[v2] drm/i915/hdmi: add debugfs to contorl HDMI bpc

[Imre Deak]

On Mon, Aug 11, 2025 at 09:46:55AM +0300, Lee, Shawn C wrote:
> On Fri, Aug 08, 2025 at 09:40:00AM +0000, Imre Deak C wrote:
> >On Fri, Aug 08, 2025 at 09:16:02AM +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.
> >> 
> >> Introduce a new debugfs that allows user to configure dedicated bpc 
> >> manually, and making HDMI compliance test much easier.
> >> 
> >> v2: Using exist variable max_requested_bpc.
> >
> > How come this doesn't get reset after a hotplug as you described for
> > the case when the property is used, even though both the property and
> > this debug entries use the same state variable? (Not saying that the
> > reset happing after a hotplug is a valid justification for a new
> > debugfs entry, the hotplug could be also handled by the user, but you
> > could argue the debugfs entry is more convenient.)
> 
> Hi Imre, thank you for the prompt response.
> 
> https://elixir.bootlin.com/linux/v6.16/source/drivers/gpu/drm/i915/display/intel_hdmi.c#L2672
>
> The max_bpc_property and max_bpc values are initialized during
> connector initialization process.
> 
> https://elixir.bootlin.com/linux/v6.16/source/drivers/gpu/drm/drm_atomic.c#L468
>
> The max_bpc will be restored to info->bpc (from EDID) at
> drm_atomic_connector_check() everytime.
> 
> When max_bpc_property is available, this funciton also compares
> max_bpc with max_requested_bpc and updates max_bpc to the smaller of
> the two values.
> 
> The i915 display driver then relies on this max_bpc value to determine
> whether to update the pipe bpp value in compute_sink_pipe_bpp().
> Therefore, we can simply update max_requested_bpc to affect pipe bpp
> output. And no additional driver changes are required.

The point above was to figure out how the "max bpc" connector property
gets reset after a hotplug, based on your earlier explanation for the
reason to add a debugfs entry instead. But based on the above and also
based on my tests, that connector property actually keeps its value
across hotplugs.

So I think that the property could be used for your tests then, or if
you wanted to have a debugfs entry for convenience, then the
intel_force_link_bpp debugfs entry could be added for all HDMI
connectors, as suggested earlier, instead of adding a new debugfs entry.
The following would do that, could you give it a go?

diff --git a/drivers/gpu/drm/i915/display/g4x_hdmi.c b/drivers/gpu/drm/i915/display/g4x_hdmi.c
index 108ebd97f9e4..b31fb1e4bc1a 100644
--- a/drivers/gpu/drm/i915/display/g4x_hdmi.c
+++ b/drivers/gpu/drm/i915/display/g4x_hdmi.c
@@ -136,11 +136,8 @@ static int g4x_hdmi_compute_config(struct intel_encoder *encoder,
 	struct intel_atomic_state *state = to_intel_atomic_state(crtc_state->uapi.state);
 	struct intel_crtc *crtc = to_intel_crtc(crtc_state->uapi.crtc);

-	if (HAS_PCH_SPLIT(display)) {
+	if (HAS_PCH_SPLIT(display))
 		crtc_state->has_pch_encoder = true;
-		if (!intel_fdi_compute_pipe_bpp(crtc_state))
-			return -EINVAL;
-	}

 	if (display->platform.g4x)
 		crtc_state->has_hdmi_sink = g4x_compute_has_hdmi_sink(state, crtc);
diff --git a/drivers/gpu/drm/i915/display/intel_hdmi.c b/drivers/gpu/drm/i915/display/intel_hdmi.c
index cbee628eb26b..027e8ed0cea8 100644
--- a/drivers/gpu/drm/i915/display/intel_hdmi.c
+++ b/drivers/gpu/drm/i915/display/intel_hdmi.c
@@ -55,6 +55,7 @@
 #include "intel_display_regs.h"
 #include "intel_display_types.h"
 #include "intel_dp.h"
+#include "intel_fdi.h"
 #include "intel_gmbus.h"
 #include "intel_hdcp.h"
 #include "intel_hdcp_regs.h"
@@ -2345,6 +2346,9 @@ int intel_hdmi_compute_config(struct intel_encoder *encoder,
 	if (adjusted_mode->flags & DRM_MODE_FLAG_DBLCLK)
 		pipe_config->pixel_multiplier = 2;

+	if (!intel_fdi_compute_pipe_bpp(pipe_config))
+		return -EINVAL;
+
 	pipe_config->has_audio =
 		intel_hdmi_has_audio(encoder, pipe_config, conn_state) &&
 		intel_audio_compute_config(encoder, pipe_config, conn_state);
diff --git a/drivers/gpu/drm/i915/display/intel_link_bw.c b/drivers/gpu/drm/i915/display/intel_link_bw.c
index 3caef7f9c7c4..9e8220d86a4a 100644
--- a/drivers/gpu/drm/i915/display/intel_link_bw.c
+++ b/drivers/gpu/drm/i915/display/intel_link_bw.c
@@ -449,6 +449,7 @@ void intel_link_bw_connector_debugfs_add(struct intel_connector *connector)
 	switch (connector->base.connector_type) {
 	case DRM_MODE_CONNECTOR_DisplayPort:
 	case DRM_MODE_CONNECTOR_eDP:
+	case DRM_MODE_CONNECTOR_HDMIA:
 		break;
 	case DRM_MODE_CONNECTOR_VGA:
 	case DRM_MODE_CONNECTOR_SVIDEO:
@@ -456,13 +457,6 @@ void intel_link_bw_connector_debugfs_add(struct intel_connector *connector)
 	case DRM_MODE_CONNECTOR_DVID:
 		if (HAS_FDI(display))
 			break;
-
-		return;
-	case DRM_MODE_CONNECTOR_HDMIA:
-		if (HAS_FDI(display) && !HAS_DDI(display))
-			break;
-
-		return;
 	default:
 		return;
 	}