On Wed, Aug 06, 2025 at 04:20:53AM +0000, Lee Shawn C wrote:
> While performing HDMI compliance testing, test equipment may request
> different bpc output for signal measurement. However, display driver
> typically determines the maximum available bpc based on HW bandwidth.
>
> This patch introduces a new debugfs that allow user to configure
> dedicated bpc manually, and making HDMI compliance test much easier.
There is already the intel_force_link_bpp connector debugfs entry, IMO
that should be enabled for all HDMI connectors (atm it's used only for
those that are on an FDI link), instead of adding a new debugfs entry.
> Cc: Shankar Uma <uma.shan...@intel.com>
> Signed-off-by: Lee Shawn C <shawn.c....@intel.com>
> ---
> .../drm/i915/display/intel_display_debugfs.c | 58 +++++++++++++++++++
> .../drm/i915/display/intel_display_types.h | 2 +
> drivers/gpu/drm/i915/display/intel_hdmi.c | 7 +++
> 3 files changed, 67 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/display/intel_display_debugfs.c
> b/drivers/gpu/drm/i915/display/intel_display_debugfs.c
> index ce3f9810c42d..cca115a6f130 100644
> --- a/drivers/gpu/drm/i915/display/intel_display_debugfs.c
> +++ b/drivers/gpu/drm/i915/display/intel_display_debugfs.c
> @@ -1208,6 +1208,56 @@ static const struct file_operations
> i915_dsc_fractional_bpp_fops = {
> .write = i915_dsc_fractional_bpp_write
> };
>
> +static int i915_hdmi_req_bpc_show(struct seq_file *m, void *data)
> +{
> + struct intel_connector *connector = m->private;
> +
> + seq_printf(m, "request bpc: %u\n", connector->force_hdmi_bpc);
> +
> + return 0;
> +}
> +
> +static ssize_t i915_hdmi_req_bpc_write(struct file *file,
> + const char __user *ubuf,
> + size_t len, loff_t *offp)
> +{
> + struct seq_file *m = file->private_data;
> + struct intel_connector *intel_connector = m->private;
> + struct intel_display *display = to_intel_display(intel_connector);
> + int new_bpc = 0, ret = 0;
> + char *kbuf;
> + const char *p;
> +
> + kbuf = memdup_user_nul(ubuf, len);
> + if (IS_ERR(kbuf))
> + return PTR_ERR(kbuf);
> +
> + p = strim(kbuf);
> +
> + ret = kstrtoint(p, 0, &new_bpc);
> + if (ret < 0)
> + return -EINVAL;
> +
> + switch (new_bpc) {
> + case 0:
> + case 8:
> + case 10:
> + case 12:
> + break;
> + default:
> + drm_dbg(display->drm,
> + "HDMI bpc (%u) may exceed the max value (12)\n",
> new_bpc);
> + return -EINVAL;
> + }
> +
> + intel_connector->force_hdmi_bpc = new_bpc;
> +
> + *offp += len;
> + kfree(kbuf);
> + return len;
> +}
> +DEFINE_SHOW_STORE_ATTRIBUTE(i915_hdmi_req_bpc);
> +
> /*
> * Returns the Current CRTC's bpc.
> * Example usage: cat /sys/kernel/debug/dri/0/crtc-0/i915_current_bpc
> @@ -1359,6 +1409,13 @@ void intel_connector_debugfs_add(struct
> intel_connector *connector)
> connector_type == DRM_MODE_CONNECTOR_HDMIB)
> debugfs_create_file("i915_lpsp_capability", 0444, root,
> connector, &i915_lpsp_capability_fops);
> +
> + if (connector_type == DRM_MODE_CONNECTOR_HDMIA ||
> + connector_type == DRM_MODE_CONNECTOR_HDMIB) {
> + connector->force_hdmi_bpc = 0;
> + debugfs_create_file("i915_force_hdmi_bpc", 0644, root,
> + connector, &i915_hdmi_req_bpc_fops);
> + }
> }
>
> /**
> diff --git a/drivers/gpu/drm/i915/display/intel_display_types.h
> b/drivers/gpu/drm/i915/display/intel_display_types.h
> index 4d9df803ad47..e1e42d701f68 100644
> --- a/drivers/gpu/drm/i915/display/intel_display_types.h
> +++ b/drivers/gpu/drm/i915/display/intel_display_types.h
> @@ -564,6 +564,8 @@ struct intel_connector {
> struct work_struct modeset_retry_work;
>
> struct intel_hdcp hdcp;
> +
> + u8 force_hdmi_bpc;
> };
>
> struct intel_digital_connector_state {
> diff --git a/drivers/gpu/drm/i915/display/intel_hdmi.c
> b/drivers/gpu/drm/i915/display/intel_hdmi.c
> index cbee628eb26b..0aa4044e2854 100644
> --- a/drivers/gpu/drm/i915/display/intel_hdmi.c
> +++ b/drivers/gpu/drm/i915/display/intel_hdmi.c
> @@ -2118,6 +2118,7 @@ static int intel_hdmi_compute_bpc(struct intel_encoder
> *encoder,
> int clock, bool respect_downstream_limits)
> {
> struct intel_hdmi *intel_hdmi = enc_to_intel_hdmi(encoder);
> + struct intel_connector *intel_connector =
> intel_hdmi->attached_connector;
> int bpc;
>
> /*
> @@ -2134,6 +2135,12 @@ static int intel_hdmi_compute_bpc(struct intel_encoder
> *encoder,
> if (!respect_downstream_limits)
> bpc = 8;
>
> + /*
> + * overwrite bpc per user's request
> + */
> + if (intel_connector->force_hdmi_bpc)
> + bpc = intel_connector->force_hdmi_bpc;
> +
> for (; bpc >= 8; bpc -= 2) {
> int tmds_clock = intel_hdmi_tmds_clock(clock, bpc,
> crtc_state->sink_format);
> --
> 2.34.1
>
