On Fri, 01 Aug 2025, Marius Vlad <marius.v...@collabora.com> wrote:
> From: Derek Foreman <derek.fore...@collabora.com>
>
> Add a way to know the actual bpc of a running link.
>
> Drivers might change the current bpc link value due to changes in mode
> line or refresh rates. For example when enabling VRR the underlying
> hardware might not be able sustain the same bandwidth for a particular
> mode line, and it might attempt to lower the bpc. Another example can be
> found when switching the color output format, part of YUV420 fallback.
>
> This means we might be displaying a stale bpc value although it was
> modified for different reasons -- like a refresh rate or an output
> color format.
>
> This patch introduces a new property 'link bpc' that user-space can
Nitpick, s/This patch introduces/Introduce/.
> use to get the current bpc value of a running link. In the same
> time this would allow user-space set up bpc using 'max_bpc' property.
>
> Signed-off-by: Derek Foreman <derek.fore...@collabora.com>
> Signed-off-by: Marius Vlad <marius.v...@collabora.com>
> ---
> drivers/gpu/drm/drm_atomic_uapi.c | 5 +++++
> drivers/gpu/drm/drm_connector.c | 26 ++++++++++++++++++++++++++
> include/drm/drm_connector.h | 8 ++++++++
> 3 files changed, 39 insertions(+)
>
> diff --git a/drivers/gpu/drm/drm_atomic_uapi.c
> b/drivers/gpu/drm/drm_atomic_uapi.c
> index ecc73d52bfae..3a2ffb957ade 100644
> --- a/drivers/gpu/drm/drm_atomic_uapi.c
> +++ b/drivers/gpu/drm/drm_atomic_uapi.c
> @@ -776,6 +776,9 @@ static int drm_atomic_connector_set_property(struct
> drm_connector *connector,
> fence_ptr);
> } else if (property == connector->max_bpc_property) {
> state->max_requested_bpc = val;
> + } else if (property == connector->link_bpc_property) {
> + drm_dbg_kms(dev, "only drivers can set link bpc property. Use
> max_bpc instead\n");
> + return -EINVAL;
> } else if (property == connector->privacy_screen_sw_state_property) {
> state->privacy_screen_sw_state = val;
> } else if (property == connector->broadcast_rgb_property) {
> @@ -861,6 +864,8 @@ 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->link_bpc_property) {
> + *val = state->hdmi.output_bpc;
> } else if (property == connector->privacy_screen_sw_state_property) {
> *val = state->privacy_screen_sw_state;
> } else if (property == connector->broadcast_rgb_property) {
> diff --git a/drivers/gpu/drm/drm_connector.c b/drivers/gpu/drm/drm_connector.c
> index 272d6254ea47..7ed27aec0ccc 100644
> --- a/drivers/gpu/drm/drm_connector.c
> +++ b/drivers/gpu/drm/drm_connector.c
> @@ -542,6 +542,28 @@ int drmm_connector_init(struct drm_device *dev,
> }
> EXPORT_SYMBOL(drmm_connector_init);
>
> +static int
> +drm_connector_attach_link_bpc_property(struct drm_connector *connector,
> + int min, int max)
> +{
> + struct drm_device *dev = connector->dev;
> + struct drm_property *prop;
> +
> + prop = connector->link_bpc_property;
> + if (prop)
> + return 0;
> +
> + prop = drm_property_create_range(dev, 0, "link bpc", min, max);
> + if (!prop)
> + return -ENOMEM;
> +
> + connector->link_bpc_property = prop;
> +
> + drm_object_attach_property(&connector->base, prop, max);
> +
> + return 0;
> +}
> +
> /**
> * drmm_connector_hdmi_init - Init a preallocated HDMI connector
> * @dev: DRM device
> @@ -618,6 +640,10 @@ int drmm_connector_hdmi_init(struct drm_device *dev,
> drm_connector_attach_max_bpc_property(connector, 8, max_bpc);
> connector->max_bpc = max_bpc;
>
> + ret = drm_connector_attach_link_bpc_property(connector, 8, max_bpc);
> + if (ret)
> + return ret;
> +
> if (max_bpc > 8)
> drm_connector_attach_hdr_output_metadata_property(connector);
>
> diff --git a/include/drm/drm_connector.h b/include/drm/drm_connector.h
> index 8f34f4b8183d..4a50198aa7c0 100644
> --- a/include/drm/drm_connector.h
> +++ b/include/drm/drm_connector.h
> @@ -2079,6 +2079,14 @@ struct drm_connector {
> */
> struct drm_property *max_bpc_property;
>
> + /**
> + * @link_bpc_property: Current connector link bpc set by the driver
> + *
> + * This property can be used to retrieve the current link bpc from
> + * connector_state::hdmi:output_bpc
It doesn't have to be just HDMI, does it?
> + */
> + struct drm_property *link_bpc_property;
> +
> /** @privacy_screen: drm_privacy_screen for this connector, or NULL. */
> struct drm_privacy_screen *privacy_screen;
--
Jani Nikula, Intel
