Re: [PATCH] tty: 8250: Add UART_BUG_TXRACE workaround for Aspeed VUART

[Andrew Jeffery]

On Tue, 18 May 2021, at 16:08, Greg Kroah-Hartman wrote:
> On Tue, May 18, 2021 at 11:00:39AM +0930, Andrew Jeffery wrote:
> > 
> > 
> > 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 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         | 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).
> 
> Ick, my fault, I meant "BIT()"?  To perhaps use that macro instead of the <<
> symbol.

Ah, that makes a lot more sense.

I'll send two patches. I'll leave the explicit shift in the bug fix for 
the VUARTs for consistency, then in a subsequent patch I'll convert the 
UART_{CAP,BUG}_* macros to use BIT() (which will also clean up 
UART_BUG_TXRACE).

Andrew