On Mon, Sep 29, 2014 at 05:50:57PM +0000, Eoff, Ullysses A wrote: > > -----Original Message----- > > From: Intel-gfx [mailto:intel-gfx-bounces@xxxxxxxxxxxxxxxxxxxxx] On Behalf Of Jani Nikula > > Sent: Monday, September 29, 2014 6:07 AM > > To: Joe Konno; intel-gfx@xxxxxxxxxxxxxxxxxxxxx > > Subject: Re: [PATCH] drm/i915: intel_backlight scale() math WA v2 > > > > On Wed, 24 Sep 2014, Joe Konno <joe.konno@xxxxxxxxxxxxxxx> 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 > > > > > > 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); > > > > This fails to build with CONFIG_X86_32=y: > > > > drivers/built-in.o: In function `scale': > > intel_panel.c:(.text+0x12ab38): undefined reference to `__udivdi3' > > make: *** [vmlinux] Error 1 > > > > > > Do you have a recommended workaround? Should we just > use the v1 technique instead? Compromise and write DIV_ROUND_CLOSEST_ULL(). :| -Chris -- Chris Wilson, Intel Open Source Technology Centre _______________________________________________ Intel-gfx mailing list Intel-gfx@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/intel-gfx
