[PATCH v2 2/2] drm/connector: automatically set immutable flag for max_bpc property

[Maxime Ripard]

Hi,

On Tue, Jun 25, 2024 at 09:21:27AM GMT, Dmitry Baryshkov wrote:
> On Tue, 25 Jun 2024 at 01:56, Abhinav Kumar <quic_abhinavk at quicinc.com> wrote:
> >
> >
> >
> > On 6/24/2024 3:46 PM, Dmitry Baryshkov wrote:
> > > On Tue, 25 Jun 2024 at 01:39, Abhinav Kumar <quic_abhinavk at quicinc.com> wrote:
> > >>
> > >> + IGT dev
> > >>
> > >> On 6/22/2024 10:40 PM, Dmitry Baryshkov wrote:
> > >>> With the introduction of the HDMI Connector framework the driver might
> > >>> end up creating the max_bpc property with min = max = 8. IGT insists
> > >>> that such properties carry the 'immutable' flag. Automatically set the
> > >>> flag if the driver asks for the max_bpc property with min == max.
> > >>>
> > >>
> > >> This change does not look right to me.
> > >>
> > >> I wonder why we need this check because DRM_MODE_PROP_IMMUTABLE means
> > >> that as per the doc, userspace cannot change the property.
> > >>
> > >>            * DRM_MODE_PROP_IMMUTABLE
> > >>            *     Set for properties whose values cannot be changed by
> > >>            *     userspace. The kernel is allowed to update the value of
> > >> these
> > >>            *     properties. This is generally used to expose probe state to
> > >>            *     userspace, e.g. the EDID, or the connector path property
> > >> on DP
> > >>            *     MST sinks. Kernel can update the value of an immutable
> > >> property
> > >>            *     by calling drm_object_property_set_value().
> > >>            */
> > >>
> > >> Here we are allowing userspace to change max_bpc
> > >>
> > >>
> > >> drm_atomic_connector_set_property()
> > >> {
> > >>          **********
> > >>
> > >>           } else if (property == connector->max_bpc_property) {
> > >>                   state->max_requested_bpc = val;
> > >>
> > >>          **********
> > >> }
> > >>
> > >> I believe you are referring to this IGT check right?
> > >>
> > >> https://gitlab.freedesktop.org/drm/igt-gpu-tools/-/blob/master/tests/kms_properties.c#L428
> > >
> > > Yes
> > >
> > >>
> > >> I think we should fix IGT in this case unless there is some reason we
> > >> are missing. Because just because it has the same min and max does not
> > >> mean its immutable by the doc of the IMMUTABLE flag.
> > >
> > > Well, having the same min and max means that it is impossible to
> > > change the property. So the property is immutable, but doesn't have
> > > the flag.
> > >
> >
> > True, then does DRM_MODE_PROP_IMMUTABLE need a doc update too indicating
> > that even if the min and max is same, property will be interpreted as
> > immutable.
> 
> Granted that I'm only doing it for max_bpc property I don't think so.

Yeah, I have to agree with Abhinav here, it does look fishy to me too,
even more so that it's only ever "documented" through an igt routine
that has never documented why we want that.

I'm fine with the change if it's indeed what we expect, and it might
very well be, but I'd like to clear that up and document it first.

Maxime
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