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

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Date:	Mon, 23 Jun 2014 12:18:51 -0400
From:	Ilia Mirkin <imirkin@...m.mit.edu>
To:	Greg KH <greg@...ah.com>
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 Mon, Jun 23, 2014 at 12:07 PM, Greg KH <greg@...ah.com> wrote:
> 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.

I think that's a little inconsistent. There are *tons* of files in
sysfs with multiple values. For example "local_cpulist" which contains
the string "0-3" for me, the per-connector "modes" file which has a
list of modes supported, and a "resource" file which has a list of hex
values (which probably have something to do with PCI resources?). [I
purposely picked values coming from different parts of the kernel not
to focus on one subsystem...]

  -ilia
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