[PATCH v2 3/3] drm/scdc-helper: Implement parsing and printing HDMI 2.1 fields
From: Nicolas Frattaroli
Date: Wed May 20 2026 - 09:39:12 EST
HDMI 2.1 redefines previously reserved fields in SCDC for various new
uses. No version check needs to be performed, as an HDMI 2.0 sink's
reserved SCDC fields are well-defined to be 0, and any zero-ness of
these fields for an HDMI 2.0 sink is not a surprise for SCDC parsers for
HDMI 2.1.
Implement reading and outputting these fields over debugfs.
Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@xxxxxxxxxxxxx>
---
drivers/gpu/drm/display/drm_scdc_helper.c | 130 +++++++++++++++++++++++++++---
include/drm/display/drm_scdc.h | 9 +++
include/drm/display/drm_scdc_helper.h | 85 +++++++++++++++----
3 files changed, 201 insertions(+), 23 deletions(-)
diff --git a/drivers/gpu/drm/display/drm_scdc_helper.c b/drivers/gpu/drm/display/drm_scdc_helper.c
index d582ba38a610..e9e59c7553a4 100644
--- a/drivers/gpu/drm/display/drm_scdc_helper.c
+++ b/drivers/gpu/drm/display/drm_scdc_helper.c
@@ -21,6 +21,7 @@
* DEALINGS IN THE SOFTWARE.
*/
+#include <linux/bitfield.h>
#include <linux/export.h>
#include <linux/i2c.h>
#include <linux/slab.h>
@@ -63,6 +64,38 @@ struct scdc_debugfs_priv {
struct drm_scdc_state state;
};
+static __pure const char *drm_scdc_frl_rate_str(enum drm_scdc_frl_rate rate)
+{
+ switch (rate) {
+ case SCDC_FRL_RATE_OFF:
+ return "Off";
+ case SCDC_FRL_RATE_3X3:
+ return "3 Gbit/s x 3 lanes";
+ case SCDC_FRL_RATE_6X3:
+ return "6 Gbit/s x 3 lanes";
+ case SCDC_FRL_RATE_6X4:
+ return "6 Gbit/s x 4 lanes";
+ case SCDC_FRL_RATE_8X4:
+ return "8 Gbit/s x 4 lanes";
+ case SCDC_FRL_RATE_10X4:
+ return "10 Gbit/s x 4 lanes";
+ case SCDC_FRL_RATE_12X4:
+ return "12 Gbit/s x 4 lanes";
+ case SCDC_FRL_RATE_RESV_7:
+ case SCDC_FRL_RATE_RESV_8:
+ case SCDC_FRL_RATE_RESV_9:
+ case SCDC_FRL_RATE_RESV_10:
+ case SCDC_FRL_RATE_RESV_11:
+ case SCDC_FRL_RATE_RESV_12:
+ case SCDC_FRL_RATE_RESV_13:
+ case SCDC_FRL_RATE_RESV_14:
+ case SCDC_FRL_RATE_RESV_15:
+ return "(Reserved)";
+ default:
+ return NULL;
+ }
+}
+
/**
* drm_scdc_read - read a block of data from SCDC
* @adapter: I2C controller
@@ -309,10 +342,41 @@ int drm_scdc_read_status0_flags(struct drm_connector *connector,
}
EXPORT_SYMBOL(drm_scdc_read_status0_flags);
+/**
+ * drm_scdc_read_status1_2_flags - Read SCDC "Status Flags" 1 and 2 Registers
+ * @connector: pointer to &struct drm_connector to issue the scdc request on
+ * @flags: pointer to the caller's &struct drm_scdc_status_flags to output to
+ *
+ * Reads the SCDC Status Flags 1 and 2 registers, and outputs their contents to
+ * the destination @flags. Contents of @flags.status1 and @flags.status2 are
+ * only valid if function returns 0.
+ *
+ * Returns 0 on success, negative errno on error.
