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Re: [PATCH] Fix STM32F2XX USART data register readout

From: Olivier Heriveaux
Subject: Re: [PATCH] Fix STM32F2XX USART data register readout
Date: Mon, 29 Nov 2021 17:35:06 +0100

If I understand correctly (I'm not a QEmu internals expert), yes this is what happens.
Maybe stm32f2xx_usart_can_receive() is also called but since the USART_SR_RXNE flag is reset before the USART_DR is read, it does not prevent reading the next character.

Best regards,
Olivier Hériveaux

Le lun. 29 nov. 2021 à 11:46, Peter Maydell <peter.maydell@linaro.org> a écrit :
On Sun, 28 Nov 2021 at 12:07, Olivier Hériveaux
<olivier.heriveaux@ledger.fr> wrote:
> Fix issue where the data register may be overwritten by next character
> reception before being read and returned.
> Signed-off-by: Olivier Hériveaux <olivier.heriveaux@ledger.fr>
> ---
>  hw/char/stm32f2xx_usart.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> diff --git a/hw/char/stm32f2xx_usart.c b/hw/char/stm32f2xx_usart.c
> index 8df0832424..fde67f4f03 100644
> --- a/hw/char/stm32f2xx_usart.c
> +++ b/hw/char/stm32f2xx_usart.c
> @@ -103,10 +103,11 @@ static uint64_t stm32f2xx_usart_read(void *opaque, hwaddr addr,
>          return retvalue;
>      case USART_DR:
>          DB_PRINT("Value: 0x%" PRIx32 ", %c\n", s->usart_dr, (char) s->usart_dr);
> +        retvalue = s->usart_dr & 0x3FF;
>          s->usart_sr &= ~USART_SR_RXNE;
>          qemu_chr_fe_accept_input(&s->chr);
>          qemu_set_irq(s->irq, 0);
> -        return s->usart_dr & 0x3FF;
> +        return retvalue;
>      case USART_BRR:
>          return s->usart_brr;
>      case USART_CR1:
> --
> 2.17.1

The bug happens because qemu_chr_fe_accept_input() can cause
stm32f2xx_usart_receive() to be called, right ?

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>

I'll put this in my list of patches to take via target-arm.next for the
7.0 release.

-- PMM

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