linux-kernel - Re: [PATCH 2/2] pm_trace: Add sysfs attr for rechecking dev hash.

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Date:	Mon, 11 Oct 2010 00:12:06 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	James Hogan <james@...anarts.com>
Cc:	Pavel Machek <pavel@....cz>, Greg KH <greg@...ah.com>,
	linux-pm@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
	linux-doc@...r.kernel.org, Len Brown <len.brown@...el.com>,
	Randy Dunlap <rdunlap@...otime.net>,
	"Greg Kroah-Hartman" <gregkh@...e.de>,
	Alan Stern <stern@...land.harvard.edu>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	markgross <markgross@...gnar.org>
Subject: Re: [PATCH 2/2] pm_trace: Add sysfs attr for rechecking dev hash.

On Sunday, October 10, 2010, James Hogan wrote:
> On Sunday 10 October 2010 21:47:19 Pavel Machek wrote:
> > On Sun 2010-10-10 20:47:01, James Hogan wrote:
> > > Hi Pavel,
> > > 
> > > On Sunday 10 October 2010 19:04:00 Pavel Machek wrote:
> > > > Hi!
> > > > 
> > > > > If the device which fails to resume is part of a loadable kernel
> > > > > module it won't be checked at startup against the magic number
> > > > > stored in the RTC.
> > > > > 
> > > > > Add a read-only sysfs attribute /sys/power/pm_trace_dev_hash which
> > > > > contains a list of newline separated devices (usually just the one)
> > > > > which currently match the last magic number. This allows the device
> > > > > which is failing to resume to be found after the modules are loaded
> > > > > again.
> > > > > 
> > > > > --- a/Documentation/power/s2ram.txt
> > > > > +++ b/Documentation/power/s2ram.txt
> > > > > 
> > > > > @@ -49,6 +49,13 @@ machine that doesn't boot) is:
> > > > >     device (lspci and /sys/devices/pci* is your friend), and see if
> > > > >     you can fix it, disable it, or trace into its resume function.
> > > > > 
> > > > > +   If no device matches the hash, it may be a device from a loadable
> > > > > kernel +   module that is not loaded until after the hash is checked.
> > > > > You can check +   the hash against the current devices again after
> > > > > more modules are loaded +   using sysfs:
> > > > > +
> > > > > +	cat /sys/power/pm_trace_dev_hash
> > > > > +
> > > > 
> > > > Yep, but exact semantics of that sysfs file should probably be linked
> > > > in the sysfs documentation...
> > > > 
> > > > 								Pavel
> > > 
> > > To clarify, do you mean I should link to
> > > Documentation/ABI/testing/sysfs-power from
> > > Documentation/power/s2ram.txt, or just make sure the syfs file is
> > > documented in Documentation/ABI/testing/sysfs-power (which is done in v2
> > > of this patch)?
> > 
> > v2 of the patch is probably ok.
> > 
> > Also, sysfs should be one entry per file, and strictly speaking, this
> > one is not. That may be fine... but as this is debugging facility,
> > perhaps it should go to debugfs? Maybe cc gregkh...
> > 									Pavel
> 
> I thought about the one entry per file issue.
> Documentation/filesystems/sysfs.txt says:
> > It is noted that it may not be efficient to contain only one
> > value per file, so it is socially acceptable to express an array of
> > values of the same type.
> 
> which appears to cover my use of it as they're all the same type (and 
> considering that ideally there will be exactly 1 device listed anyway).
> 
> I wonder if I should separate them with spaces rather than newlines too 
> (newlines just seemed more appropriate for a variable sized list at the time).
> 
> I ackowledge your point that debugfs may be a more appropriate place for it, 
> but would that also apply to the pm_trace file?

Yes, it would.  Let's keep the things cosistent, ie. v2 of your patch is fine
and I'm going to apply it.

Thanks,
Rafael
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