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

[Daniel Vetter]

On Tue, Sep 30, 2014 at 04:52:43PM +0000, Eoff, Ullysses A wrote:
> > -----Original Message-----
> > From: Daniel Vetter [mailto:daniel.vetter at ffwll.ch] On Behalf Of Daniel Vetter
> > Sent: Tuesday, September 30, 2014 9:32 AM
> > To: Eoff, Ullysses A
> > Cc: daniel at ffwll.ch; intel-gfx at lists.freedesktop.org
> > Subject: Re: [Intel-gfx] [PATCH 1/2 v3] drm/i915: intel_backlight scale() math WA
> > 
> > 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 ...
> 
> The reason for doing it this way is so we can cherry-pick patch 1 into the
> chromeos 3.10 kernel cleanly (with some other backlight patches).
> That is, patch 1 is decoupled from the changes in intel_display.c.
> Hope that makes sense.

Yeah. Would be good to mention that in the commit message, e.g. "For
easier backporting Duplicate the division macro here and refactor the code
in a follow-up patch". I'll add that when merging if you don't resend.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch