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

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Date: Mon, 24 Jun 2024 15:55:56 -0700
From: Abhinav Kumar <quic_abhinavk@...cinc.com>
To: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
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 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.

>>
>>
>>> 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