From mboxrd@z Thu Jan 1 00:00:00 1970 From: Kevin Hilman Subject: Re: [linux-pm] [PATCH 0/8] Suspend block api (version 8) Date: Thu, 03 Jun 2010 07:42:37 -0700 Message-ID: <87d3w818ki.fsf@deeprootsystems.com> References: <20100527232357.6d14fdb2@lxorguk.ukuu.org.uk> <20100601135102.GA8098@srcf.ucam.org> <1275426085.21962.967.camel@mulgrave.site> <201006020024.14220.rjw@sisk.pl> <1275431816.21962.1108.camel@mulgrave.site> <1275451342.21962.1777.camel@mulgrave.site> <1275491111.2799.110.camel@mulgrave.site> <20100602214748.7742e3ae@schatten.dmk.lab> <1275511271.2799.516.camel@mulgrave.site> <20100603010607.5baf82a6@schatten.dmk.lab> <20100603110312.48a508dc@lxorguk.ukuu.org.uk> <1275559512.27810.35287.camel@twins> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: In-Reply-To: <1275559512.27810.35287.camel@twins> (Peter Zijlstra's message of "Thu\, 03 Jun 2010 12\:05\:12 +0200") Sender: linux-kernel-owner@vger.kernel.org To: Peter Zijlstra Cc: Alan Cox , "Gross, Mark" , Florian Mickler , James Bottomley , Arve =?iso-8859-1?Q?Hj=F8nnev=E5g?= , Neil Brown , "tytso@mit.edu" , LKML , Thomas Gleixner , Linux OMAP Mailing List , Linux PM , "felipe.balbi@nokia.com" List-Id: linux-omap@vger.kernel.org Peter Zijlstra writes: > On Thu, 2010-06-03 at 11:03 +0100, Alan Cox wrote: >> > [mtg: ] This has been a pain point for the PM_QOS implementation. >> They change the constrain back and forth at the transaction level of >> the i2c driver. The pm_qos code really wasn't made to deal with such >> hot path use, as each such change triggers a re-computation of what >> the aggregate qos request is. >> >> That should be trivial in the usual case because 99% of the time you can >> hot path >> >> the QoS entry changing is the latest one >> there have been no other changes >> If it is valid I can use the cached previous aggregate I cunningly >> saved in the top QoS entry when I computed the new one >> >> (ie most of the time from the kernel side you have a QoS stack) > > Why would the kernel change the QoS state of a task? Why not have two > interacting QoS variables, one for the task, one for the subsystem in > question, and the action depends on their relative value? Yes, having a QoS parameter per-subsystem (or even per-device) is very important for SoCs that have independently controlled powerdomains. If all devices/subsystems in a particular powerdomain have QoS parameters that permit, the power state of that powerdomain can be lowered independently from system-wide power state and power states of other power domains. Kevin