* [PATCH v3 1/2] drm/display: hdmi: Only allow BPC values of 8, 10, 12 and 16
2026-06-19 14:37 [PATCH v3 0/2] Make HDMI state helpers handle odd max bpc requests Nicolas Frattaroli
@ 2026-06-19 14:37 ` Nicolas Frattaroli
2026-06-22 9:06 ` Maxime Ripard
2026-06-19 14:37 ` [PATCH v3 2/2] drm/display: hdmi: Round odd max_bpc down to even numbers Nicolas Frattaroli
1 sibling, 1 reply; 4+ messages in thread
From: Nicolas Frattaroli @ 2026-06-19 14:37 UTC (permalink / raw)
To: Maarten Lankhorst, Maxime Ripard, Thomas Zimmermann,
David Airlie, Simona Vetter, Dmitry Baryshkov
Cc: dri-devel, linux-kernel, kernel, Nicolas Frattaroli
As per the comment in sink_supports_format_bpc(), CTA-861-F defines that
only bits-per-channel values of 8, 10, 12 and 16 are allowed for HDMI.
Allowing more than this has surprising consequences for the atomic check
phase. The HDMI state helpers may accidentally conclude that a sink
supports 11bpc if a caller asks for it.
Fix this by exiting early if the bpc value doesn't match one of those
given in the standard.
Fixes: 26ff1c38fc29 ("drm/connector: hdmi: Compute bpc and format automatically")
Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
---
drivers/gpu/drm/display/drm_hdmi_state_helper.c | 11 +++++++++++
1 file changed, 11 insertions(+)
diff --git a/drivers/gpu/drm/display/drm_hdmi_state_helper.c b/drivers/gpu/drm/display/drm_hdmi_state_helper.c
index ce17eeefc2da..6d02cf85f7c9 100644
--- a/drivers/gpu/drm/display/drm_hdmi_state_helper.c
+++ b/drivers/gpu/drm/display/drm_hdmi_state_helper.c
@@ -405,6 +405,17 @@ sink_supports_format_bpc(const struct drm_connector *connector,
struct drm_device *dev = connector->dev;
u8 vic = drm_match_cea_mode(mode);
+ switch (bpc) {
+ case 8:
+ case 10:
+ case 12:
+ case 16:
+ break;
+ default:
+ drm_dbg_kms(dev, "bpc of %u is not supported by standard\n", bpc);
+ return false;
+ }
+
/*
* CTA-861-F, section 5.4 - Color Coding & Quantization states
* that the bpc must be 8, 10, 12 or 16 except for the default
--
2.54.0
^ permalink raw reply [flat|nested] 4+ messages in thread* [PATCH v3 2/2] drm/display: hdmi: Round odd max_bpc down to even numbers
2026-06-19 14:37 [PATCH v3 0/2] Make HDMI state helpers handle odd max bpc requests Nicolas Frattaroli
2026-06-19 14:37 ` [PATCH v3 1/2] drm/display: hdmi: Only allow BPC values of 8, 10, 12 and 16 Nicolas Frattaroli
@ 2026-06-19 14:37 ` Nicolas Frattaroli
1 sibling, 0 replies; 4+ messages in thread
From: Nicolas Frattaroli @ 2026-06-19 14:37 UTC (permalink / raw)
To: Maarten Lankhorst, Maxime Ripard, Thomas Zimmermann,
David Airlie, Simona Vetter, Dmitry Baryshkov
Cc: dri-devel, linux-kernel, kernel, Nicolas Frattaroli
The HDMI state helpers will count down from the max bpc to 8 in steps of
2, trying each value as a possible output bpc. This goes awry if max bpc
is restricted by userspace to an odd number with the "max bpc" connector
property.
Prevent this, without introducing any additional bpc format trial steps,
by simply rounding down to the next multiple of 2 as the starting point
for the for loop.
Additionally, add a KUnit test to validate the handling of this.
