tty: 8250: Add UART_BUG_TXRACE workaround for Aspeed VUART

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@aspeedtech.com>
Signed-off-by: Andrew Jeffery <andrew@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         | 2 ++
 3 files changed, 4 insertions(+)

On Mon, 17 May 2021, at 23:45, Greg KH wrote:
> On Mon, May 17, 2021 at 10:11:05PM +0930, 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@aspeedtech.com>
> > Signed-off-by: Andrew Jeffery <andrew@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         | 2 ++
> >  3 files changed, 4 insertions(+)
> > 
> > diff --git a/drivers/tty/serial/8250/8250.h b/drivers/tty/serial/8250/8250.h
> > index 52bb21205bb6..4d6f5e0ecd4c 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 */
> 
> BUG()?

Can you please expand on what you mean here? I don't follow.

At least, I think there might be a formatting issue (spaces vs tabs).

> 
> >  #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..6c032abfc321 100644
> > --- a/drivers/tty/serial/8250/8250_port.c
> > +++ b/drivers/tty/serial/8250/8250_port.c
> > @@ -1809,6 +1809,8 @@ 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)
> > +			serial_in(up, UART_SCR);
> 
> Can you document why you are doing a call here to serial_in(), otherwise
> someone running "automated checking scripts" will remove it later as it
> seems to be doing nothing.

Good point, I'll add a comment.

Thanks,

Andrew