Re: [PATCH v4 03/17] drm/uAPI: Add "active bpc" as feedback channel for "max bpc" drm property

[Pekka Paalanen]

On Tue, 29 Jun 2021 13:02:05 +0200
Werner Sembach <wse@xxxxxxxxxxxxxxxxxxx> wrote:

> Am 28.06.21 um 19:03 schrieb Werner Sembach:
> > Am 18.06.21 um 11:11 schrieb Werner Sembach:  
> >> 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 being available.
> >>
> >> Signed-off-by: Werner Sembach <wse@xxxxxxxxxxxxxxxxxxx>
> >> ---
> >>  drivers/gpu/drm/drm_connector.c | 51 +++++++++++++++++++++++++++++++++
> >>  include/drm/drm_connector.h     |  8 ++++++
> >>  2 files changed, 59 insertions(+)
> >>
> >> diff --git a/drivers/gpu/drm/drm_connector.c b/drivers/gpu/drm/drm_connector.c
> >> index da39e7ff6965..943f6b61053b 100644
> >> --- a/drivers/gpu/drm/drm_connector.c
> >> +++ b/drivers/gpu/drm/drm_connector.c
> >> @@ -1197,6 +1197,14 @@ 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 tells userspace the pixel color bit depth
> >> + *	actually used by the hardware display engine on "the cable" on a
> >> + *	connector. The chosen value depends on hardware capabilities, both
> >> + *	display engine and connected monitor, and the "max bpc" property.
> >> + *	Drivers shall use drm_connector_attach_active_bpc_property() to install
> >> + *	this property.
> >> + *  
> > Regarding "on the cable" and dithering: As far as I can tell, what the dithering option does, is setting a hardware
> > register here:
> >
> > - https://elixir.bootlin.com/linux/v5.13/source/drivers/gpu/drm/i915/display/intel_display.c#L4534
> >
> > - https://elixir.bootlin.com/linux/v5.13/source/drivers/gpu/drm/i915/display/intel_display.c#L4571
> >
> > So dithering seems to be calculated by fixed purpose hardware/firmware outside of the driver?
> >
> > The Intel driver does not seem to set a target bpc/bpp for this hardware so I guess it defaults to 6 or 8 bpc?  
> 
> Never mind it does. This switch-case does affect the dithering output:
> https://elixir.bootlin.com/linux/v5.13/source/drivers/gpu/drm/i915/display/intel_display.c#L4537

Hi,

I obviously do not know the intel driver or hardware at all, but
to me that just looks like translating from bits per pixel to bits per
channel in RGB mapping?

> As found in this documentation p.548:
> https://01.org/sites/default/files/documentation/intel-gfx-prm-osrc-lkf-vol02c-commandreference-registers-part2.pdf
> 
> So max bpc and active bpc are affecting/affected by the bpc after dithering.

By definition, if the cable carries N bpc, then dithering does not
change that. The cable still carries N bpc, but due to spatial or
temporal dithering, the *observed* color resolution may or may not be
higher than the cable bpc.

Of course, if the cable bpc is 8, and dithering targets 6 bpc, then 2
LSB on the cable are always zero, right?

Maybe one would want to do that if the monitor has a 6 bit panel and it
simply ignored the 2 LSB, and the cable cannot go down to 6 bpc.

So, what does "max bpc" mean right now?

It seems like dither on/off is insufficient information, one would also
need to control the dithering target bpc. I suppose the driver has a
policy on how it chooses the target bpc, but what is that policy? Is
the dither target bpc the cable bpc or the sink bpc?

Needless to say, I'm quite confused.


