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Message-ID: <20251031204534.659180-1-sashamcintosh@google.com>
Date: Fri, 31 Oct 2025 16:45:34 -0400
From: Sasha McIntosh <sashamcintosh@...gle.com>
To: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>,
Thomas Zimmermann <tzimmermann@...e.de>
Cc: David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>, dri-devel@...ts.freedesktop.org,
linux-kernel@...r.kernel.org, trivial@...nel.org,
Sasha McIntosh <sashamcintosh@...gle.com>
Subject: [PATCH] drm: Add "min bpc" connector property
[Why]
When playing HDR or WCG content, bandwidth constraints may force the
driver to downgrade to 6bpc, resulting in visual artifacts like banding.
Userspace should be able to configure a minimum allowed bpc.
[How]
Introduce the "min bpc" connector property so the user can limit the
minimum bpc. Mirror the "mac bpc" implementation.
Signed-off-by: Sasha McIntosh <sashamcintosh@...gle.com>
---
drivers/gpu/drm/drm_atomic.c | 12 +++++++++
drivers/gpu/drm/drm_atomic_helper.c | 4 +++
drivers/gpu/drm/drm_atomic_uapi.c | 4 +++
drivers/gpu/drm/drm_connector.c | 41 +++++++++++++++++++++++++++++
include/drm/drm_connector.h | 20 ++++++++++++++
5 files changed, 81 insertions(+)
diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_atomic.c
index be2cb6e43cb0..f85ad9c55e69 100644
--- a/drivers/gpu/drm/drm_atomic.c
+++ b/drivers/gpu/drm/drm_atomic.c
@@ -468,6 +468,17 @@ static int drm_atomic_connector_check(struct drm_connector *connector,
state->max_bpc = info->bpc ? info->bpc : 8;
if (connector->max_bpc_property)
state->max_bpc = min(state->max_bpc, state->max_requested_bpc);
+ if (connector->min_bpc_property)
+ state->min_bpc = state->min_requested_bpc;
+ if (connector->max_bpc_property && connector->min_bpc_property &&
+ state->max_requested_bpc < state->min_requested_bpc) {
+ drm_dbg_atomic(connector->dev,
+ "[CONNECTOR:%d:%s] max bpc %d < min bpc %d\n",
+ connector->base.id, connector->name,
+ state->max_requested_bpc,
+ state->min_requested_bpc);
+ return -EINVAL;
+ }
if ((connector->connector_type != DRM_MODE_CONNECTOR_WRITEBACK) || !writeback_job)
return 0;
@@ -1195,6 +1206,7 @@ static void drm_atomic_connector_print_state(struct drm_printer *p,
drm_printf(p, "\tinterlace_allowed=%d\n", connector->interlace_allowed);
drm_printf(p, "\tycbcr_420_allowed=%d\n", connector->ycbcr_420_allowed);
drm_printf(p, "\tmax_requested_bpc=%d\n", state->max_requested_bpc);
+ drm_printf(p, "\tmin_requested_bpc=%d\n", state->min_requested_bpc);
drm_printf(p, "\tcolorspace=%s\n", drm_get_colorspace_name(state->colorspace));
if (connector->connector_type == DRM_MODE_CONNECTOR_HDMIA ||
diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
index 5a473a274ff0..75659d46c6fe 100644
--- a/drivers/gpu/drm/drm_atomic_helper.c
+++ b/drivers/gpu/drm/drm_atomic_helper.c
@@ -736,6 +736,10 @@ drm_atomic_helper_check_modeset(struct drm_device *dev,
if (old_connector_state->max_requested_bpc !=
new_connector_state->max_requested_bpc)
new_crtc_state->connectors_changed = true;
+
+ if (old_connector_state->min_requested_bpc !=
+ new_connector_state->min_requested_bpc)
+ new_crtc_state->connectors_changed = true;
}
if (funcs->atomic_check)
diff --git a/drivers/gpu/drm/drm_atomic_uapi.c b/drivers/gpu/drm/drm_atomic_uapi.c
index 85dbdaa4a2e2..f99649f9c51f 100644
--- a/drivers/gpu/drm/drm_atomic_uapi.c
+++ b/drivers/gpu/drm/drm_atomic_uapi.c
@@ -776,6 +776,8 @@ 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->min_bpc_property) {
+ state->min_requested_bpc = val;
} else if (property == connector->privacy_screen_sw_state_property) {
state->privacy_screen_sw_state = val;
} else if (property == connector->broadcast_rgb_property) {
@@ -861,6 +863,8 @@ 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->min_bpc_property) {
+ *val = state->min_requested_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..2d9cfd4f5118 100644
--- a/drivers/gpu/drm/drm_connector.c
+++ b/drivers/gpu/drm/drm_connector.c
@@ -1699,6 +1699,13 @@ EXPORT_SYMBOL(drm_hdmi_connector_get_output_format_name);
* drm_connector_attach_max_bpc_property() to create and attach the
* property to the connector during initialization.
