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 <w...@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.
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