linux-kernel - Re: [RFC: PATCH 2/2] iio: adc: exynos_adc: Handle timeout and race conditions

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Message-ID: <CAD=FV=VBYHyD6yXBrM1BV3gazDFT0oHgvJM0+5LU1w4AO16Yzg@mail.gmail.com>
Date:	Wed, 3 Apr 2013 10:06:14 -0700
From:	Doug Anderson <dianders@...omium.org>
To:	Lars-Peter Clausen <lars@...afoo.de>
Cc:	Naveen Krishna Chatradhi <ch.naveen@...sung.com>,
	linux-iio <linux-iio@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-samsung-soc@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Naveen Krishna <naveenkrishna.ch@...il.com>
Subject: Re: [RFC: PATCH 2/2] iio: adc: exynos_adc: Handle timeout and race conditions

Lars,

On Sat, Mar 16, 2013 at 7:41 AM, Lars-Peter Clausen <lars@...afoo.de> wrote:
> I think you still need the mutex for serialization, otherwise the requests
> would just cancel each other out. Btw. what happens if you start a conversion
> while another is still in progress? Is it possible to abort a conversion?

I was thinking that the spinlock would just replace the mutex for the
purposes of serialization.

I stepped back a bit, though, and I'm wondering if we're over-thinking
things.  The timeout case should certainly be handled properly (thanks
for pointing it out), but getting a timeout is really not expected and
adding a lot of extra overhead to handle it elegantly seems a bit
much?

Specifically, the mutex means that we have one user of the ADC at a
time, and ADC conversion has nothing variable about it.  The user
manual that I have access to talks about 12-bit conversion happening
in 1 microsecond with a 5MHz input clock or 5 microseconds with a 1MHz
input clock.  Even if someone has clocks configured very differently,
it would be hard to imagine a conversion actually taking a full
second.

...so that means that if the timeout actually fires then something
else fairly drastic has gone wrong.  It's _very_ unlikely that the IRQ
will still go off for this conversion sometime in the future.

To me, total modifications to what's landed already ought to be:

* Change timeout to long (from unsigned long)

* Make sure we return errors (negative results) from
wait_for_completion_interruptible_timeout() properly.

* If we get back a value of 0 from
wait_for_completion_interruptible_timeout() then we should print a
warning and attempt machinations to reset the ADC.  Without ever
seeing real-world situtations that would cause a real timeout these
machinations would be a bit of a guess (is resetting the adc useful
when it's more likely that someone accidentally messed with the clock
tree or power gated the ADC?)...  ...or perhaps a warning and a TODO
in the code would be enough?

Thoughts?

-Doug
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