linux-kernel - Re: [PATCH] i2c: exynos5: Properly use the "noirq" variants of suspend/resume

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Message-ID: <53A8AAC9.8030407@gmail.com>
Date:	Tue, 24 Jun 2014 00:31:37 +0200
From:	Tomasz Figa <tomasz.figa@...il.com>
To:	Doug Anderson <dianders@...omium.org>,
	Kevin Hilman <khilman@...aro.org>
CC:	Wolfram Sang <wsa@...-dreams.de>,
	Kukjin Kim <kgene.kim@...sung.com>,
	Javier Martinez Canillas <javier.martinez@...labora.co.uk>,
	naveen krishna <ch.naveen@...sung.com>,
	Jingoo Han <jg1.han@...sung.com>,
	Jean Delvare <jdelvare@...e.de>, Simon Glass <sjg@...gle.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Masanari Iida <standby24x7@...il.com>,
	Sachin Kamat <sachin.kamat@...aro.org>,
	"linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] i2c: exynos5: Properly use the "noirq" variants of suspend/resume



On 24.06.2014 00:27, Doug Anderson wrote:
> Kevin,
> 
> On Mon, Jun 23, 2014 at 3:19 PM, Kevin Hilman <khilman@...aro.org> wrote:
>> Doug Anderson <dianders@...omium.org> writes:
>>
>> [...]
>>
>>> On Fri, Jun 20, 2014 at 4:59 PM, Tomasz Figa <tomasz.figa@...il.com> wrote:
>>>>
>>>> I'm not sure noirq is going to work correctly, at least not with current
>>>> callbacks. I can see a call to clk_prepare_enable() there which needs to
>>>> acquire a mutex.
>>>
>>> Nice catch, thanks!  :)
>>>
>>> OK, looking at that now.  Interestingly this doesn't seem to cause us
>>> problems in our ChromeOS 3.8 tree.  I just tried enabling:
>>>   CONFIG_DEBUG_ATOMIC_SLEEP=y
>>>
>>> ...and confirmed that I got it on right:
>>>
>>> # zgrep -i atomic /proc/config.gz
>>> CONFIG_DEBUG_ATOMIC_SLEEP=y
>>>
>>> I can suspend/resume with no problems.  My bet is that it works fine because:
>>>
>>> * resume_noirq is not considered "atomic" in the sense enforced by
>>> CONFIG_DEBUG_ATOMIC_SLEEP (at least not in 3.8--I haven't tried on
>>> ToT)
>>
>> The reason is because "noirq" in the suspend/resume path actually means
>> no *device* IRQs for that specific device.
>>
>> It's often assumed that the "noirq" callbacks are called with *all*
>> interrupts disabled, but that's not the case.  Only the IRQs for that
>> specific device are disabled when its noirq callbacks run.
> 
> Ah, so even with my fix of moving to noirq we could still be broken if
> the system decided to enable interrupts for the device before the i2c
> controller get resumed then we'd still be SOL.
> 
> ...oh, but if it matches probe order then maybe we're guaranteed for
> that not to happen?  We know that we will probe the i2c bus before the
> devices on it, right?

If the mentioned device is a child of the I2C controller then the
parent-child relation determines the order. Otherwise (e.g. another,
non-I2C interrupt source that just triggers some operation on an I2C
device like voltage regulator) we're doomed. ;)

Best regards,
Tomasz
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