+ */
+int drm_scdc_read_status1_2_flags(struct drm_connector *connector,
+ struct drm_scdc_status_flags *flags)
+{
+ int ret;
+
+ ret = drm_scdc_writeb(connector->ddc, SCDC_UPDATE_0, SCDC_FLT_UPDATE);
+ if (ret)
+ return ret;
+
+ ret = drm_scdc_readb(connector->ddc, SCDC_STATUS_FLAGS_1, &flags->status1.data);
+ if (ret)
+ return ret;
+
+ return drm_scdc_readb(connector->ddc, SCDC_STATUS_FLAGS_2, &flags->status2.data);
+}
+EXPORT_SYMBOL(drm_scdc_read_status1_2_flags);
+
+#define ERR_DET_OFF(x) ((x) - SCDC_ERR_DET_0_L)
+
/**
* drm_scdc_read_error_counters - Read and clear SCDC error counters
* @connector: pointer to &struct drm_connector to issue the scdc request on
- * @counter: Caller's u16 array with 3 elements to write the counter values into
+ * @counter: Caller's u16 array with 4 elements to write the counter values into
+ * @num_lanes: number of active lanes, either 3 or 4
*
* Read the SCDC channel error counters. If the count of channel *n* is valid,
* write it into counter[n]. Otherwise, set counter[n] to 0. Reads all counters
@@ -320,19 +384,33 @@ EXPORT_SYMBOL(drm_scdc_read_status0_flags);
*
* Returns: %0 on success, negative errno on error.
*/
-int drm_scdc_read_error_counters(struct drm_connector *connector, u16 counter[3])
+int drm_scdc_read_error_counters(struct drm_connector *connector, u16 counter[4],
+ unsigned int num_lanes)
{
- u8 buf[7] = { 0 };
+ unsigned int buf_sz;
+ u8 buf[9] = { 0 };
ssize_t ret;
u8 sum = 0;
int i;
+ switch (num_lanes) {
+ case 0:
+ case 3:
+ buf_sz = 7;
+ break;
+ case 4:
+ buf_sz = 9;
+ break;
+ default:
+ return -EINVAL;
+ }
+
ret = drm_scdc_writeb(connector->ddc, SCDC_UPDATE_0, SCDC_CED_UPDATE);
if (ret)
return ret;
/* why in the fucking fuck does this return a ssize_t */
- ret = drm_scdc_read(connector->ddc, SCDC_ERR_DET_0_L, buf, ARRAY_SIZE(buf));
+ ret = drm_scdc_read(connector->ddc, SCDC_ERR_DET_0_L, buf, buf_sz);
if (ret)
return ret;
@@ -340,13 +418,13 @@ int drm_scdc_read_error_counters(struct drm_connector *connector, u16 counter[3]
* Verify the "checksum", i.e. sum up everything including the checksum
* register as a wrapping unsigned 8-bit addition and verify it's 0.
