linux-kernel - Re: [PATCH] pm_qos: Add system bus performance parameter

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Date:	Fri, 27 Aug 2010 11:33:02 -0700
From:	Bryan Huntsman <bryanh@...eaurora.org>
To:	Kevin Hilman <khilman@...prootsystems.com>
CC:	Saravana Kannan <skannan@...eaurora.org>,
	linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	James Bottomley <James.Bottomley@...e.de>,
	mark gross <markgross@...gnar.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Jonathan Corbet <corbet@....net>,
	Matthew Garrett <mjg59@...f.ucam.org>
Subject: Re: [PATCH] pm_qos: Add system bus performance parameter

Kevin Hilman wrote:
> Saravana Kannan <skannan@...eaurora.org> writes:
> 
>> Some drivers/devices might need some minimum system bus performance to
>> provide acceptable service. Provide a PM QoS parameter to send these requests
>> to.
>>
>> The new parameter is named "system bus performance" since it is generic enough
>> for the unit of the request to be frequency, bandwidth or something else that
>> might be appropriate. It's up to each implementation of the QoS provider to
>> define what the unit of the request would be.
>>
>> Signed-off-by: Saravana Kannan <skannan@...eaurora.org>
> 
> With this current design, only one system-wide bus would be managed.
> What if a platform has more than one independently scalable bus?
> 
> I think the only scalable way to handle this kind of thing is to have
> per-device QoS constraints that can then be combined/aggregated by parent
> devices/busses.
> 
> At LPC this year, I've proposed per-device QoS constraints[1] as a topic
> for the PM mini-conf.  I hope some folks from the MSM camp can be there
> for these discussions.
> 
> Kevin
> 
> [1] http://www.linuxplumbersconf.org/2010/ocw/proposals/819

Yeah, I'm planning on rounding up some MSM folks for LPC this year. 
Power is a big concern for us so it would be good to join the 
discussion.  Initially, I was very keen on the per-device QoS contraints 
but I've since cooled on it.  For our HW, there's not a generic unit 
that can convey enough data for us to act on.  At least not w/o lookup 
tables, etc., at which point the unit loses it's value and becomes a 
generic handle.  I'm looking forward to a good group discussion on this 
topic.  Thanks.

- Bryan


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