Fixes: 26ff1c38fc29 ("drm/connector: hdmi: Compute bpc and format automatically")
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
---
drivers/gpu/drm/display/drm_hdmi_state_helper.c | 2 +-
drivers/gpu/drm/tests/drm_hdmi_state_helper_test.c | 71 ++++++++++++++++++++++
2 files changed, 72 insertions(+), 1 deletion(-)
diff --git a/drivers/gpu/drm/display/drm_hdmi_state_helper.c b/drivers/gpu/drm/display/drm_hdmi_state_helper.c
index 6d02cf85f7c9..f3824fa2bf29 100644
--- a/drivers/gpu/drm/display/drm_hdmi_state_helper.c
+++ b/drivers/gpu/drm/display/drm_hdmi_state_helper.c
@@ -650,7 +650,7 @@ hdmi_compute_format_bpc(const struct drm_connector *connector,
unsigned int bpc;
int ret;
- for (bpc = max_bpc; bpc >= 8; bpc -= 2) {
+ for (bpc = rounddown(max_bpc, 2); bpc >= 8; bpc -= 2) {
ret = hdmi_try_format_bpc(connector, conn_state, mode, bpc, fmt);
if (!ret)
continue;
diff --git a/drivers/gpu/drm/tests/drm_hdmi_state_helper_test.c b/drivers/gpu/drm/tests/drm_hdmi_state_helper_test.c
index 353a261d42da..31ac93d9be98 100644
--- a/drivers/gpu/drm/tests/drm_hdmi_state_helper_test.c
+++ b/drivers/gpu/drm/tests/drm_hdmi_state_helper_test.c
@@ -1361,6 +1361,76 @@ static void drm_test_check_tmds_char_rate_rgb_12bpc(struct kunit *test)
drm_modeset_acquire_fini(&ctx);
}
+/*
+ * Test that given a request for an odd-numbered max bpc, the HDMI state helpers
+ * will succeed an atomic check but round down to the even-numbered bpc on the
+ * output, while leaving the requested value alone.
+ */
+static void drm_test_check_odd_max_bpc(struct kunit *test)
+{
+ struct drm_atomic_helper_connector_hdmi_priv *priv;
+ struct drm_connector_state *conn_state;
+ struct drm_modeset_acquire_ctx ctx;
+ struct drm_display_mode *preferred;
+ struct drm_atomic_commit *state;
+ struct drm_connector *conn;
+ struct drm_device *drm;
+ int ret;
+
+ priv = drm_kunit_helper_connector_hdmi_init_with_edid_funcs(test,
+ BIT(DRM_OUTPUT_COLOR_FORMAT_RGB444),
+ 12,
+ &dummy_connector_hdmi_funcs,
+ test_edid_hdmi_1080p_rgb_yuv_dc_max_340mhz);
+ KUNIT_ASSERT_NOT_NULL(test, priv);
+
+ drm = &priv->drm;
+ conn = &priv->connector;
+ preferred = find_preferred_mode(conn);
+ KUNIT_ASSERT_NOT_NULL(test, preferred);
+
+ KUNIT_ASSERT_TRUE(test, conn->display_info.is_hdmi);
+
+ drm_modeset_acquire_init(&ctx, 0);
+
+retry_conn_enable:
+ ret = drm_kunit_helper_enable_crtc_connector(test, drm, priv->crtc,
+ conn, preferred, &ctx);
+ if (ret == -EDEADLK) {
+ ret = drm_modeset_backoff(&ctx);
+ if (!ret)
+ goto retry_conn_enable;
+ }
+ KUNIT_ASSERT_EQ(test, ret, 0);
+
+ state = drm_kunit_helper_atomic_state_alloc(test, drm, &ctx);
+ KUNIT_ASSERT_NOT_ERR_OR_NULL(test, state);
+
+retry_conn_state:
+ conn_state = drm_atomic_get_connector_state(state, conn);
+ KUNIT_ASSERT_NOT_ERR_OR_NULL(test, conn_state);
+
+ conn_state->max_requested_bpc = 11;
+
+ ret = drm_atomic_check_only(state);
+ if (ret == -EDEADLK) {
+ drm_atomic_commit_clear(state);
+ ret = drm_modeset_backoff(&ctx);
+ if (!ret)
+ goto retry_conn_state;
+ }
+ KUNIT_ASSERT_EQ(test, ret, 0);
+
+ conn_state = drm_atomic_get_new_connector_state(state, conn);
+ KUNIT_ASSERT_NOT_ERR_OR_NULL(test, conn_state);
+
+ KUNIT_EXPECT_EQ(test, conn_state->hdmi.output_bpc, 10);
+ KUNIT_EXPECT_EQ(test, conn_state->max_requested_bpc, 11);
+
+ drm_modeset_drop_locks(&ctx);
+ drm_modeset_acquire_fini(&ctx);
+}
+
/*
* Test that if we filter a rate through our hook, it's indeed rejected
* by the whole atomic_check logic.
@@ -2476,6 +2546,7 @@ static struct kunit_case drm_atomic_helper_connector_hdmi_check_tests[] = {
KUNIT_CASE(drm_test_check_tmds_char_rate_rgb_8bpc),
KUNIT_CASE(drm_test_check_tmds_char_rate_rgb_10bpc),
KUNIT_CASE(drm_test_check_tmds_char_rate_rgb_12bpc),
+ KUNIT_CASE(drm_test_check_odd_max_bpc),
KUNIT_CASE_PARAM(drm_test_check_hdmi_color_format,
check_hdmi_color_format_gen_params),
KUNIT_CASE_PARAM(drm_test_check_hdmi_color_format_420_only,
--
2.54.0
^ permalink raw reply [flat|nested] 4+ messages in thread