*/
- for (i = 0; i < ARRAY_SIZE(buf); i++)
+ for (i = 0; i < buf_sz; i++)
sum = wrapping_add(u8, sum, buf[i]);
if (sum)
return -EPROTO;
- for (i = 0; i < ARRAY_SIZE(buf) - 1; i += 2) {
+ for (i = 0; i <= ERR_DET_OFF(SCDC_ERR_DET_2_H); i += 2) {
if (buf[i + 1] & SCDC_CHANNEL_VALID)
counter[i / 2] = buf[i] | (buf[i + 1] & ~SCDC_CHANNEL_VALID) << 8;
else
@@ -355,9 +433,15 @@ int drm_scdc_read_error_counters(struct drm_connector *connector, u16 counter[3]
buf[i] = 0;
buf[i + 1] = 0;
}
- buf[ARRAY_SIZE(buf) - 1] = 0;
+ buf[ERR_DET_OFF(SCDC_ERR_DET_CHECKSUM)] = 0;
+
+ if (num_lanes == 4)
+ counter[3] = buf[ERR_DET_OFF(SCDC_ERR_DET_3_L)] |
+ (buf[ERR_DET_OFF(SCDC_ERR_DET_3_H)] & ~SCDC_CHANNEL_VALID) << 8;
+ else
+ counter[3] = 0;
- return drm_scdc_write(connector->ddc, SCDC_ERR_DET_0_L, buf, ARRAY_SIZE(buf));
+ return drm_scdc_write(connector->ddc, SCDC_ERR_DET_0_L, buf, buf_sz);
}
EXPORT_SYMBOL(drm_scdc_read_error_counters);
@@ -377,6 +461,7 @@ int drm_scdc_read_state(struct drm_connector *connector, struct drm_scdc_state *
u8 upd_flags[2] = { 0 };
struct i2c_adapter *ddc;
struct drm_scdc *scdc;
+ int num_lanes;
int ret;
u8 val;
@@ -398,6 +483,17 @@ int drm_scdc_read_state(struct drm_connector *connector, struct drm_scdc_state *
state->scrambling_detected = drm_scdc_get_scrambling_status(connector);
+ ret = drm_scdc_readb(ddc, SCDC_CONFIG_1, &val);
+ if (ret)
+ return ret;
+
+ state->rate = FIELD_GET(SCDC_FRL_RATE, val);
+ num_lanes = drm_scdc_num_frl_lanes(state->rate);
+ if (num_lanes < 0)
+ return num_lanes;
+
+ state->ffe_levels = FIELD_GET(SCDC_FFE_LEVELS, val);
+
ret = drm_scdc_read(ddc, SCDC_UPDATE_0, &upd_flags, sizeof(upd_flags));
if (ret)
return ret;
@@ -408,8 +504,15 @@ int drm_scdc_read_state(struct drm_connector *connector, struct drm_scdc_state *
return ret;
}
+ if (upd_flags[0] & SCDC_FLT_UPDATE) {
+ ret = drm_scdc_read_status1_2_flags(connector, &state->stf);
+ if (ret)
+ return ret;
+ }
+
if (upd_flags[0] & SCDC_CED_UPDATE) {
- ret = drm_scdc_read_error_counters(connector, state->error_count);
+ ret = drm_scdc_read_error_counters(connector, state->error_count,
+ num_lanes);
if (ret)
return ret;
}
@@ -457,6 +560,8 @@ static int scdc_status_show(struct seq_file *m, void *data)
scdc_print_flag(m, "Scrambling Enabled", st->scrambling_enabled);
scdc_print_flag(m, "Scrambling Detected", st->scrambling_detected);
+ scdc_print_str(m, "FRL Rate", drm_scdc_frl_rate_str(st->rate));
+ scdc_print_dec(m, "FFE Levels", st->ffe_levels);
if (st->tmds_bclk_x40)
scdc_print_str(m, "TMDS Bit Clock Ratio", "1/40");
@@ -467,10 +572,17 @@ static int scdc_status_show(struct seq_file *m, void *data)
scdc_print_flag(m, "Channel 0 Locked", st->stf.status0.flags.ch0_locked);
