| Message ID | 535DC8C1.5090407@intel.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
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@intel.com> > 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@schottelius.org> > Signed-off-by: Aaron Lu <aaron.lu@intel.com> > --- > 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
On 05/04/2014 03:22 PM, Chris Wilson wrote: > 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@intel.com> >> 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@schottelius.org> >> Signed-off-by: Aaron Lu <aaron.lu@intel.com> >> --- >> 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. Ah...yes, my fault, thanks. > > Also this still has the same rounding error as before. I didn't get this, care to explain?
On Sun, May 04, 2014 at 03:31:01PM +0800, Aaron Lu wrote: > On 05/04/2014 03:22 PM, Chris Wilson wrote: > > Also this still has the same rounding error as before. > > I didn't get this, care to explain? The calculation you use, truncates, rather than say round to nearest, would is the same discrepancy in your changelog. -Chris
On 05/04/2014 06:41 PM, Chris Wilson wrote: > On Sun, May 04, 2014 at 03:31:01PM +0800, Aaron Lu wrote: >> On 05/04/2014 03:22 PM, Chris Wilson wrote: >>> Also this still has the same rounding error as before. >> >> I didn't get this, care to explain? > > The calculation you use, truncates, rather than say round to nearest, > would is the same discrepancy in your changelog. This patch is meant to fix the problem that when user specify the max backlight level in acpi_video interface, we will get the hardware max level too in i915. With this patch, the hardware level we will get is: level * freq / max = 255 * 937 / 255 = 937, which is correct. For other values, the truncates is OK: 254 * 937 / 255 = 933 253 * 937 / 255 = 929 ... ...
diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c index a953b081ee38..bdd2f24b7a6b 100644 --- a/drivers/gpu/drm/i915/intel_panel.c +++ b/drivers/gpu/drm/i915/intel_panel.c @@ -502,10 +502,7 @@ 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; + level = (u64)level * freq / max; panel->backlight.level = level; if (panel->backlight.device)
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-and-tested-by: Nico Schottelius <nico-bugzilla.kernel.org@schottelius.org> Signed-off-by: Aaron Lu <aaron.lu@intel.com> --- drivers/gpu/drm/i915/intel_panel.c | 5 +---- 1 file changed, 1 insertion(+), 4 deletions(-)