linux-kernel - Re: [linux-pm] [RFC 1/4] cpuidle: define the enter function in the driver structure

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Message-ID: <4FD89144.7000203@linaro.org>
Date:	Wed, 13 Jun 2012 15:10:28 +0200
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	Jean Pihet <jean.pihet@...oldbits.com>
CC:	Deepthi Dharwar <deepthi@...ux.vnet.ibm.com>,
	linux-acpi@...r.kernel.org, linux-pm@...ts.linux-foundation.org,
	linaro-dev@...ts.linaro.org, linux-kernel@...r.kernel.org,
	Kevin Hilman <khilman@...com>
Subject: Re: [linux-pm] [RFC 1/4] cpuidle: define the enter function in the
 driver structure

On 06/13/2012 02:44 PM, Jean Pihet wrote:
> Hi Daniel,
> 
> On Fri, Jun 8, 2012 at 11:34 PM, Daniel Lezcano
> <daniel.lezcano@...aro.org> wrote:
>> On 06/08/2012 07:33 PM, Deepthi Dharwar wrote:
>>> Hi Daniel,
>>
>> Hi Deepthi,
>>
>>> On 06/08/2012 09:32 PM, Daniel Lezcano wrote:
>>>
>>>> We have the state index passed as parameter to the 'enter' function.
>>>> Most of the drivers assign their 'enter' functions several times in
>>>> the cpuidle_state structure, as we have the index, we can delegate
>>>> to the driver to handle their own callback array.
>>>>
>>>> That will have the benefit of removing multiple lines of code in the
>>>> different drivers.
>>>>
>>>> In order to smoothly modify the driver, the 'enter' function are in
>>>> the driver structure and in the cpuidle state structure. That will
>>>> let the time to modify the different drivers one by one.
>>>> So the 'cpuidle_enter' function checks if the 'enter' callback is
>>>> assigned in the driver structure and use it, otherwise it invokes
>>>> the 'enter' assigned to the cpuidle_state.
>>>
>>>
>>> Currently, the backend driver initializes
>>> all the cpuidle states supported on the platform,
>>> and each state can have its own enter routine
>>> which can be unique This is a clean approach.
>>
>> Yes, I perfectly understood the purpose of this field but as clean it is
>> it does not make sense as it is not used in this way. If it is supposed
>> to be done in the way you are describing here, we should have the same
>> number of states and enter functions. Here it is how it is used:
>>
>>  --------------------------------------------------
>> | Arch             | nr states | nr enter function |
>>  --------------------------------------------------
>> | x86 (nehalem)    |    3      |         1         |
>>  --------------------------------------------------
>> | x86 (snb)        |    4      |         1         |
>>  --------------------------------------------------
>> | x86 (atom)       |    4      |         1         |
>>  --------------------------------------------------
>> | ARM tegra        |    1      |         1         |
>>  --------------------------------------------------
>> | ARM omap3        |    7      |         2         |
>>  --------------------------------------------------
>> | ARM omap4        |    3      |         1         |
>>  --------------------------------------------------
>> | ARM ux500        |    2      |         1         |
>>  --------------------------------------------------
>> | ARM shmobile     |    1      |         1         |
>>  --------------------------------------------------
>> | ARM davinci      |    2      |         1         |
>>  --------------------------------------------------
>> | ARM at91         |    2      |         1         |
>>  --------------------------------------------------
>> | ARM s3c64xx      |    1      |         1         |
>>  --------------------------------------------------
>> | ARM exynos       |    2      |         1         |
>>  --------------------------------------------------
>> | ARM kirkwood     |    2      |         1         |
>>  --------------------------------------------------
>> | SH               |    3      |         1         |
>>  --------------------------------------------------
>> | PPC              |    2      |         2         |
>>  --------------------------------------------------
>> |                  |           |                   |
>> | TOTAL            |    39     |        17         |
>> |                  |           |                   |
>>  --------------------------------------------------
>>
>>
>> As you can see most of the enter functions are only used as one.
>> The Omap3 cpuidle driver enter function for C2 calls the enter function
>> of C1. Other arch, already use a table of callbacks or the index.
> There is a plan to remove the extra enter function as part of an
> optimization, cf. [1]. The fix is planned to reach the 3.6 mainline
> kernel via Kevin's tree [2].
> 
> [1] http://marc.info/?l=linux-omap&m=133856365818099&w=2
> [2] http://git.kernel.org/?p=linux/kernel/git/khilman/linux-omap-pm.git;a=shortlog;h=refs/heads/for_3.6/pm/performance
> 
> The result is that there will be only one enter function for OMAP3.

Another argument in favor of moving this field to the driver structure.

Thanks Jean for the information.

  -- Daniel


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