Re: [RESEND] drm/i915/hdmi: Poll for 200 msec for TMDS_Scrambler_Status

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Tue, Jan 13, 2026 at 07:08:45PM +0530, Ankit Nautiyal wrote:
> From: Jerome Tollet <jerome.tollet@gmail.com>
> 
> As per HDMI 2.0 specification, after scrambled video transmission begins,
> the source must poll the TMDS_Scrambler_Status bit until it reads 1 or
> until a timeout of 200 ms.
> 
> Add a polling step after enabling the HDMI port to verify scrambling
> status, following the spec requirement.
> 
> Without the wait for the scrambling bit to set, some HDMI 2.0 monitors fail
> to decode the signal at 4K@60Hz (594 MHz) when SCDC scrambling is not yet
> fully configured by the sink.
> 
> v2:
>  - Instead of the fixed delay, poll for TMDS scramble status for 200 msec
>    as per the HDMI spec. (Ankit)
> 
> Reported-by: Jerome Tollet <jerome.tollet@gmail.com>
> Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/6868
> Link: https://lore.kernel.org/dri-devel/20251230091037.5603-1-jerome.tollet@gmail.com/
> Signed-off-by: Jerome Tollet <jerome.tollet@gmail.com>
> Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
> ---
> 
> Rebased and resend to intel-gfx and intel-xe so that the patch is picked
> up by intel gfx CI.
> 
> ---
>  drivers/gpu/drm/i915/display/intel_ddi.c  |  2 ++
>  drivers/gpu/drm/i915/display/intel_hdmi.c | 25 +++++++++++++++++++++++
>  drivers/gpu/drm/i915/display/intel_hdmi.h |  2 ++
>  3 files changed, 29 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_ddi.c b/drivers/gpu/drm/i915/display/intel_ddi.c
> index cb91d07cdaa6..c708b713f0e8 100644
> --- a/drivers/gpu/drm/i915/display/intel_ddi.c
> +++ b/drivers/gpu/drm/i915/display/intel_ddi.c
> @@ -3506,6 +3506,8 @@ static void intel_ddi_enable_hdmi(struct intel_atomic_state *state,
>  	}
>  
>  	intel_ddi_buf_enable(encoder, buf_ctl);
> +
> +	intel_hdmi_poll_for_scrambling_enable(crtc_state, connector);

This is the last thing we do in the modeset (apart for some HDCP stuff),
so it's not at all clear why this would fix anything. The only thing that
will change after this is the actual video data or some infoframes (audio
in particular). It would be good to find out what exactly is the thing
that needs the delay (ie. keep moving the delay further forward in the
modeset sequence until it fails again).

>  }
>  
>  static void intel_ddi_enable(struct intel_atomic_state *state,
> diff --git a/drivers/gpu/drm/i915/display/intel_hdmi.c b/drivers/gpu/drm/i915/display/intel_hdmi.c
> index 055e68810d0d..6f7dcd7365a4 100644
> --- a/drivers/gpu/drm/i915/display/intel_hdmi.c
> +++ b/drivers/gpu/drm/i915/display/intel_hdmi.c
> @@ -3363,3 +3363,28 @@ intel_hdmi_dsc_get_bpp(int src_fractional_bpp, int slice_width, int num_slices,
>  
>  	return 0;
>  }
> +
> +/*
> + * As Per HDMI 2.0 spec: after scrambled video transmission begins,
> + * poll TMDS_Scrambler_Status until it reads 1, for up to 200 ms.
> + */
> +void
> +intel_hdmi_poll_for_scrambling_enable(const struct intel_crtc_state *crtc_state,
> +				      struct drm_connector *_connector)
> +{
> +	struct intel_connector *connector = to_intel_connector(_connector);
> +	struct intel_display *display = to_intel_display(crtc_state);
> +	bool scrambling_enabled = false;
> +	int ret;
> +
> +	if (!crtc_state->hdmi_scrambling)
> +		return;
> +
> +	/* Poll for a max of 200 msec as per HDMI spec */
> +	ret = poll_timeout_us(scrambling_enabled = drm_scdc_get_scrambling_status(&connector->base),
> +			      scrambling_enabled, 1000, 200 * 1000, false);
> +	if (ret)
> +		drm_dbg_kms(display->drm,
> +			    "[CONNECTOR:%d:%s] Timed out waiting for scrambling enable\n",
> +			    connector->base.base.id, connector->base.name);
> +}
> diff --git a/drivers/gpu/drm/i915/display/intel_hdmi.h b/drivers/gpu/drm/i915/display/intel_hdmi.h
> index be2fad57e4ad..0fa3661568e8 100644
> --- a/drivers/gpu/drm/i915/display/intel_hdmi.h
> +++ b/drivers/gpu/drm/i915/display/intel_hdmi.h
> @@ -70,5 +70,7 @@ void hsw_read_infoframe(struct intel_encoder *encoder,
>  			const struct intel_crtc_state *crtc_state,
>  			unsigned int type,
>  			void *frame, ssize_t len);
> +void intel_hdmi_poll_for_scrambling_enable(const struct intel_crtc_state *crtc_state,
> +					   struct drm_connector *_connector);
>  
>  #endif /* __INTEL_HDMI_H__ */
> -- 
> 2.45.2

-- 
Ville Syrjälä
Intel