Re: [RFC PATCH v4] tty: pl011: Avoid spuriously stuck-off interrupts

From: Dave Martin <Dave.Martin@arm.com>
To: Andrew Jones <drjones@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>,
	Ciro Santilli <ciro.santilli@gmail.com>,
	Linus Walleij <linus.walleij@linaro.org>,
	Russell King <linux@armlinux.org.uk>,
	Wei Xu <xuwei5@hisilicon.com>,
	linux-serial@vger.kernel.org,
	arm-mail-list <linux-arm-kernel@lists.infradead.org>
Subject: Re: [RFC PATCH v4] tty: pl011: Avoid spuriously stuck-off interrupts
Date: Thu, 26 Apr 2018 16:25:01 +0100	[thread overview]
Message-ID: <20180426152500.GS16308@e103592.cambridge.arm.com> (raw)
In-Reply-To: <20180426134201.mwnkib4ch4dcqnqi@kamzik.brq.redhat.com>

On Thu, Apr 26, 2018 at 03:42:01PM +0200, Andrew Jones wrote:
> On Thu, Apr 26, 2018 at 01:11:41PM +0100, Peter Maydell wrote:
> > On 26 April 2018 at 11:54, Dave Martin <Dave.Martin@arm.com> wrote:
> > > This is an update to a previous RFC v3 [1], to fix a problem observed by
> > > the qemu community that causes serial input to hang when booting a
> > > simulated system with data already queued in the UART FIFO [2].
> > >
> > > RFC v3 did not solve the problem by itself, due to the problem being
> > > triggered again in pl011_enable_interrupts() after working around it
> > > in pl011_hwinit().  See the updated commit message in the patch for
> > > details.
> > >
> > > This patch is intended to supersede the previous RFCs, so please test
> > > _without_ RFC v2 (or 3) applied.
> > >
> > > If you can, please:
> > >
> > >  a) Check that you can still reproduce the bug on mainline without this
> > >     patch.
> > >
> > >  b) Check whether this patch fixes the problem.
> > 
> > Adding back to the CC list the people who might be able to do
> > this testing...
> > Link to v4 patch:
> > https://patchwork.kernel.org/patch/10365551/
> >
> 
> Tested-by: Andrew Jones <drjones@redhat.com>
> 
> And
>  
> Reviewed-by: Andrew Jones <drjones@redhat.com>
> 
> as this patch is consistent with what I expected was needed back
> when I reported the issue[*]. I was just too dense to realize
> that we didn't need to "reset" the FIFO, but rather just drain it.

Thanks for the testing and review!

There seem to be a few ways to solve this, and it's hard to know which 
ne is best.  This at least looks simple now.

Possibly there is a way to reset the FIFO on a real PL011, but I ignored
the possibility since there is definitely no way to do that on e.g.,
SBSA UART.  Draining the FIFO by hand ought to work on all variants IIUC.

Cheers
---Dave

> 
> Thanks,
> drew
> 
> [*] https://lkml.org/lkml/2016/8/1/247
> 
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