linux-kernel - Re: [PATCH v6 0/4] block layer runtime pm

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Message-ID: <Pine.LNX.4.44L0.1301141040020.1782-100000@iolanthe.rowland.org>
Date:	Mon, 14 Jan 2013 10:41:47 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Aaron Lu <aaron.lu@...el.com>
cc:	Jens Axboe <axboe@...nel.dk>, "Rafael J. Wysocki" <rjw@...k.pl>,
	James Bottomley <James.Bottomley@...senpartnership.com>,
	<linux-pm@...r.kernel.org>, <linux-scsi@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, Aaron Lu <aaron.lwe@...il.com>,
	Shane Huang <shane.huang@....com>
Subject: Re: [PATCH v6 0/4] block layer runtime pm

On Mon, 14 Jan 2013, Aaron Lu wrote:

> On Tue, Jan 08, 2013 at 10:27:54AM -0500, Alan Stern wrote:
> > On Tue, 8 Jan 2013, Aaron Lu wrote:
> > 
> > > So this also reminds me that as long as CONFIG_PM_RUNTIME is selected,
> > > the blk_pm_add/put/peek_request functions will be in the block IO path.
> > > Shall we introduce a new config option to selectively build block
> > > runtime PM functionality? something like CONFIG_BLK_PM_RUNTIME perhaps?
> > > 
> > > Just some condition checks in those functions, not sure if it is worth a
> > > new config though. Please suggest, thanks.
> > 
> > I don't think it is needed.  The new routines will be very quick when 
> > blk_pm_runtime_init() hasn't been called, once you add back the checks 
> > for the queue's device.
> 
> Is it necessary to also add the q->dev check in the following case?
> 
> static void blk_pm_requeue_request(struct request *rq)
> {
> 	if (rq->q->dev && !(rq->cmd_flags & REQ_PM))
> 		rq->q->nr_pending--;
> }
> 
> So that we do not have a meanlingless value of nr_pending for those
> drivers that don't use block runtime PM, and this also has the benefit
> of making those drivers return early.

It doesn't really matter.  Since nr_pending doesn't get used when 
q->dev isn't set, it doesn't hurt for it to have a meaningless value.

Alan Stern

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