Re: [PATCH 4/7] drm/omap: Implement CTM property for CRTC using OVL managers CPR matrix

[Ilia Mirkin <imirkin@alum.mit.edu>]

On Wed, Sep 4, 2019 at 4:08 PM Jyri Sarha <jsarha@ti.com> wrote:
>
> On 04/09/2019 14:11, Laurent Pinchart wrote:
> > Hi Jyri,
> >
> > On Wed, Sep 04, 2019 at 10:17:00AM +0300, Jyri Sarha wrote:
> >> On 03/09/2019 18:24, Laurent Pinchart wrote:
> >>> On Mon, Sep 02, 2019 at 03:53:56PM +0300, Tomi Valkeinen wrote:
> >>>> From: Jyri Sarha <jsarha@ti.com>
> >>>>
> >>>> Implement CTM color management property for OMAP CRTC using DSS
> >>>> overlay manager's Color Phase Rotation matrix. The CPR matrix does not
> >>>> exactly match the CTM property documentation. On DSS the CPR matrix is
> >>>> applied after gamma table look up. However, it seems stupid to add a
> >>>> custom property just for that.
> >>>
> >>> In that case the DRM documentation should be updated to mention that
> >>> both options are allowed.
> >>
> >> Ok, if that is alright. But if we do that, then I guess all the drivers
> >> implementing CTM should document the point where it is applied in the
> >> pipeline.
> >
> > Whatever solution we end up picking, I think it should at least be
> > discussed with a broader upstream audience and not just swept under the
> > omapdrm carpet :-)
> >
>
> I'll try to write something and send the next series to wider audience.
> Let's see what jury says.

In case it's useful ... the pipeline normally goes degamma -> ctm ->
gamma. If your ctm is applied after gamma, perhaps you can just rename
"gamma" to "degamma" and be done? (There's the unfortunate case of
legacy gamma which does end up in "GAMMA" when using atomic helpers.
But in such a case, you won't have a CTM.)

>
> >>>> Signed-off-by: Jyri Sarha <jsarha@ti.com>
> >>>> Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
> >>>> ---
> >>>>  drivers/gpu/drm/omapdrm/omap_crtc.c | 39 +++++++++++++++++++++++++++--
> >>>>  1 file changed, 37 insertions(+), 2 deletions(-)
> >>>>
> >>>> diff --git a/drivers/gpu/drm/omapdrm/omap_crtc.c b/drivers/gpu/drm/omapdrm/omap_crtc.c
> >>>> index 3c5ddbf30e97..d63213dd7d83 100644
> >>>> --- a/drivers/gpu/drm/omapdrm/omap_crtc.c
> >>>> +++ b/drivers/gpu/drm/omapdrm/omap_crtc.c
> >>>> @@ -391,6 +391,32 @@ static void omap_crtc_manual_display_update(struct work_struct *data)
> >>>>    }
> >>>>  }
> >>>>
> >>>> +static s16 omap_crtc_S31_32_to_s2_8(s64 coef)
> >>>> +{
> >>>> +  uint64_t sign_bit = 1ULL << 63;
> >>>> +  uint64_t cbits = (uint64_t) coef;
> >>>
> >>> s/uint64_t/u64/ for both lines as we're dealing with kernel code. And
> >>> there's no need for a space before coef.
> >>>
> >>>> +  s16 ret = clamp_val(((cbits & ~sign_bit) >> 24), 0, 0x1FF);
> >>>> +
> >>>> +  if (cbits & sign_bit)
> >>>> +          ret = -ret;
> >>>> +
> >>>> +  return ret;
> >>>
> >>> Can't this be simplified to
> >>>
> >>>     s16 ret = (coef >> 24) & 0x1ff;
> >>>
> >>>     return coef < 0 ? -ret : ret;
> >>>
> >>
> >> No. Clamping is different thing. If the original value is greater than
> >> what we can present with our 2 magnitude bit, we want to use the maximum
> >> value, not something that we may have in the LSB end of bits. e.g if
> >> user-space tries to set the value to 2.0 (= 0x200) we rather present it
> >> as 1.996 (= 0x1FF) than 0.0 (= 0x000).
> >
> > Of course, my bad.
> >
> > Perhaps a stupid question, should we reject out of range values at
> > atomic check time ?
> >
>
> I've at least seen CSC matrices with 2.0 values, so I think we should
> accept those and use clamping, but maybe we should reject CTMs with
> values far bigger than what we can represent. Such matrices would hardly
> work the way they were intended.

I clamped in nouveau. Not 100% sure it's the right policy. Having
something consistent across drivers is probably good. I don't think I
came up with clamping all by myself, so someone else must have been
doing it. (https://cgit.freedesktop.org/drm/drm/commit/drivers/gpu/drm/nouveau?id=88b703527ba70659365d989f29579f1292ebf9c3
-- see csc_drm_to_base.)

Cheers,

  -ilia
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