[PATCH] drm/i915/hdmi: add debugfs to contorl HDMI bpc

[Imre Deak]

On Wed, Aug 06, 2025 at 05:05:39PM +0300, Imre Deak wrote:
> 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.

There is also the "max bpc" connector property. If that doesn't work in
your usecase could you explain why?

> > Cc: Shankar Uma <uma.shankar at intel.com>
> > Signed-off-by: Lee Shawn C <shawn.c.lee at 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
> >