Re: [PATCH 2/4] drm/uAPI: Add "active bpc" as feedback channel for "max bpc" drm property

[Werner Sembach <wse@tuxedocomputers.com>]

Am 07.06.21 um 09:40 schrieb Maxime Ripard:
> Hi,
>
> On Fri, Jun 04, 2021 at 07:17:21PM +0200, Werner Sembach wrote:
>> Add a new general drm property "active bpc" which can be used by graphic drivers
>> to report the applied bit depth per pixel back to userspace.
> Just a heads up, we'll need an open source project that has accepted it
> before merging it.
>
> See https://www.kernel.org/doc/html/latest/gpu/drm-uapi.html#open-source-userspace-requirements
Yes, I know.
>
>> While "max bpc" can be used to change the color depth, there was no way to check
>> which one actually got used. While in theory the driver chooses the best/highest
>> color depth within the max bpc setting a user might not be fully aware what his
>> hardware is or isn't capable off. This is meant as a quick way to double check
>> the setup.
>>
>> In the future, automatic color calibration for screens might also depend on this
>> information available.
>>
>> Signed-off-by: Werner Sembach <wse@tuxedocomputers.com>
>> ---
>>   drivers/gpu/drm/drm_atomic_uapi.c |  2 ++
>>   drivers/gpu/drm/drm_connector.c   | 40 +++++++++++++++++++++++++++++++
>>   include/drm/drm_connector.h       | 15 ++++++++++++
>>   3 files changed, 57 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/drm_atomic_uapi.c b/drivers/gpu/drm/drm_atomic_uapi.c
>> index 268bb69c2e2f..7ae4e40936b5 100644
>> --- a/drivers/gpu/drm/drm_atomic_uapi.c
>> +++ b/drivers/gpu/drm/drm_atomic_uapi.c
>> @@ -873,6 +873,8 @@ drm_atomic_connector_get_property(struct drm_connector *connector,
>>   		*val = 0;
>>   	} else if (property == connector->max_bpc_property) {
>>   		*val = state->max_requested_bpc;
>> +	} else if (property == connector->active_bpc_property) {
>> +		*val = state->active_bpc;
>>   	} else if (connector->funcs->atomic_get_property) {
>>   		return connector->funcs->atomic_get_property(connector,
>>   				state, property, val);
>> diff --git a/drivers/gpu/drm/drm_connector.c b/drivers/gpu/drm/drm_connector.c
>> index 7631f76e7f34..5f42a5be5822 100644
>> --- a/drivers/gpu/drm/drm_connector.c
>> +++ b/drivers/gpu/drm/drm_connector.c
>> @@ -1195,6 +1195,13 @@ static const struct drm_prop_enum_list dp_colorspaces[] = {
>>    *	drm_connector_attach_max_bpc_property() to create and attach the
>>    *	property to the connector during initialization.
>>    *
>> + * active bpc:
>> + *	This read-only range property is used by userspace check the bit depth
>> + *	actually applied by the GPU driver after evaluation all hardware
>                                  ^ display
>
> Depending on the system, the display component might have a GPU attached
> or not, and the GPU might have a display component or not.
Ok, I try to bee more clear in the wording
>
>> + *	capabilities and max bpc. Drivers to use the function
>> + *	drm_connector_attach_active_bpc_property() to create and attach the
>> + *	property to the connector during initialization.
>> + *
>>    * Connectors also have one standardized atomic property:
>>    *
>>    * CRTC_ID:
>> @@ -2150,6 +2157,39 @@ int drm_connector_attach_max_bpc_property(struct drm_connector *connector,
>>   }
>>   EXPORT_SYMBOL(drm_connector_attach_max_bpc_property);
>>   
>> +/**
>> + * drm_connector_attach_active_bpc_property - attach "active bpc" property
>> + * @connector: connector to attach active bpc property on.
>> + * @min: The minimum bit depth supported by the connector.
>> + * @max: The maximum bit depth supported by the connector.
>> + *
>> + * This is used to check the applied bit depth on a connector.
>> + *
>> + * Returns:
>> + * Zero on success, negative errno on failure.
>> + */
>> +int drm_connector_attach_active_bpc_property(struct drm_connector *connector,
>> +					  int min, int max)
>> +{
>> +	struct drm_device *dev = connector->dev;
>> +	struct drm_property *prop;
>> +
>> +	prop = connector->active_bpc_property;
>> +	if (!prop) {
>> +		prop = drm_property_create_range(dev, 0, "active bpc", min, max);
>> +		if (!prop)
>> +			return -ENOMEM;
>> +
>> +		connector->active_bpc_property = prop;
>> +	}
>> +
>> +	drm_object_attach_property(&connector->base, prop, 0);
>> +	connector->state->active_bpc = 0;
> I guess we want to default to 8?
>
> Maxime
The default state should only be active when no display is connected or 
the display is off, so I thought 0 makes more sense, as no active 
display = no active bpc.