linux-kernel - Re: [PATCH v2 2/2] drm/connector: automatically set immutable flag for max

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Message-ID: <CAA8EJpo_dBDqLUVH-SkufhFchu64rhC+vkhVBFdt++E4pdCrQg@mail.gmail.com>
Date: Tue, 25 Jun 2024 09:21:27 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: Abhinav Kumar <quic_abhinavk@...cinc.com>
Cc: Andrzej Hajda <andrzej.hajda@...el.com>, Neil Armstrong <neil.armstrong@...aro.org>, 
	Robert Foss <rfoss@...nel.org>, Laurent Pinchart <Laurent.pinchart@...asonboard.com>, 
	Jonas Karlman <jonas@...boo.se>, Jernej Skrabec <jernej.skrabec@...il.com>, 
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>, 
	Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>, 
	Dave Stevenson <dave.stevenson@...pberrypi.com>, 
	"igt-dev@...ts.freedesktop.org" <igt-dev@...ts.freedesktop.org>, Petri Latvala <adrinael@...inael.net>, 
	Kamil Konieczny <kamil.konieczny@...ux.intel.com>, 
	Ville Syrjälä <ville.syrjala@...ux.intel.com>, 
	Rob Clark <robdclark@...il.com>, dri-devel@...ts.freedesktop.org, 
	freedreno@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] drm/connector: automatically set immutable flag
 for max_bpc property

On Tue, 25 Jun 2024 at 01:56, Abhinav Kumar <quic_abhinavk@...cinc.com> wrote:
>
>
>
> On 6/24/2024 3:46 PM, Dmitry Baryshkov wrote:
> > On Tue, 25 Jun 2024 at 01:39, Abhinav Kumar <quic_abhinavk@...cinc.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.

>
> >>
> >>
> >>> Fixes: aadb3e16b8f3 ("drm/connector: hdmi: Add output BPC to the connector state")
> >>> Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
> >>> ---
> >>>    drivers/gpu/drm/drm_connector.c | 7 ++++++-
> >>>    1 file changed, 6 insertions(+), 1 deletion(-)
> >
> > With best wishes
> > Dmitry



-- 
With best wishes
Dmitry