linux-kernel - Re: [QUEUED v20160630 1/4] stm class: Add runtime power management handling

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Date:	Thu, 30 Jun 2016 09:59:37 -0600
From:	Mathieu Poirier <mathieu.poirier@...aro.org>
To:	Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc:	Greg KH <greg@...ah.com>, Chunyan Zhang <zhang.chunyan@...aro.org>,
	laurent.fert@...el.com, yann.fouassier@...el.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [QUEUED v20160630 1/4] stm class: Add runtime power management handling

On 30 June 2016 at 09:30, Alexander Shishkin
<alexander.shishkin@...ux.intel.com> wrote:
> Mathieu Poirier <mathieu.poirier@...aro.org> writes:
>
>> On 30 June 2016 at 06:56, Alexander Shishkin
>> <alexander.shishkin@...ux.intel.com> wrote:
>>> Currently, there's no runtime pm in stm class devices, which makes it
>>> harder for the underlying hardware drivers to handle their power
>>> management.
>>>
>>> This patch applies the following runtime pm policy to stm class devices,
>>> which their parents can rely on for their power management tracking:
>>>
>>>   * device is in use during character device writes,
>>>   * delayed autosuspend is used to keep it active between adjacent
>>>   writes,
>>>   * device is in use while mmio regions are mapped,
>>>   * device is is use while any stm_source devices are linked to it.
>>>
>>> Signed-off-by: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
>>> Cc: Mathieu Poirier <mathieu.poirier@...aro.org>
>>> Cc: Chunyan Zhang <zhang.chunyan@...aro.org>
>>
>> Coresight power management on my Juno board (the only device with an
>> STM I have access to) is broken and as such, can't test if this code
>> does what is intended.  But theoretically it looks good.
>
> Thanks for taking a look.
>
>> Throughout the driver, wouldn't it be better to use
>> pm_runtime_put_sync() rather than autosuspending with a hard coded
>> value?
>
> Yeah, the autosuspend is for the char write()ers that are likely to send
> multiple consequent write()s, so that we don't have to go in and out of
> suspend every time that happens.
>

Yes, it's a trade off.  Please add a comment in stm_register_device()
that explains the usage of the autosuspend functions and the choice of
'2000' value.

Thanks,

Reviewed-by: Mathieu Poirier <mathieu.poirier@...aro.org>

> Thanks,
> --
> Alex