 
%" PRIu32 ")"
 
 # cmsdk-apb-uart.c
-- 
2.38.1