linux-kernel - Re: [PATCH v5] power: introduce library for device-specific OPPs

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Message-ID: <4CAB92B0.2030707@ti.com>
Date:	Tue, 5 Oct 2010 16:03:44 -0500
From:	Nishanth Menon <nm@...com>
To:	Kevin Hilman <khilman@...prootsystems.com>
CC:	"Rafael J. Wysocki" <rjw@...k.pl>,
	linux-pm <linux-pm@...ts.linux-foundation.org>,
	lkml <linux-kernel@...r.kernel.org>,
	linux-arm <linux-arm-kernel@...ts.infradead.org>,
	linux-omap <linux-omap@...r.kernel.org>,
	Paul <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH v5] power: introduce library for device-specific OPPs

Kevin Hilman had written, on 10/05/2010 03:50 PM, the following:
> "Rafael J. Wysocki" <rjw@...k.pl> writes:
> 
>> On Tuesday, October 05, 2010, Nishanth Menon wrote:
>>> Rafael J. Wysocki had written, on 10/04/2010 05:36 PM, the following:
>>>> On Friday, October 01, 2010, Nishanth Menon wrote:
> 
> [...]
> 
>>>> I'm not really sure why so many mutexes are needed here.  I don't think you
>>>> need a separate mutex in every struct device_opp object.  I'd just use
>>>> dev_opp_list_lock for everything.
>>> I did consider using  dev_opp_list_lock to lock everything *but* here is 
>>> the contention:
>>>
>>> dev_opp_list_lock locks modification for addition of domains device. 
>>> This operation happens usually during init stage.
>>>
>>> each domain device has multiple opps, new opps can be added, but the 
>>> more often usage will probably be opp_enable and disable. domain are 
>>> usually modifiable independent of each other - device_opp->lock provides 
>>> device level lock allowing for each domain device opp list to be 
>>> modified independent of each other. e.g. on thermal overage we may 
>>> choose to lower mpu domain while a coprocessor driver in parallel might 
>>> choose to disable co-processor domain in parallel.
>>>
>>> Wondering why you'd like a single lock for all domains and restrict 
>>> parallelization?
>> Because of the simplicity, mostly.  If there's only a relatively short period
>> when the lock will be contended for, that still is not too bad and it's much
>> easier to get the synchronization right with just one lock for starters.
> 
> FWIW, I agree with Rafael
> 
> These are not going be highly contended locks, and the lock durations
> are very short, so simplifying the locking is a big win for readability.
> 
> Kevin
Fair enough. we can relook if the lock becomes a contended lock in the 
future. I do agree that simplifying the locking will benefit 
readability. Will post a v6 with a singular lock and updated 
documentation for the same.

-- 
Regards,
Nishanth Menon
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