[Intel-gfx] [PATCH] drm/i915: expose a fixed brightness range to userspace

[Eoff, Ullysses A]

Jani,

First of all, thank you for your explanation.  I was unaware of the
motivations behind the current implementation.  You are exactly
right that the whole reason for all this is that userspace is expecting
actual_brightness to always match the brightness it set.

I would like to point out that I never meant to suggest there was a
"functional" bug in the driver.  And my motivations were more about
testability via sysfs than anything else and the assumption that brightness
was supposed to always equal actual_brightness.  I really should have
been more explicit in pointing this out much sooner if it wasn't clear.

Thanks for providing a link to the documentation... I wish I had been
more diligent in looking for this in the first place.  I'm always more
inclined to defer to the documentation.  And in light of it, I stand
corrected.

Although your new patch would likely work, I don't think it's necessary
anymore to make brightness==actual_brightness always hold true; since
testing for that is the incorrect thing to do in the first place (based on
the documentation).  Nonetheless, testing brightness from userspace will
just have to get a little more creative.

Apologies for all the noise... now I'll go make some elsewhere. ;)

U. Artie

> -----Original Message-----
> From: Jani Nikula [mailto:jani.nikula at linux.intel.com]
> Sent: Wednesday, November 19, 2014 12:57 AM
> To: Eoff, Ullysses A; Stéphane Marchesin
> Cc: Jesse Barnes; intel-gfx at lists.freedesktop.org
> Subject: RE: [Intel-gfx] [PATCH] drm/i915: expose a fixed brightness range to userspace
> 
> On Wed, 19 Nov 2014, "Eoff, Ullysses A" <ullysses.a.eoff at intel.com> wrote:
> >> -----Original Message-----
> >> From: Stéphane Marchesin [mailto:stephane.marchesin at gmail.com]
> >> Sent: Tuesday, November 18, 2014 3:53 PM
> >> To: Eoff, Ullysses A
> >> Cc: Jani Nikula; Jesse Barnes; intel-gfx at lists.freedesktop.org
> >> Subject: Re: [Intel-gfx] [PATCH] drm/i915: expose a fixed brightness range to userspace
> >>
> >> On Tue, Nov 18, 2014 at 3:18 PM, Eoff, Ullysses A
> >> <ullysses.a.eoff at intel.com> wrote:
> >> > Thanks Jesse for the ack.
> >> >
> >> > Unfortunately I just learned from Stéphane that there
> >> > are certain devices which only support 256 levels, so this
> >> > patch would do us no good at solving the real issue for
> >> > such devices.
> >> >
> >> > Why can't we just use a dynamic 1:1 mapping as was
> >> > suggested before?  I would vote for that instead.
> >> >
> >>
> >> Right, from my (consumer's) perspective, a 1:1 mapping is simpler. But
> >> the confusing part for me is that (as far as I can see) the current
> >> mapping should be 1:1 (because the user and hw ranges are the same),
> >> even though it goes through the scale logic. Is the scale() function
> >> maybe not the identity? If it isn't, maybe we just need to make it
> >> so...
> >>
> >
> > Yes, if the user and hw ranges are the same, then there will be a
> > 1:1 mapping, currently, and no issue.  It's the other case where
> > the hw range is smaller than the user range we end up with
> > brightness != actual_brightness in sysfs.  The scale logic rounds
> > into discrete values of the ranges where multiple user values can
> > scale to the same hw value in this case.  Right now, user range is
> > [0..max hw] and hw range is [min_hw..max_hw].  If min_hw > 0,
> > then we encounter the problem.  The proposal is to set the user
> > range to [0..(hw_max - hw_min)].
> 
> Some things to consider.
> 
> Have you heard of any requirements to support changing backlight PWM
> frequency run time? We currently don't support it, and it would require
> a fixed range. The backlight class interface does not support changing
> max brightness on the fly. Sure, we can implement this later if
> required, but we now have most of what's needed for this in place.
> 
> The luminance of the backlight is not a linear function of the
> brightness value set. Currently a single brightness step has a different
> luminance change depending on the absolute value. There's been talk
> about letting userspace fix this, but I'm not convinced the userspace
> has any chance of abstracting the plethora of hardware out there. As it
> happens, the ACPI opregion the driver has access to, does have a lookup
> table for this. We could fix this in the driver, but not if we commit to
> having 1:1 mapping.
> 
> Another thing to consider is that the max value we currently expose is
> quite meaningless to the userspace. I question the point of exposing a
> range of, say, 0..10000 when in reality you'll only get maybe 100
> distinct levels of brightness, depending on the backlight frequency.
> 
> An interesting and perhaps counter intuitive detail, the higher the PWM
> frequency, i.e. the higher the exposed max, the fewer user
> distinguishable levels you can actually get from the backlight. This is
> due to the rise and fall times in the backlight following the PWM
> signal.
> 
> Finally, it seems to me the problem with the scaling boils down to
> userspace expecting actual_brightness to always match the brightness it
> set. That's the value read back from the hardware. The ABI explicitly
> says the brightness stored in the driver may not be the actual
> brightness [1]. I don't think there are guarantees that all hardware
> would or could maintain the precision either. I think that's broken in
> userspace, but we're not supposed to say such things.
> 
> Soo... here's an attempt to be constructive after all the whining
> above. ;) How about this to always return the same value if the actual
> brightness duty cycle in the hardware has not changed? Totally untested,
> of course.
> 
> diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
> index 4d63839bd9b4..8678467d5d83 100644
> --- a/drivers/gpu/drm/i915/intel_panel.c
> +++ b/drivers/gpu/drm/i915/intel_panel.c
> @@ -1024,7 +1024,12 @@ static int intel_backlight_device_get_brightness(struct backlight_device *bd)
>  	drm_modeset_lock(&dev->mode_config.connection_mutex, NULL);
> 
>  	hw_level = intel_panel_get_backlight(connector);
> -	ret = scale_hw_to_user(connector, hw_level, bd->props.max_brightness);
> +	if (hw_level == scale_user_to_hw(connector, bd->props.brightness,
> +					 bd->props.max_brightness))
> +		ret = bd->props.brightness;
> +	else
> +		ret = scale_hw_to_user(connector, hw_level,
> +				       bd->props.max_brightness);
> 
>  	drm_modeset_unlock(&dev->mode_config.connection_mutex);
>  	intel_runtime_pm_put(dev_priv);
> 
> 
> BR,
> Jani.
> 
> 
> 
> [1] http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documentation/ABI/stable/sysfs-class-backlight
> 
> 
> --
> Jani Nikula, Intel Open Source Technology Center