linux-kernel - Re: [PATCH] soc/tegra: pmc: Fix "scheduling while atomic"

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Date:	Fri, 27 May 2016 13:46:37 +0100
From:	Jon Hunter <jonathanh@...dia.com>
To:	Dmitry Osipenko <digetx@...il.com>
CC:	Stephen Warren <swarren@...dotorg.org>,
	Thierry Reding <thierry.reding@...il.com>,
	Alexandre Courbot <gnurou@...il.com>,
	Peter De Schrijver <pdeschrijver@...dia.com>,
	Prashant Gaikwad <pgaikwad@...dia.com>,
	<linux-clk@...r.kernel.org>, <linux-tegra@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] soc/tegra: pmc: Fix "scheduling while atomic"


On 26/05/16 18:01, Dmitry Osipenko wrote:
> On 26.05.2016 18:27, Jon Hunter wrote:
>> On 26/05/16 15:57, Dmitry Osipenko wrote:

...

>>> That's how I see it:
>>>
>>> +----------------------------------------------+
>>> |                    CPU 0                     |
>>> +-------------------+--------------------------+
>>> |    Idle thread    | Interactive gov. thread  |
>>> +----------------------------------------------+
>>> |     inactive      |                          |
>>> |                   |                          |
>>> |                   |   CPU freq. change       |
>>> |                   |                          |
>>> |                   |   clk_set_rate()         |
>>> |                   |                          |
>>> |       ...         |   clk_prepare_lock()     |
>>> |                   |                          |
>>> |                   |   PRE rate notifier call |
>>> |                   |                          |
>>> |                   |   schedule               |
>>
>> What is this notifier doing? Is there some sort of hardware activity
>> that it is waiting for to complete?
>>
> 
> It changes regulator voltage if required. So at least I2C would cause
> scheduling on wait_for_completion_timeout().

Yes, of course that would make sense. What is interesting/odd in this
case is that the frequency is increasing (voltage scaled pre frequency
change) but yet you are entering LP2. May be that is possible? I guess
this problem may also occur on reducing frequency as well?

What are you using in the v3.18 kernel for exit_latency and
target_residency? The current mainline has 5000us and 10000us,
respectively.

It does seem that this could be triggered in the right circumstances and
I have to say I don't like the fact that this could be fragile as it is
today. Have you thought about adding a post clock notifier for pclk in
the PMC driver as an alternative to the change you are suggesting?

Cheers
Jon

-- 
nvpublic