linux-kernel - Re: [PATCH] DRM: Armada: fixup wait_event

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Date:	Wed, 10 Jun 2015 13:48:45 +0200
From:	Nicholas Mc Guire <der.herr@...r.at>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Nicholas Mc Guire <hofrat@...dl.org>,
	David Airlie <airlied@...ux.ie>,
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] DRM: Armada: fixup wait_event_timeout being ignored

On Wed, 10 Jun 2015, Russell King - ARM Linux wrote:

> On Wed, Jun 10, 2015 at 01:07:08PM +0200, Nicholas Mc Guire wrote:
> > The calling side seems to assume 0 as success and <0 as error so 
> > returning -ETIME should be fine here.
> 
> The idea here is to allow the remainder of the code to execute when
> the condition succeeds _or_ times out.  If it times out, that is
> not a failure - it merely means that the display has been blanked
> and we're not seeing frame done interrupts anymore.
> 
> The code should not be checking the returned value at all - in fact
> I have updates to this code which (in part) remove this, and fix a
> glaring problem that the wait queue is never woken.
> 
> I wonder how many places you've made this same mistake... please
> ensure that you review the code you're changing carefully.
>

Sorry for that - I do try my best to understand the code - my obviously
wrong understanding of the code was that a negative return was being 
expected as being possible and then handed back to the caller so I 
assumed that would be the timeout case - but as this can never happen it 
was basically ignoring the timeout - that the execution should continue 
in the case of timeout being reached was not clear to me (it might be 
worth a comment ?)

I did find similar cases in other drivers 
./drivers/media/platform/s5p-tv/mixer_reg.c:364
        incorrect check for negative return
checking for < 0 and returning (so unreachable return statement with no 
effect but no side-effect in that condition ither) or 
./drivers/media/pci/ddbridge/ddbridge-core.c:89
        incorrect check for negative return
which checked for <= 0 and was fixed up to == 0 which is correct as the < 0
case simply is unreachable - so no change of error handling logic.

but those two other cases I think are correctly fixed up.

thx!
hofrat
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