[PATCH v2 1/2] serial: 8250: Add UART_BUG_TXRACE workaround for Aspeed VUART

[Joel Stanley]

On Wed, 19 May 2021 at 00:07, Andrew Jeffery <andrew at aj.id.au> wrote:
>
> Aspeed Virtual UARTs directly bridge e.g. the system console UART on the
> LPC bus to the UART interface on the BMC's internal APB. As such there's
> no RS-232 signalling involved - the UART interfaces on each bus are
> directly connected as the producers and consumers of the one set of
> FIFOs.
>
> The APB in the AST2600 generally runs at 100MHz while the LPC bus peaks
> at 33MHz. The difference in clock speeds exposes a race in the VUART
> design where a Tx data burst on the APB interface can result in a byte
> lost on the LPC interface. The symptom is LSR[DR] remains clear on the
> LPC interface despite data being present in its Rx FIFO, while LSR[THRE]
> remains clear on the APB interface as the host has not consumed the data
> the BMC has transmitted. In this state, the UART has stalled and no
> further data can be transmitted without manual intervention (e.g.
> resetting the FIFOs, resulting in loss of data).
>
> The recommended work-around is to insert a read cycle on the APB
> interface between writes to THR.
>
> Cc: ChiaWei Wang <chiawei_wang at aspeedtech.com>
> Signed-off-by: Andrew Jeffery <andrew at aj.id.au>

Reviewed-by: Joel Stanley <joel at jms.id.au>

> ---
>  drivers/tty/serial/8250/8250.h              |  1 +
>  drivers/tty/serial/8250/8250_aspeed_vuart.c |  1 +
>  drivers/tty/serial/8250/8250_port.c         | 10 ++++++++++
>  3 files changed, 12 insertions(+)
>
> diff --git a/drivers/tty/serial/8250/8250.h b/drivers/tty/serial/8250/8250.h
> index 52bb21205bb6..34aa2714f3c9 100644
> --- a/drivers/tty/serial/8250/8250.h
> +++ b/drivers/tty/serial/8250/8250.h
> @@ -88,6 +88,7 @@ struct serial8250_config {
>  #define UART_BUG_NOMSR (1 << 2)        /* UART has buggy MSR status bits (Au1x00) */
>  #define UART_BUG_THRE  (1 << 3)        /* UART has buggy THRE reassertion */
>  #define UART_BUG_PARITY        (1 << 4)        /* UART mishandles parity if FIFO enabled */
> +#define UART_BUG_TXRACE        (1 << 5)        /* UART Tx fails to set remote DR */
>
>
>  #ifdef CONFIG_SERIAL_8250_SHARE_IRQ
> diff --git a/drivers/tty/serial/8250/8250_aspeed_vuart.c b/drivers/tty/serial/8250/8250_aspeed_vuart.c
> index a28a394ba32a..4caab8714e2c 100644
> --- a/drivers/tty/serial/8250/8250_aspeed_vuart.c
> +++ b/drivers/tty/serial/8250/8250_aspeed_vuart.c
> @@ -440,6 +440,7 @@ static int aspeed_vuart_probe(struct platform_device *pdev)
>         port.port.status = UPSTAT_SYNC_FIFO;
>         port.port.dev = &pdev->dev;
>         port.port.has_sysrq = IS_ENABLED(CONFIG_SERIAL_8250_CONSOLE);
> +       port.bugs |= UART_BUG_TXRACE;

A future enhancement would be to have this depend on the ast2600
compatible string, so we don't enable the feature for ast2400/ast2500.

That would also mean adding a compatible string for the ast2600.

Cheers,

Joel