> -----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 > 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
