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

[Jiri Slaby]

On 19. 05. 21, 2:07, Andrew Jeffery 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@xxxxxxxxxxxxxx>
> Signed-off-by: Andrew Jeffery <andrew@xxxxxxxx>
> ---
>   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;
>   
>   	rc = sysfs_create_group(&vuart->dev->kobj, &aspeed_vuart_attr_group);
>   	if (rc < 0)
> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> index d45dab1ab316..9d44b2b2ff18 100644
> --- a/drivers/tty/serial/8250/8250_port.c
> +++ b/drivers/tty/serial/8250/8250_port.c
> @@ -1809,6 +1809,16 @@ void serial8250_tx_chars(struct uart_8250_port *up)
>   	count = up->tx_loadsz;
>   	do {
>   		serial_out(up, UART_TX, xmit->buf[xmit->tail]);
> +		if (up->bugs & UART_BUG_TXRACE) {
> +			/* The Aspeed BMC virtual UARTs have a bug where data

This is not how a multiline comment should start. It should have been:
			/*
			 * The Aspeed BMC virtual...

> +			 * may get stuck in the BMC's Tx FIFO from bursts of
> +			 * writes on the APB interface.
> +			 *
> +			 * Delay back-to-back writes by a read cycle to avoid
> +			 * stalling the VUART.
> +			 */
> +			(void)serial_in(up, UART_SCR);

(void) is useless here. It's only syntactic sugar which wouldn't even 
filter out a warning about unused result (if serial_in was marked w/ 
__must_check/warn_unused_result attribute).

> +		}
>   		xmit->tail = (xmit->tail + 1) & (UART_XMIT_SIZE - 1);
>   		port->icount.tx++;
>   		if (uart_circ_empty(xmit))

thanks,
--
js
suse labs