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

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Message-ID: <20240625-enlightened-optimistic-fossa-fdd128@houat>
Date: Tue, 25 Jun 2024 15:58:25 +0200
From: Maxime Ripard <mripard@...nel.org>
To: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
Cc: Abhinav Kumar <quic_abhinavk@...cinc.com>, 
	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>, 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, Jun 25, 2024 at 10:23:14AM GMT, Dmitry Baryshkov wrote:
> On Tue, 25 Jun 2024 at 10:19, Maxime Ripard <mripard@...nel.org> wrote:
> >
> > 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@...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.
> >
> > 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.
> 
> Should I also move the setting of the IMMUTABLE flag to a more generic code?

Possibly, but I guess that will depend on the outcome of that discussion

Maxime

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