linux-kernel - Re: unparseable, undocumented /sys/class/drm/.../pstate

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Date:	Mon, 23 Jun 2014 12:07:27 -0400
From:	Greg KH <greg@...ah.com>
To:	Ilia Mirkin <imirkin@...m.mit.edu>
Cc:	Pavel Machek <pavel@....cz>,
	kernel list <linux-kernel@...r.kernel.org>,
	Ben Skeggs <bskeggs@...hat.com>,
	Alexandre Courbot <acourbot@...dia.com>,
	David Airlie <airlied@...ux.ie>,
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>
Subject: Re: unparseable, undocumented /sys/class/drm/.../pstate

On Sun, Jun 22, 2014 at 10:12:14PM -0400, Ilia Mirkin wrote:
> On Sat, Jun 21, 2014 at 3:45 PM, Greg KH <greg@...ah.com> wrote:
> > On Sat, Jun 21, 2014 at 02:22:59PM -0400, Ilia Mirkin wrote:
> >> On Sat, Jun 21, 2014 at 2:02 PM, Pavel Machek <pavel@....cz> wrote:
> >> > Hi!
> >> >
> >> > AFAICT, pstate file will contain something like
> >> >
> >> > 07: core 100 MHz memory 123 MHz *
> >> > 08: core 100-200 MHz memory 123 MHz
> >> >
> >> > ...which does not look exactly like one-value-per-file, and I'm pretty
> >> > sure userspace will get it wrong if it tries to parse it. Plus, I
> >> > don't see required documentation in Documentation/ABI.
> >> >
> >> > Should we disable it for now, so that userspace does not start
> >> > depending on it and we'll not have to maintain it forever?
> >> >
> >> > I guess better interface would be something like
> >> >
> >> > pstate/07/core_clock_min
> >> >           core_clock_max
> >> >           memory_clock_min
> >> >           memory_clock_max
> >> >
> >> > and then pstate/active containing just the number of active state?
> >> >
> >> > Thanks,
> >> >                                                                 Pavel
> >> >
> >> > PS: I have no nvidia, got the news at
> >> >
> >> > http://www.phoronix.com/scan.php?page=article&item=nouveau_try_linux316&num=2
> >>
> >> FTR, this file has been in place since 3.13, and there was a different
> >> file before it (performance_levels), with a comparable format since
> >> much earlier (definitely 3.8, probably earlier). I think it's meant a
> >> lot more for people looking at it and echo'ing stuff to it to modify
> >> the levels (where supported), than for programs parsing it. Perhaps
> >> sysfs is the wrong place for this -- what is the right place? debugfs?
> >
> > Yes, please move it to debugfs.
> 
> Could we just say that the format of this file is one-per-line of
> 
> level: information-for-the-user
> 
> And you can echo a level into it to switch to that level? That seems
> like a reasonable ABI to have... would be happy to throw it into a
> file somewhere... not sure where though.

sysfs files are "one value per file", that's it.  Do anything other than
that, and it can not be in sysfs, sorry.

greg k-h
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