[Intel-gfx] [PATCH 1/2 v3] drm/i915: intel_backlight scale() math WA

[Daniel Vetter]

On Tue, Sep 30, 2014 at 02:58:54PM +0000, Eoff, Ullysses A wrote:
> On Tue, 2014-09-30 at 10:04 +0200, Daniel Vetter wrote:
> > On Mon, Sep 29, 2014 at 03:49:32PM -0700, U. Artie Eoff wrote:
> > > 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
> > > 
> > > v3: -DIV_ROUND_CLOSEST() fails to build with CONFIG_X86_32=y. (Jani)
> > >     -Use DIV_ROUND_CLOSEST_ULL() instead. (Damien)
> > >     -v1 and v2 originally authored by Joe Konno.
> > > 
> > > Signed-off-by: U. Artie Eoff <ullysses.a.eoff at intel.com>
> > 
> > Is there some bug report, internal jira, mailing list reference or similar
> > about this?
> > 
> > Note that at least for OTC jira tasks we now want them to be added to
> > commit message with e.g.
> > 
> > OTC-Jria: VIZ-4932
> > 
> 
> Yes, the OTC-Jira task is: VIZ-4395.  I'll resubmit with amended commit
> message.
> 
> > And I guess I should merge patch 2 before patch 1, right?
> 
> No, patch 1 before patch 2.

Oh, I didn't notice that your first add a duplicated version of the macro
and then unify it. That's a bit backwards ...
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch