[RFC PATCH] tty: pl011: Avoid stuck-off spurious interrupts

[Wei Xu]

Hi Dave,

On 2018/1/29 17:50, Wei Xu wrote:
> Hi Dave,
> 
> On 2018/1/29 16:09, 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 spuriously
>> 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 attempts to handle (suspected) spurious interrupts more
>> robustly, by allowing the interrupt(s) to fire but quenching the
>> scream.
>>
>> pl011_int() runs and attempts to drain the FIFO anyway just as if
>> the interrupts were real.  If the FIFO is already empty, great.  To
>> avoid a screaming spurious interrupt, the RX FIFO and timeout
>> interrupts are now explicitly cleared in between committing to
>> drain the RX FIFO and actually draining it.  We do not have to
>> worry about lost interrupts here, because we are effectively in
>> polled mode inside pl011_int() until the RX FIFO becomes empty:
>>
>>   * A new char received before the RX FIFO is fully drained will be
>>     drained out synchronously by pl011_int() along with the other
>>     chars already pending.  A new char received after the RX FIFO
>>     is drained will result in correct RX FIFO interrupt assertion,
>>     because emptying the RX FIFO guarantees that the RX FIFO /
>>     timeout interrupt state machines are back in a sane state.
>>
>>   * A new RX timeout before the RX FIFO is fully drained is no
>>     problem, because pl011_int() has already committed to emptying
>>     the FIFO at this point, guaranteeing that no stray chars will
>>     be left behind.  A new RX timeout after the RX FIFO is fully
>>     drained will result in correct interrupt assertion.
>>
>> This patch does not attempt to address the case where the RX FIFO
>> fills faster than it can be drained: that is a pathological
>> condition that is beyond the scope of the driver to work around.
>> Users cannot expect this to work unless they enable hardware flow
>> control.
>>
>> [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>
>> ---
>>  drivers/tty/serial/amba-pl011.c | 6 +-----
>>  1 file changed, 1 insertion(+), 5 deletions(-)
>>
> 
> After commented the 1549 line in the amba-pl011.c[1] and applied this patch,
> the console is not hanged any more.

Sorry, it should be line 1678[1] for the 4.15-rc9.

> 
> The UART011_ICR is cleared in the pl011_hwinit that will clear the
> RX interruption as well.
> Is it OK to remove 1549 as well?
> Thanks!
> 
> [1]:http://elixir.free-electrons.com/linux/v4.4/source/drivers/tty/serial/amba-pl011.c#L1549

[1]: http://elixir.free-electrons.com/linux/v4.15-rc9/source/drivers/tty/serial/amba-pl011.c#L1678

Best Regards,
Wei

> 
> Best Regards,
> Wei
>