Re: Questions about clocks emulation

[Philippe Mathieu-Daudé]

Hi Arnaud,

(Cc'ing Peter and Luc)

On 12/12/23 14:05, Arnaud Minier wrote:
> Hi all,
>
> I tried to implement the RCC (Reset and Clock Control) for the STM32L4x5_SoC 
> but ran into some problems regarding clock emulation in Qemu.
> In this SoC, it is possible to change the source of several clocks used for 
> devices like the CPU, the USART, and approximately every other device on the 
> SoC.
> This change can be made at runtime by writing into a specific register.
> I tried to model this by using the clocks in hw/core/clock.c by I noticed that 
> it is not possible to change a clock's source once it has been assigned (see 
> clock_set_source()).

Ouch.

I thought this was what connect_mux_sources() what doing (see
hw/misc/bcm2835_cprman.c) but it is only called (along the
clock_set_source() calls) from the realize() handler.

> It prevents me from implementing a clock tree similar to the one on the 
> hardware.
>
> Is this limitation there for some reason or has it simply not been implemented?

There is a /* changing clock source is not supported */ comment in
clock_set_source(), so likely not implemented :)

IIUC the only issue is how to update the src->children qlist.

> Thanks,
> Arnaud Minier