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

[Andrew Jeffery]

On Wed, 19 May 2021, at 15:40, Jiri Slaby wrote:
> 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 at aspeedtech.com>
> > Signed-off-by: Andrew Jeffery <andrew at aj.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;
> >   
> >   	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...

Sure.

> 
> > +			 * 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).

Yes. It was just meant as a visual indicator that we didn't care for 
the result.

I will remove it.

Thanks,

Andrew