[PATCH] tty: pl011: Avoid spuriously stuck-off interrupts

[Dave Martin]

On Tue, May 08, 2018 at 12:01:19PM +0100, Russell King - ARM Linux wrote:
> On Fri, Apr 27, 2018 at 11:05:45AM +0100, Dave Martin wrote:
> > Commit 9b96fbacda34 ("serial: PL011: clear pending interrupts")
> > clears the RX and receive timeout interrupts on pl011 startup, to
> > avoid a screaming-interrupt scenario that can occur when the
> > firmware or bootloader leaves these interrupts asserted.
> > 
> > This has been noted as an issue when running Linux on qemu [1].
> > 
> > Unfortunately, the above fix seems to lead to potential
> > misbehaviour if the RX FIFO interrupt is asserted _non_ spuriously
> > on driver startup, if the RX FIFO is also already full to the
> > trigger level.
> > 
> > Clearing the RX FIFO interrupt does not change the FIFO fill level.
> > In this scenario, because the interrupt is now clear and because
> > the FIFO is already full to the trigger level, no new assertion of
> > the RX FIFO interrupt can occur unless the FIFO is drained back
> > below the trigger level.  This never occurs because the pl011
> > driver is waiting for an RX FIFO interrupt to tell it that there is
> > something to read, and does not read the FIFO at all until that
> > interrupt occurs.
> > 
> > Thus, simply clearing "spurious" interrupts on startup may be
> > misguided, since there is no way to be sure that the interrupts are
> > truly spurious, and things can go wrong if they are not.
> > 
> > This patch instead clears the interrupt condition by draining the
> > RX FIFO during UART startup, after clearing any potentially
> > spurious interrupt.  This should ensure that an interrupt will
> > definitely be asserted if the RX FIFO subsequently becomes
> > sufficiently full.
> > 
> > The drain is done at the point of enabling interrupts only.  This
> > means that it will occur any time the UART is newly opened through
> > the tty layer.  It will not apply to polled-mode use of the UART by
> > kgdboc: since that scenario cannot use interrupts by design, this
> > should not matter.  kgdboc will interact badly with "normal" use of
> > the UART in any case: this patch makes no attempt to paper over
> > such issues.
> > 
> > This patch does not attempt to address the case where the RX FIFO
> > fills faster than it can be drained: that is a pathological
> > hardware design problem that is beyond the scope of the driver to
> > work around.
> > 
> > [1] [Qemu-devel] [Qemu-arm] [PATCH] pl011: do not put into fifo
> > before enabled the interruption
> > https://lists.gnu.org/archive/html/qemu-devel/2018-01/msg06446.html
> > 
> > Reported-by: Wei Xu <xuwei5 at hisilicon.com>
> > Cc: Russell King <linux at armlinux.org.uk>
> > Cc: Linus Walleij <linus.walleij at linaro.org>
> > Cc: Peter Maydell <peter.maydell at linaro.org>
> > Fixes: 9b96fbacda34 ("serial: PL011: clear pending interrupts")
> > Signed-off-by: Dave Martin <Dave.Martin at arm.com>
> > Tested-by: Andrew Jones <drjones at redhat.com>
> > Reviewed-by: Andrew Jones <drjones at redhat.com>
> > ---
> >  drivers/tty/serial/amba-pl011.c | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> > 
> > diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
> > index 4b40a5b..a6ccb85 100644
> > --- a/drivers/tty/serial/amba-pl011.c
> > +++ b/drivers/tty/serial/amba-pl011.c
> > @@ -1731,6 +1731,16 @@ static void pl011_enable_interrupts(struct uart_amba_port *uap)
> >  
> >  	/* Clear out any spuriously appearing RX interrupts */
> >  	pl011_write(UART011_RTIS | UART011_RXIS, uap, REG_ICR);
> > +
> > +	/*
> > +	 * RXIS is asserted only when the RX FIFO transitions from below
> > +	 * to above the trigger threshold.  If the RX FIFO is already
> > +	 * full to the threshold this can't happen and RXIS will now be
> > +	 * stuck off.  Drain the RX FIFO explicitly to fix this:
> > +	 */
> > +	while (!(pl011_read(uap, REG_FR) & UART01x_FR_RXFE))
> > +		pl011_read(uap, REG_DR);
> 
> You probably ought to limit this in case of a stuck receive not empty
> case, maybe to (eg) twice the depth of the FIFO?

My argument would have been that if the FIFO fills too fast to be
drained, then you have more serious problems... but it would be better
for this to show as character loss than a lockup.

So, I guess draining twice the FIFO depth before giving up is a
reasonable compromise.


I'll hack that in and repost.

Cheers
---Dave