On Sun, May 04, 2014 at 03:16:05PM +0800, Aaron Lu wrote: > On 04/28/2014 09:41 PM, Daniel Vetter wrote: > > 64bit divisions won't compile on 32bit. You need one of the DO_DIV macros, > > or whatever they're called again. I pain, I know ;-) > > Thanks for the correction, here is an updated patch :-) > > From: Aaron Lu <aaron.lu@xxxxxxxxx> > Date: Mon, 28 Apr 2014 11:02:52 +0800 > Subject: [PATCH v2] drm/i915: restore backlight precision when converting from > ACPI > > When we set backlight on behalf of ACPI opregion, we will convert the > backlight value in the 0-255 range defined in opregion to the actual > hardware level. Commit 22505b82a2 (drm/i915: avoid brightness overflow > when doing scale) is meant to fix the overflow problem when doing the > conversion, but it also caused a problem that the converted hardware > level doesn't quite represent the intended value: say user wants maximum > backlight level(255 in opregion's range), then we will calculate the > actual hardware level to be: level = freq / max * level, where freq is > the hardware's max backlight level(937 on an user's box), and max and > level are all 255. The converted value should be 937 but the above > calculation will yield 765. > > To fix this issue, just use 64 bits to do the calculation to keep the > precision and avoid overflow at the same time. > > Buglink: https://bugzilla.kernel.org/show_bug.cgi?id=72491 > Reported-by: Nico Schottelius <nico-bugzilla.kernel.org@xxxxxxxxxxxxxxx> > Signed-off-by: Aaron Lu <aaron.lu@xxxxxxxxx> > --- > v2: use do_div as reminded by Daniel. > > drivers/gpu/drm/i915/intel_panel.c | 8 ++++---- > 1 file changed, 4 insertions(+), 4 deletions(-) > > diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c > index a953b081ee38..8725917a3d0d 100644 > --- a/drivers/gpu/drm/i915/intel_panel.c > +++ b/drivers/gpu/drm/i915/intel_panel.c > @@ -492,6 +492,7 @@ void intel_panel_set_backlight(struct intel_connector *connector, u32 level, > enum pipe pipe = intel_get_pipe_from_connector(connector); > u32 freq; > unsigned long flags; > + u64 n; > > if (!panel->backlight.present || pipe == INVALID_PIPE) > return; > @@ -502,10 +503,9 @@ void intel_panel_set_backlight(struct intel_connector *connector, u32 level, > > /* scale to hardware max, but be careful to not overflow */ > freq = panel->backlight.max; > - if (freq < max) > - level = level * freq / max; > - else > - level = freq / max * level; > + n = level * freq; 32b * 32b = 32b n = (u64)level * freq; to avoid overflow as you claim. Also this still has the same rounding error as before. -Chris -- Chris Wilson, Intel Open Source Technology Centre _______________________________________________ Intel-gfx mailing list Intel-gfx@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/intel-gfx
