[PATCH libdrm] tests: fix MAKE_RGBA macro for 10bpp modes

[Ville Syrjälä]

On Thu, Nov 23, 2017 at 03:14:46PM -0500, Ilia Mirkin wrote:
> On Thu, Nov 23, 2017 at 2:09 PM, Ville Syrjälä
> <ville.syrjala at linux.intel.com> wrote:
> > On Thu, Nov 23, 2017 at 01:59:28PM -0500, Ilia Mirkin wrote:
> >> We need to shift the values up, otherwise we'd end up with a negative
> >> shift. This works for up-to 16-bit components, which is fine for now.
> >
> > Shouldn't we actually replicate the high bits in the low bits?
> 
> Not entirely sure what you're proposing...
> 
> Ideally we wouldn't be lazy and pass e.g. 16-bit values to MAKE_RGBA
> which would then shift down as necessary (and even there, you could
> end up with off-by-1's maybe?). For e.g. 0xff, that should become
> 0x3ff but with my code will become 0x3fc.

Exactly the issue.

> But for other values, it's
> less clear what to do with the low bits. I figured it didn't really
> matter.
> 
> Do you have a concrete proposal?

The usual thing would be just

(value) * 0x3ff / 0xff

or

((value) << 2) | ((value) >> 6)

-- 
Ville Syrjälä
Intel OTC