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

["Nautiyal, Ankit K" <ankit.k.nautiyal@intel.com>]

On 5/20/2026 7:55 AM, Ankit Nautiyal wrote:
> From: Jerome Tollet <jerome.tollet@gmail.com>
>
> HDMI 2.0 section 6.1.3.1 specifies that after enabling
> Scrambling_Enable and starting scrambled video transmission, the source
> should poll Scrambling_Status until it reads 1 or until a timeout of
> 200 ms expires.
>
> Add a polling step after enabling the HDMI port to check the scrambling
> status when HDMI scrambling is enabled.
>
> On some HDMI 2.0 sinks, omitting this check can result in 4K@60Hz
> (594 MHz) failing to come up correctly because the sink has not yet
> finished its scrambling setup. In practice, waiting for the scrambling
> status here fixes such sinks.
>
> While this synchronous polling is not itself explicitly required for
> correct modeset sequencing, HDMI 2.0 section 6.1.3.1 does recommend it
> as the way for the source to verify that the TMDS link is functioning
> correctly with scrambling enabled.
>
> v3:
>   - Add explicit HDMI 2.0 section reference in code comment
>   - Clarify commit message around the observed sink fix
>
> v2:
>   - Poll TMDS_Scrambler_Status for up to 200 ms instead of using a fixed
>     delay
>
> Reported-by: Jerome Tollet <jtollet@cisco.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>
> Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>


Thanks for the patch, comments and reviews. Pushed to drm-intel-next.

Regards,

Ankit

> ---
>
> 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 | 26 +++++++++++++++++++++++
>   drivers/gpu/drm/i915/display/intel_hdmi.h |  2 ++
>   3 files changed, 30 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/display/intel_ddi.c b/drivers/gpu/drm/i915/display/intel_ddi.c
> index 86520848892e..ac307a53576b 100644
> --- a/drivers/gpu/drm/i915/display/intel_ddi.c
> +++ b/drivers/gpu/drm/i915/display/intel_ddi.c
> @@ -3505,6 +3505,8 @@ 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);
>   }
>   
>   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 9076c2b176ec..b9d11fb8559d 100644
> --- a/drivers/gpu/drm/i915/display/intel_hdmi.c
> +++ b/drivers/gpu/drm/i915/display/intel_hdmi.c
> @@ -2728,6 +2728,32 @@ intel_hdmi_add_properties(struct intel_hdmi *intel_hdmi, struct drm_connector *_
>   		drm_connector_attach_max_bpc_property(&connector->base, 8, 12);
>   }
>   
> +/*
> + * HDMI 2.0 spec, section 6.1.3.1 (Scrambling Control): after
> + * enabling Scrambling_Enable and starting scrambled video
> + * transmission, poll Scrambling_Status 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);
> +}
> +
>   /*
>    * intel_hdmi_handle_sink_scrambling: handle sink scrambling/clock ratio setup
>    * @encoder: intel_encoder
> 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__ */