scdc_print_flag(m, "Channel 1 Locked", st->stf.status0.flags.ch1_locked);
scdc_print_flag(m, "Channel 2 Locked", st->stf.status0.flags.ch2_locked);
+ if (drm_scdc_num_frl_lanes(st->rate) == 4)
+ scdc_print_flag(m, "Lane 3 Locked", st->stf.status0.flags.ln3_locked);
+
+ scdc_print_flag(m, "Sink Ready For Link Training", st->stf.status0.flags.flt_ready);
+ scdc_print_flag(m, "Sink Failed To Decode DSC", st->stf.status0.flags.dsc_fail);
scdc_print_dec(m, "Channel 0 Errors", st->error_count[0]);
scdc_print_dec(m, "Channel 1 Errors", st->error_count[1]);
scdc_print_dec(m, "Channel 2 Errors", st->error_count[2]);
+ if (drm_scdc_num_frl_lanes(st->rate) == 4)
+ scdc_print_dec(m, "Lane 3 Errors", st->error_count[3]);
return 0;
diff --git a/include/drm/display/drm_scdc.h b/include/drm/display/drm_scdc.h
index 3d58f37e8ed8..56cd909edac9 100644
--- a/include/drm/display/drm_scdc.h
+++ b/include/drm/display/drm_scdc.h
@@ -29,6 +29,7 @@
#define SCDC_SOURCE_VERSION 0x02
#define SCDC_UPDATE_0 0x10
+#define SCDC_FLT_UPDATE (1 << 5)
#define SCDC_READ_REQUEST_TEST (1 << 2)
#define SCDC_CED_UPDATE (1 << 1)
#define SCDC_STATUS_UPDATE (1 << 0)
@@ -46,6 +47,10 @@
#define SCDC_CONFIG_0 0x30
#define SCDC_READ_REQUEST_ENABLE (1 << 0)
+#define SCDC_CONFIG_1 0x31
+#define SCDC_FRL_RATE 0x0f
+#define SCDC_FFE_LEVELS 0xf0
+
#define SCDC_STATUS_FLAGS_0 0x40
#define SCDC_CH2_LOCK (1 << 3)
#define SCDC_CH1_LOCK (1 << 2)
@@ -54,6 +59,7 @@
#define SCDC_CLOCK_DETECT (1 << 0)
#define SCDC_STATUS_FLAGS_1 0x41
+#define SCDC_STATUS_FLAGS_2 0x42
#define SCDC_ERR_DET_0_L 0x50
#define SCDC_ERR_DET_0_H 0x51
@@ -65,6 +71,9 @@
#define SCDC_ERR_DET_CHECKSUM 0x56
+#define SCDC_ERR_DET_3_L 0x57
+#define SCDC_ERR_DET_3_H 0x58
+
#define SCDC_TEST_CONFIG_0 0xc0
#define SCDC_TEST_READ_REQUEST (1 << 7)
#define SCDC_TEST_READ_REQUEST_DELAY(x) ((x) & 0x7f)
diff --git a/include/drm/display/drm_scdc_helper.h b/include/drm/display/drm_scdc_helper.h
index b597e24a2976..1a95da3da5fe 100644
--- a/include/drm/display/drm_scdc_helper.h
+++ b/include/drm/display/drm_scdc_helper.h
@@ -24,6 +24,7 @@
#ifndef DRM_SCDC_HELPER_H
#define DRM_SCDC_HELPER_H
+#include <linux/errno.h>
#include <linux/types.h>
#include <drm/display/drm_scdc.h>
@@ -40,10 +41,10 @@ struct drm_scdc_status_flags {
bool ch0_locked : 1;
bool ch1_locked : 1;
bool ch2_locked : 1;
- bool rsvd_4 : 1;
+ bool ln3_locked : 1;
bool rsvd_5 : 1;
- bool rsvd_6 : 1;
- bool rsvd_7 : 1;
+ bool flt_ready : 1;
+ bool dsc_fail : 1;
} flags __packed;
u8 data;
} status0;
@@ -51,19 +52,68 @@ struct drm_scdc_status_flags {
/* Status Register 1 */
union {
struct {
- bool rsvd_0 : 1;
- bool rsvd_1 : 1;
- bool rsvd_2 : 1;
- bool rsvd_3 : 1;
- bool rsvd_4 : 1;
- bool rsvd_5 : 1;
- bool rsvd_6 : 1;
- bool rsvd_7 : 1;
+ u8 ln0_training_pattern : 4;