*
+ * min bpc:
+ * This range property is used by userspace to set a lower bound for the bit
+ * depth. When used the driver would set the bpc in accordance with the
+ * valid range supported by the hardware and sink. Drivers to use the function
+ * drm_connector_attach_min_bpc_property() to create and attach the
+ * property to the connector during initialization.
+ *
* Connectors also have one standardized atomic property:
*
* CRTC_ID:
@@ -2845,6 +2852,40 @@ int drm_connector_attach_max_bpc_property(struct drm_connector *connector,
}
EXPORT_SYMBOL(drm_connector_attach_max_bpc_property);
+/**
+ * drm_connector_attach_min_bpc_property - attach "min bpc" property
+ * @connector: connector to attach min 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 add support for limiting the bit depth on a connector.
+ *
+ * Returns:
+ * Zero on success, negative errno on failure.
+ */
+int drm_connector_attach_min_bpc_property(struct drm_connector *connector,
+ int min, int max)
+{
+ struct drm_device *dev = connector->dev;
+ struct drm_property *prop;
+
+ prop = connector->min_bpc_property;
+ if (!prop) {
+ prop = drm_property_create_range(dev, 0, "min bpc", min, max);
+ if (!prop)
+ return -ENOMEM;
+
+ connector->min_bpc_property = prop;
+ }
+
+ drm_object_attach_property(&connector->base, prop, min);
+ connector->state->min_requested_bpc = min;
+ connector->state->min_bpc = min;
+
+ return 0;
+}
+EXPORT_SYMBOL(drm_connector_attach_min_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 8f34f4b8183d..7581f965b015 100644
--- a/include/drm/drm_connector.h
+++ b/include/drm/drm_connector.h
@@ -1126,12 +1126,24 @@ struct drm_connector_state {
*/
u8 max_requested_bpc;
+ /**
+ * @min_requested_bpc: Connector property to limit the minimum bit
+ * depth of the pixels.
+ */
+ u8 min_requested_bpc;
+
/**
* @max_bpc: Connector max_bpc based on the requested max_bpc property
* and the connector bpc limitations obtained from edid.
*/
u8 max_bpc;
+ /**
+ * @min_bpc: Connector min_bpc based on the requested min_bpc property
+ * and the connector bpc limitations obtained from edid.
+ */
+ u8 min_bpc;
+
/**
* @privacy_screen_sw_state: See :ref:`Standard Connector
* Properties<standard_connector_properties>`
@@ -2079,6 +2091,12 @@ struct drm_connector {
*/
struct drm_property *max_bpc_property;
+ /**
+ * @min_bpc_property: Default connector property for the min bpc to be
+ * driven out of the connector.
+ */
+ struct drm_property *min_bpc_property;
+
/** @privacy_screen: drm_privacy_screen for this connector, or NULL. */
struct drm_privacy_screen *privacy_screen;
@@ -2482,6 +2500,8 @@ int drm_connector_set_orientation_from_panel(
struct drm_panel *panel);
int drm_connector_attach_max_bpc_property(struct drm_connector *connector,
int min, int max);
+int drm_connector_attach_min_bpc_property(struct drm_connector *connector,
+ int min, int max);
void drm_connector_create_privacy_screen_properties(struct drm_connector *conn);
void drm_connector_attach_privacy_screen_properties(struct drm_connector *conn);
void drm_connector_attach_privacy_screen_provider(
base-commit: 098456f3141bf9e0c0d8973695ca38a03465ccd6
--
2.51.1.851.g4ebd6896fd-goog
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