Re: [igt-dev] [v4] tests/kms_color: Fix CRC mismatch issues with ctm test

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Fri, Mar 29, 2019 at 10:05:14AM +0100, Daniel Vetter wrote:
> On Fri, Mar 29, 2019 at 10:00:59AM +0100, Daniel Vetter wrote:
> > On Fri, Mar 29, 2019 at 02:49:15AM +0530, Uma Shankar wrote:
> > > Due to Gamma/Degamma limitation with precision (lack of
> > > exact 1.0 representation) due to ABI restriction, applying
> > 
> > Huh, why? That sounds like a conversion bug in our gamma table handler.
> > 0xffff == 1.0 if we don't treat it like that that's a driver bug. The
> > gamma table is _not_ fixed point, but linear range from 0-0xffff. Which is
> > unlike the ctm (which due to an uapi accident has a really hilarious fixed
> > point with sign bit format).
> > 
> > Please don't paper over driver bugs :-)
> 
> Can you pls also review existing gamma igt coverage to make sure we're
> catching this? Or maybe it's just the testcase that fills the gamma table
> the wrong way.

I've been pondering if we should just do value+1 in the kernel for the
last LUT entry when using the interpolated modes.

For userspace we could probably use the odd LUT size as a hint to 
indicate that the hardware will interpolate. So userspace could just
do something like "if (size & 1) max = 1<<16; else max = (1<<16)-1;"
when generating the curve (+ clamp to 0xffff). Looks like there's
some kind of kludge for CHV in kms_color atm, but maybe we can just
replace that with the generic logic above.

I'm also not sure the gamma tests actually are testing things
sufficiently. IIRC we have the 0 vs. max value type of tests
but is there anything to make sure eg. a LUT value of 0.5 does
what it's supposed to?

-- 
Ville Syrjälä
Intel
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