+ u8 ln1_training_pattern : 4;
} flags __packed;
u8 data;
} status1;
+
+ /* Status Register 2 */
+ union {
+ struct {
+ u8 ln2_training_pattern : 4;
+ u8 ln3_training_pattern : 4;
+ } flags __packed;
+ u8 data;
+ } status2;
+};
+
+enum drm_scdc_frl_rate {
+ SCDC_FRL_RATE_OFF = 0,
+ SCDC_FRL_RATE_3X3 = 1,
+ SCDC_FRL_RATE_6X3 = 2,
+ SCDC_FRL_RATE_6X4 = 3,
+ SCDC_FRL_RATE_8X4 = 4,
+ SCDC_FRL_RATE_10X4 = 5,
+ SCDC_FRL_RATE_12X4 = 6,
+ SCDC_FRL_RATE_RESV_7 = 7,
+ SCDC_FRL_RATE_RESV_8 = 8,
+ SCDC_FRL_RATE_RESV_9 = 9,
+ SCDC_FRL_RATE_RESV_10 = 10,
+ SCDC_FRL_RATE_RESV_11 = 11,
+ SCDC_FRL_RATE_RESV_12 = 12,
+ SCDC_FRL_RATE_RESV_13 = 13,
+ SCDC_FRL_RATE_RESV_14 = 14,
+ SCDC_FRL_RATE_RESV_15 = 15
};
+/**
+ * drm_scdc_num_frl_lanes - get number of lanes for a given FRL rate
+ * @rate: one of &enum drm_scdc_frl_rate
+ *
+ * For a given @rate, return the number of lanes it uses.
+ *
+ * Returns: %-EINVAL if @rate is not a valid FRL rate, or the number of lanes
+ * for a given &enum drm_scdc_frl_rate on success (including %0 for "off")
+ */
+static inline __pure int drm_scdc_num_frl_lanes(enum drm_scdc_frl_rate rate)
+{
+ switch (rate) {
+ case SCDC_FRL_RATE_OFF:
+ return 0;
+ case SCDC_FRL_RATE_3X3:
+ case SCDC_FRL_RATE_6X3:
+ return 3;
+ case SCDC_FRL_RATE_6X4:
+ case SCDC_FRL_RATE_8X4:
+ case SCDC_FRL_RATE_10X4:
+ case SCDC_FRL_RATE_12X4:
+ return 4;
+ default:
+ return -EINVAL;
+ }
+}
+
struct drm_scdc_state {
/** @stf: contents of the status flag registers */
struct drm_scdc_status_flags stf;
@@ -76,8 +126,12 @@ struct drm_scdc_state {
* clock period, false if it's 1/10th of the clock period.
*/
bool tmds_bclk_x40;
- /** @error_count: character error counts for each channel */
- u16 error_count[3];
+ /** @rate: FRL rate set by the source */
+ enum drm_scdc_frl_rate rate : 4;
+ /** @ffe_levels: The FFE levels for @rate set by the source */
+ u8 ffe_levels : 4;
+ /** @error_count: character error counts for each channel/link */
+ u16 error_count[4];
};
ssize_t drm_scdc_read(struct i2c_adapter *adapter, u8 offset, void *buffer,
@@ -128,9 +182,12 @@ bool drm_scdc_set_high_tmds_clock_ratio(struct drm_connector *connector, bool se
int drm_scdc_read_status0_flags(struct drm_connector *connector,
struct drm_scdc_status_flags *flags);
+int drm_scdc_read_status1_2_flags(struct drm_connector *connector,
+ struct drm_scdc_status_flags *flags);
int drm_scdc_read_state(struct drm_connector *connector,
struct drm_scdc_state *state);
-int drm_scdc_read_error_counters(struct drm_connector *connector, u16 counter[3]);
+int drm_scdc_read_error_counters(struct drm_connector *connector, u16 counter[4],
+ unsigned int num_lanes);
void drm_scdc_debugfs_init(struct drm_connector *connector, struct dentry *root);
#endif
--
2.54.0