Re: [PATCH 0/5] PowerManagement Toggle for PowerTOP

From: Jani Nikula <jani.nikula@linux.intel.com>
To: Alexandra Yates <alexandra.yates@linux.intel.com>,
	intel-gfx@lists.freedesktop.org, rodrigo.vivi@intel.com,
	nivedita.swaminathan@intel.com, joe.konno@intel.com
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Subject: Re: [PATCH 0/5] PowerManagement Toggle for PowerTOP
Date: Wed, 13 Apr 2016 13:24:11 +0300	[thread overview]
Message-ID: <871t69u7qs.fsf@intel.com> (raw)
In-Reply-To: <1460488728-23319-1-git-send-email-alexandra.yates@linux.intel.com>

On Tue, 12 Apr 2016, Alexandra Yates <alexandra.yates@linux.intel.com> wrote:
> Permits the user to identify and toggle values for PSR, FBC, RC6,
> DRRS, and IPS under /sys/class/drm/card0/power/.  By enabling these
> features I'm looking to empower our customers, such as, power team,
> chrome OS, and platform integration teams to debug graphics power
> management features.

Most of these features have module parameters to enable or disable them,
with platform specific defaults. We enable and support each feature on
each platform where we have confidence it actually works. Conversely, we
don't support the features on platforms where we know they don't work,
and are therefore disabled by default. To underline this, the module
parameters are labeled "unsafe", and setting them taints the kernel. You
change the defaults, you're on your own.

If you google for the i915 module parameters, you'll find plenty of
pages recommending the users to set the various module parameters,
typically to gain power savings. It seems to me more often than not
people don't understand what they're doing when they enable the
features. It's not uncommon to see dmesgs in bug reports with all kinds
of module parameters set, regardless of whether they are relevant on the
platform in question or not, and it's not uncommon for the module
parameters to be the reason for the issue the bug reporter is seeing.

With this background, I am feeling rather hesitant about exposing all of
these features as an ABI through the sysfs. I fear the end result is
going to be PowerTOP recommending users to change the settings, leading
to more bug reports. I understand your intention is to help developers,
but from the ABI perspective there's really no such thing as "sysfs for
developers only". I'd probably feel less uneasy about having these in
the debugfs, because that sets the expectations right.

BR,
Jani.

-- 
Jani Nikula, Intel Open Source Technology Center
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