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

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Mon, Jun 07, 2021 at 08:46:47AM +0200, Werner Sembach wrote:
> 
> Am 04.06.21 um 19:26 schrieb Ville Syrjälä:
> > 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.
> >>
> >> 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
> >> + *	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);
> > Should be immutable.
> Yes. I didn't know if there is a way to do this (or just don't define a 
> set-function), but I think I found the define for this.
> >
> > Also wondering what the semantics of this should be when eg. DSC
> > is active?
> I'm unfamiliar how the inner workings of DSC (I guess Display Stream 
> Compression?) are. But doesn't it also have color depth?

Some number of bits go into the compressor, smaller/equal number
of bits come out. Not sure if the choice of the input bpc affects
what the sink does when decompressing or not.

> 
> The active bpc should be what the GPU tells the display to actually show 
> the user when he looks at just one pixel.
> 
> So dithering computed on the host should not be included (aka when the 
> gpu sends a premade picture to the screen and tells it the lesser pbc), 
> while FRC dithering computed in the display firmware should be included 
> (since the GPU can't really tell the difference between FRC displays and 
> True 10-Bit displays, can't it?)

Not sure. Haven't checked if there's something in some standard for
that.

-- 
Ville Syrjälä
Intel