[PATCH v3 1/2] serial: 8250: Add UART_BUG_TXRACE workaround for Aspeed VUART

[ChiaWei Wang]

> -----Original Message-----
> From: Jiri Slaby <jirislaby at kernel.org>
> Sent: Thursday, May 20, 2021 1:25 PM
> 
> On 20. 05. 21, 4:13, 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>
> 
> Reviewed-by: Jiri Slaby <jirislaby at kernel.org>

Tested-by: ChiaWei Wang <chiawei_wang at aspeedtech.com>

> 
> > ---
> >   drivers/tty/serial/8250/8250.h              |  1 +
> >   drivers/tty/serial/8250/8250_aspeed_vuart.c |  1 +
> >   drivers/tty/serial/8250/8250_port.c         | 12 ++++++++++++
> >   3 files changed, 14 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..fc5ab2032282 100644
> > --- a/drivers/tty/serial/8250/8250_port.c
> > +++ b/drivers/tty/serial/8250/8250_port.c
> > @@ -1809,6 +1809,18 @@ 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
> > +			 * 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. Read a register that won't have
> > +			 * side-effects and discard the result.
> > +			 */
> > +			serial_in(up, UART_SCR);
> > +		}
> >   		xmit->tail = (xmit->tail + 1) & (UART_XMIT_SIZE - 1);
> >   		port->icount.tx++;
> >   		if (uart_circ_empty(xmit))
> >