On Tue, Feb 13, 2018 at 12:54 PM, Richard Braun <rbr...@sceen.net> wrote:
> I/O currently being synchronous, there is no reason to ever clear the
> SR_TXE bit. However the SR_TC bit may be cleared by software writing
> to the SR register, so set it on each write.
>
> In addition, fix the reset value of the USART status register.
>
> Signed-off-by: Richard Braun <rbr...@sceen.net>
Reviewed-by: Alistair Francis <alistair.fran...@xilinx.com>
Alistair
> ---
> hw/char/stm32f2xx_usart.c | 12 ++++++++----
> include/hw/char/stm32f2xx_usart.h | 7 ++++++-
> 2 files changed, 14 insertions(+), 5 deletions(-)
>
> diff --git a/hw/char/stm32f2xx_usart.c b/hw/char/stm32f2xx_usart.c
> index 07b462d4b6..032b5fda13 100644
> --- a/hw/char/stm32f2xx_usart.c
> +++ b/hw/char/stm32f2xx_usart.c
> @@ -96,12 +96,10 @@ static uint64_t stm32f2xx_usart_read(void *opaque, hwaddr
> addr,
> switch (addr) {
> case USART_SR:
> retvalue = s->usart_sr;
> - s->usart_sr &= ~USART_SR_TC;
> qemu_chr_fe_accept_input(&s->chr);
> return retvalue;
> case USART_DR:
> DB_PRINT("Value: 0x%" PRIx32 ", %c\n", s->usart_dr, (char)
> s->usart_dr);
> - s->usart_sr |= USART_SR_TXE;
> s->usart_sr &= ~USART_SR_RXNE;
> qemu_chr_fe_accept_input(&s->chr);
> qemu_set_irq(s->irq, 0);
> @@ -137,7 +135,9 @@ static void stm32f2xx_usart_write(void *opaque, hwaddr
> addr,
> switch (addr) {
> case USART_SR:
> if (value <= 0x3FF) {
> - s->usart_sr = value;
> + /* I/O being synchronous, TXE is always set. In addition, it may
> + only be set by hardware, so keep it set here. */
> + s->usart_sr = value | USART_SR_TXE;
> } else {
> s->usart_sr &= value;
> }
> @@ -151,8 +151,12 @@ static void stm32f2xx_usart_write(void *opaque, hwaddr
> addr,
> /* XXX this blocks entire thread. Rewrite to use
> * qemu_chr_fe_write and background I/O callbacks */
> qemu_chr_fe_write_all(&s->chr, &ch, 1);
> + /* XXX I/O are currently synchronous, making it impossible for
> + software to observe transient states where TXE or TC aren't
> + set. Unlike TXE however, which is read-only, software may
> + clear TC by writing 0 to the SR register, so set it again
> + on each write. */
> s->usart_sr |= USART_SR_TC;
> - s->usart_sr &= ~USART_SR_TXE;
> }
> return;
> case USART_BRR:
> diff --git a/include/hw/char/stm32f2xx_usart.h
> b/include/hw/char/stm32f2xx_usart.h
> index 9d03a7527c..7ea7448813 100644
> --- a/include/hw/char/stm32f2xx_usart.h
> +++ b/include/hw/char/stm32f2xx_usart.h
> @@ -37,7 +37,12 @@
> #define USART_CR3 0x14
> #define USART_GTPR 0x18
>
> -#define USART_SR_RESET 0x00C00000
> +/*
> + * XXX The reset value mentioned in "24.6.1 Status register" seems bogus.
> + * Looking at "Table 98 USART register map and reset values", it seems it
> + * should be 0xc0, and that's how real hardware behaves.
> + */
> +#define USART_SR_RESET (USART_SR_TXE | USART_SR_TC)
>
> #define USART_SR_TXE (1 << 7)
> #define USART_SR_TC (1 << 6)
> --
> 2.11.0
>
>
