Re: [PATCH v2] drm/i915/hdmi: Poll for 200 msec for TMDS_Scrambler_Status

[Jani Nikula <jani.nikula@linux.intel.com>]

On Thu, 08 Jan 2026, Jérôme Tollet <jerome.tollet@gmail.com> wrote:
> Hello,
> 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)

The patch is whitespace broken.

>
>  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>
>  ---
>   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);
>   }
>
>   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..958d939ae6ee 100644
>  --- a/drivers/gpu/drm/i915/display/intel_hdmi.c
>  +++ b/drivers/gpu/drm/i915/display/intel_hdmi.c
>  @@ -2694,6 +2694,31 @@ intel_hdmi_add_properties(struct intel_hdmi
> *intel_hdmi, struct drm_connector *_
>   drm_connector_attach_max_bpc_property(&connector->base, 8, 12);
>   }
>
>  +/*
>  + * 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);
>  +}
>  +
>   /*
>    * 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__ */
>  --
>  2.45.2

-- 
Jani Nikula, Intel