linux-kernel - Re: [PATCH 2/6] PM: Asynchronous resume of devices

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Date:	Fri, 28 Aug 2009 11:43:05 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
cc:	linux-pm <linux-pm@...ts.linux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>, Len Brown <lenb@...nel.org>,
	Pavel Machek <pavel@....cz>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Zhang Rui <rui.zhang@...el.com>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Linux PCI <linux-pci@...r.kernel.org>
Subject: Re: [PATCH 2/6] PM: Asynchronous resume of devices

On Wed, 26 Aug 2009, Rafael J. Wysocki wrote:

> From: Rafael J. Wysocki <rjw@...k.pl>
> 
> Theoretically, the total time of system sleep transitions (suspend
> to RAM, hibernation) can be reduced by running suspend and resume
> callbacks of device drivers in parallel with each other.  However,
> there are dependencies between devices such that, for example, we may
> not be allowed to put one device into a low power state before
> anohter one has been suspended (e.g. we cannot suspend a bridge
> before suspending all devices behind it).  In particular, we're not
> allowed to suspend the parent of a device before suspending the
> device itself.  Analogously, we're not allowed to resume a device
> before resuming its parent.

> In this version of the patch the async threads started to execute
> the resume callbacks of specific device don't exit immediately having
> done that, but search dpm_list for devices whose PM dependencies have
> already been satisfied and execute their callbacks without waiting.

Given this design, why bother to invoke device_resume() for the async 
devices?  Why not just start up a bunch of async threads, each of which 
calls async_resume() repeatedly until everything is finished?  (And 
rearrange async_resume() to scan the list first and do the actual 
resume second.)

The same goes for the noirq versions.

Alan Stern

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