Re: [PATCH 2/2] drm/i915/dp: check eDP display control capability registers

[Thierry Reding <thierry.reding@gmail.com>]

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On Mon, Nov 18, 2013 at 10:09:56AM -0500, Alex Deucher wrote:
> On Mon, Nov 18, 2013 at 9:27 AM, Thierry Reding
> <thierry.reding@gmail.com> wrote:
> > On Fri, Nov 15, 2013 at 03:01:51PM +0200, Jani Nikula wrote:
> >> Debug print the capabilities, and flag an error if the panel does not
> >> support adjusting backlight through the BL_PWM_DIM pin, requiring
> >> backlight control through DPCD.
> >>
> >> I haven't seen such panels yet, but it's a matter of time. Give
> >> ourselves a reminder when we need to fix this for real.
> >>
> >> Signed-off-by: Jani Nikula <jani.nikula@intel.com>
> >> ---
> >>  drivers/gpu/drm/i915/intel_dp.c |   14 ++++++++++++++
> >>  1 file changed, 14 insertions(+)
> >
> > I have a few general comments below, but this patch itself look fine,
> > so:
> >
> > Reviewed-by: Thierry Reding <treding@nvidia.com>
> >
> >> diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
> >> index cbf33be..ea4f3d1 100644
> >> --- a/drivers/gpu/drm/i915/intel_dp.c
> >> +++ b/drivers/gpu/drm/i915/intel_dp.c
> >> @@ -2816,6 +2816,20 @@ intel_dp_get_dpcd(struct intel_dp *intel_dp)
> >>                       dev_priv->psr.sink_support = true;
> >>                       DRM_DEBUG_KMS("Detected EDP PSR Panel.\n");
> >>               }
> >> +
> >> +             if (intel_dp->dpcd[DP_EDP_CONFIGURATION_CAP] &
> >> +                 DP_DPCD_DISPLAY_CONTROL_CAP) {
> >> +                     u8 ctrl[4] = { 0 };
> >> +
> >> +                     intel_dp_aux_native_read(intel_dp, DP_EDP_REV,
> >> +                                              ctrl, sizeof(ctrl));
> >> +                     DRM_DEBUG_KMS("eDP DPCD CTRL %02x %02x %02x %02x\n",
> >> +                                   ctrl[0], ctrl[1], ctrl[2], ctrl[3]);
> >> +
> >> +                     /* We don't support DPCD backlight control yet. */
> >> +                     if (ctrl[0] && (ctrl[1] & 1) && !(ctrl[2] & 1))
> >> +                             DRM_ERROR("eDP AUX backlight control only\n");
> >> +             }
> >>       }
> >>
> >>       if (!(intel_dp->dpcd[DP_DOWNSTREAMPORT_PRESENT] &
> >
> > I think a lot of eDP utility code could be made reusable across drivers.
> > We could probably do that by having each driver expose a drm_edp object
> > of some sort.
> >
> > Actually, the same would be true of DP in general. Accessing the DPCD is
> > something that's driver specific, but once you know how to do that a lot
> > of code can be made generic. I think a struct drm_dp could look like
> > this:
> >
> >         struct drm_dp;
> >
> >         struct drm_dpcd_ops {
> >                 ssize_t (*read)(struct drm_dp *dp, unsigned int offset,
> >                                 void *buffer, size_t size);
> >                 ssize_t (*write)(struct drm_dp *dp, unsigned int offset,
> >                                  const void *buffer, size_t size);
> >         };
> >
> >         struct drm_dp {
> >                 const struct drm_dpcd_ops *dpcd;
> >         };
> >
> > Perhaps that could even be extended with functionality to implement link
> > training in a generic way. There are already quite a few helpers to help
> > with that in drivers/gpu/drm/drm_dp_helper.c, but they assume that the
> > DPCD will be handed to them as a large buffer and therefore cannot write
> > DPCD registers.
> >
> > I suppose one could argue that it would be introducing a mid-layer, but
> > that layer would be really thin in my opinion. And it would allow a lot
> > of the algorithms to be written only once instead of multiple times.
> 
> I think it could probably be made to work.  The tricky part would be
> hw specific ordering in the training sequence.  At the very minimum,
> you need driver callbacks to set up the source side:
> 
> set_training_pattern()
> set_vs_emph()
> 
> And probably some flags to indicate whether the the hw supports
> specific features like training pattern 3.

Yes, something along those lines was what I had in mind as well. I know
that many people are unhappy about introducing this kind of mid-layer,
but quite frankly, doing this in generic code must have been one of the
primary reasons why VESA specified it that way.

The alternative will be to repeat more or less the same code in all the
drivers. I don't think that's a very nice alternative.

Thierry

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