On Mon, 10 Nov 2014, Jani Nikula <jani.nikula@xxxxxxxxxxxxxxx> wrote: > On Sat, 08 Nov 2014, "Eoff, Ullysses A" <ullysses.a.eoff@xxxxxxxxx> wrote: >> On 09/24/2014 10:42 AM, Eoff, Ullysses A wrote: >>>> -----Original Message----- >>>> From: Intel-gfx [mailto:intel-gfx-bounces@xxxxxxxxxxxxxxxxxxxxx] On Behalf Of Jani Nikula >>>> Sent: Wednesday, September 24, 2014 10:08 AM >>>> To: Hans de Goede; Joe Konno; intel-gfx@xxxxxxxxxxxxxxxxxxxxx >>>> Subject: Re: [PATCH] drm/i915: intel_backlight scale() math WA v2 >>>> >>>> On Wed, 24 Sep 2014, Hans de Goede <hdegoede@xxxxxxxxxx> wrote: >>>>> Hi, >>>>> >>>>> On 09/24/2014 05:54 PM, Joe Konno wrote: >>>>>> From: Joe Konno <joe.konno@xxxxxxxxx> >>>>>> >>>>>> Improper truncated integer division in the scale() function causes >>>>>> actual_brightness != brightness. This (partial) work-around should be >>>>>> sufficient for a majority of use-cases, but it is by no means a complete >>>>>> solution. >>>>>> >>>>>> TODO: Determine how best to scale "user" values to "hw" values, and >>>>>> vice-versa, when the ranges are of different sizes. That would be a >>>>>> buggy scenario even with this work-around. >>>>>> >>>>>> The issue was introduced in the following (v3.17-rc1) commit: >>>>>> >>>>>> 6dda730 drm/i915: respect the VBT minimum backlight brightness >>>>>> >>>>>> v2: (thanks to Chris Wilson) clarify commit message, use rounded division >>>>>> macro >>>>> I wonder why do scaling at all, why not simply shift hw_min - hw_max range >>>>> to 0 - (hw_max - hw_min) range and set max_brightness as seen by userspace >>>>> to (hw_max - hw_min) ? >>>> Mostly in preparation for a future where we expose an arbitrary range, >>>> say 0..100 or 0..255 to the userspace. >>>> >>> The problem with this scaling method is that scaling from user level to hw level and >>> back to user level is ambiguous since there isn't a 1:1 mapping between the user >>> range and the hw range. >>> >>> On the other hand, this patch does fix a bug in the currently used method (scaling). >>> That, at least, is an improvement nonetheless. >>> >>> U. Artie >> Apologies for resurrecting an old thread. But I think we still need to >> address >> this issue about not having a 1:1 mapping between user and hw levels. >> >> Right now, the problem is that the user range is larger than the hw >> range which >> results in one or more user levels mapping to the same hw level. And when >> userspace requests one of those levels, the result that is reported back to >> userspace might not be the same as what was requested. Take for example, on >> my system the hw range is [398, 7812] and the user range is [0, 7812]. >> Suppose >> userspace requests level 7017. This maps to hw level 7058. And when >> userspace requests the current level, 7018 is reported back (+1 from what >> was originally requested). In fact, with these particular ranges, there >> are exactly >> 398 values that this occurs. >> >> This problem will be compounded the larger the difference in length of the >> discrete ranges; so long as user range > hw range. >> >> Hans' solution would fix this problem, giving 1:1 mapping from hw to user >> levels. >> >> Jani's [future] solution would work too, since exposing a smaller range to >> userspace than the hw range would isolate the non 1:1 mapping inside the >> driver. > > I think we should just pick an arbitrary range, say 0..100, and be done > with it. It's not like you'd be able to get much more than 100 distinct > brightness levels out of the backlight anyway, no matter what the PWM > settings. > > BR, > Jani. PS. This (totally untested) patch should do it: diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c index b001c90312e7..a6680081415b 100644 --- a/drivers/gpu/drm/i915/intel_panel.c +++ b/drivers/gpu/drm/i915/intel_panel.c @@ -1035,7 +1035,7 @@ static int intel_backlight_device_register(struct intel_connector *connector) * Note: Everything should work even if the backlight device max * presented to the userspace is arbitrarily chosen. */ - props.max_brightness = panel->backlight.max; + props.max_brightness = 100; props.brightness = scale_hw_to_user(connector, panel->backlight.level, props.max_brightness); > > > > >> >> U. Artie >>>> BR, >>>> Jani. >>>> >>>> >>>>> Regards, >>>>> >>>>> Hans >>>>> >>>>> >>>>>> Signed-off-by: Joe Konno <joe.konno@xxxxxxxxx> >>>>>> --- >>>>>> drivers/gpu/drm/i915/intel_panel.c | 2 +- >>>>>> 1 file changed, 1 insertion(+), 1 deletion(-) >>>>>> >>>>>> diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c >>>>>> index f17ada3..dcdfbb3 100644 >>>>>> --- a/drivers/gpu/drm/i915/intel_panel.c >>>>>> +++ b/drivers/gpu/drm/i915/intel_panel.c >>>>>> @@ -421,7 +421,7 @@ static uint32_t scale(uint32_t source_val, >>>>>> /* avoid overflows */ >>>>>> target_val = (uint64_t)(source_val - source_min) * >>>>>> (target_max - target_min); >>>>>> - do_div(target_val, source_max - source_min); >>>>>> + target_val = DIV_ROUND_CLOSEST(target_val, source_max - source_min); >>>>>> target_val += target_min; >>>>>> >>>>>> return target_val; >>>>>> >>>>> _______________________________________________ >>>>> Intel-gfx mailing list >>>>> Intel-gfx@xxxxxxxxxxxxxxxxxxxxx >>>>> http://lists.freedesktop.org/mailman/listinfo/intel-gfx >>>> -- >>>> Jani Nikula, Intel Open Source Technology Center >>>> _______________________________________________ >>>> Intel-gfx mailing list >>>> Intel-gfx@xxxxxxxxxxxxxxxxxxxxx >>>> http://lists.freedesktop.org/mailman/listinfo/intel-gfx >>> _______________________________________________ >>> Intel-gfx mailing list >>> Intel-gfx@xxxxxxxxxxxxxxxxxxxxx >>> http://lists.freedesktop.org/mailman/listinfo/intel-gfx >>> >> > > -- > Jani Nikula, Intel Open Source Technology Center > _______________________________________________ > Intel-gfx mailing list > Intel-gfx@xxxxxxxxxxxxxxxxxxxxx > http://lists.freedesktop.org/mailman/listinfo/intel-gfx -- Jani Nikula, Intel Open Source Technology Center _______________________________________________ Intel-gfx mailing list Intel-gfx@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/intel-gfx
