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

[Ilia Mirkin]

On Fri, Nov 24, 2017 at 8:38 AM, Ville Syrjälä
<ville.syrjala at linux.intel.com> wrote:
> 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)

The latter method is interesting, it slowly biases from rounding down
to rounding up as you go through the range. Clever. I guess I could
change my function to use

(value << 8) | value

and then use that to shift around. I'll send a v2.

  -ilia