linux-kernel - Re: [PATCH V2 0/4] introduce device async actions mechanism

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Message-ID: <Pine.LNX.4.44L0.0908111148060.2562-100000@iolanthe.rowland.org>
Date:	Tue, 11 Aug 2009 11:53:57 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
cc:	Zhang Rui <rui.zhang@...el.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-pm <linux-pm@...ts.linux-foundation.org>,
	linux-acpi <linux-acpi@...r.kernel.org>,
	Pavel Machek <pavel@....cz>, Len Brown <lenb@...nel.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	"dtor@...l.ru" <dtor@...l.ru>
Subject: Re: [PATCH V2 0/4] introduce device async actions mechanism

On Tue, 11 Aug 2009, Rafael J. Wysocki wrote:

> In fact, we don't need the layers at all.  The only thing we have to assure is
> that, during resume, the devices given device depends on will be handled
> before we start to handle this particular device (inversely during suspend).
> 
> Please note that we're not even allowed to start executing the device's
> resume callback before the callbacks of the devices it depends on have
> returned (the same applies to the suspend callbacks, but the other way around).

The general algorithm for maximum parallelism goes as follows: Start by
resuming (in parallel) all the devices which don't depend on anything
else.  Each time a resume finishes, you go on to resume (in parallel)
all the devices which depend only on resumed devices and which haven't
yet started to resume.

As described, this can require a large number of threads.  It also
requires detailed knowledge of which devices depend on others, which we
don't have.

Alan Stern

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