Thanks,
pq

> >
> > Similar things happen on amd. Here the output dither depth seems to be written to a fixed value however:
> >
> > - https://elixir.bootlin.com/linux/v5.13/source/drivers/gpu/drm/amd/display/dc/dce/dce_transform.c#L828
> >
> > - https://elixir.bootlin.com/linux/v5.13/source/drivers/gpu/drm/amd/display/dc/dce/dce_transform.c#L769
> >
> > Does anyone know about a resource where I can read up on the used registers and what this hardware actually does?  
> Searching now for a similar register reference for AMD GPUs.
> >
> > My proposal for now: "max bpc" affects what happens before dither, so I would keep "active bpc" the same and add another
> > drm property "dither active: true/false". No additional property to control dither, as amdgpu does have one already
> > (which isn't always active?) and Intel driver does only seem prepared for dithering at 6bpc (albeit I don't know why to
> > dither at 6bpc and what depth to dither to?).
> >  
> >>   * Connectors also have one standardized atomic property:
> >>   *
> >>   * CRTC_ID:
> >> @@ -2152,6 +2160,49 @@ 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;
> >> +
> >> +	if (!connector->active_bpc_property) {
> >> +		prop = drm_property_create_range(dev, DRM_MODE_PROP_IMMUTABLE, "active bpc",
> >> +						 min, max);
> >> +		if (!prop)
> >> +			return -ENOMEM;
> >> +
> >> +		connector->active_bpc_property = prop;
> >> +		drm_object_attach_property(&connector->base, prop, 0);
> >> +	}
> >> +
> >> +	return 0;
> >> +}
> >> +EXPORT_SYMBOL(drm_connector_attach_active_bpc_property);
> >> +
> >> +/**
> >> + * drm_connector_set_active_bpc_property - sets the active bits per color property for a connector
> >> + * @connector: drm connector
> >> + * @active_bpc: bits per color for the connector currently active on "the cable"
> >> + *
> >> + * Should be used by atomic drivers to update the active bits per color over a connector.
> >> + */
> >> +void drm_connector_set_active_bpc_property(struct drm_connector *connector, int active_bpc)
> >> +{
> >> +	drm_object_property_set_value(&connector->base, connector->active_bpc_property, active_bpc);
> >> +}
> >> +EXPORT_SYMBOL(drm_connector_set_active_bpc_property);
> >> +
> >>  /**
> >>   * drm_connector_attach_hdr_output_metadata_property - attach "HDR_OUTPUT_METADA" property
> >>   * @connector: connector to attach the property on.
> >> diff --git a/include/drm/drm_connector.h b/include/drm/drm_connector.h
> >> index 714d1a01c065..eee86de62a5f 100644
> >> --- a/include/drm/drm_connector.h
> >> +++ b/include/drm/drm_connector.h
> >> @@ -1380,6 +1380,12 @@ struct drm_connector {
> >>  	 */
> >>  	struct drm_property *max_bpc_property;
> >>  
> >> +	/**
> >> +	 * @active_bpc_property: Default connector property for the active bpc
> >> +	 * to be driven out of the connector.
> >> +	 */
> >> +	struct drm_property *active_bpc_property;
> >> +
> >>  #define DRM_CONNECTOR_POLL_HPD (1 << 0)
> >>  #define DRM_CONNECTOR_POLL_CONNECT (1 << 1)
> >>  #define DRM_CONNECTOR_POLL_DISCONNECT (1 << 2)
> >> @@ -1702,6 +1708,8 @@ int drm_connector_set_panel_orientation_with_quirk(
> >>  	int width, int height);
> >>  int drm_connector_attach_max_bpc_property(struct drm_connector *connector,
> >>  					  int min, int max);
> >> +int drm_connector_attach_active_bpc_property(struct drm_connector *connector, int min, int max);
> >> +void drm_connector_set_active_bpc_property(struct drm_connector *connector, int active_bpc);
> >>  
> >>  /**
> >>   * struct drm_tile_group - Tile group metadata  
> > _______________________________________________
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> > amd-gfx@xxxxxxxxxxxxxxxxxxxxx
> > https://lists.freedesktop.org/mailman/listinfo/amd-gfx

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