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@xxxxxxxxxxxxxxx] > Sent: Wednesday, November 19, 2014 12:57 AM > To: Eoff, Ullysses A; Stéphane Marchesin > Cc: Jesse Barnes; intel-gfx@xxxxxxxxxxxxxxxxxxxxx > Subject: RE: [PATCH] drm/i915: expose a fixed brightness range to userspace > > On Wed, 19 Nov 2014, "Eoff, Ullysses A" <ullysses.a.eoff@xxxxxxxxx> wrote: > >> -----Original Message----- > >> From: Stéphane Marchesin [mailto:stephane.marchesin@xxxxxxxxx] > >> Sent: Tuesday, November 18, 2014 3:53 PM > >> To: Eoff, Ullysses A > >> Cc: Jani Nikula; Jesse Barnes; intel-gfx@xxxxxxxxxxxxxxxxxxxxx > >> Subject: Re: [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@xxxxxxxxx> 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 _______________________________________________ Intel-gfx mailing list Intel-gfx@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/intel-gfx
