Re: [PATCHv2] drm/i915/dp: On DPCD init/caps wake the DPRx

[Imre Deak <imre.deak@intel.com>]

On Tue, Feb 24, 2026 at 01:18:30PM +0530, Arun R Murthy wrote:
> Its observed that on AUX_CH failure, even if the retry is increased to
> 1000, it does not succeed. Either the command might be wrong or sink in
> an unknown/sleep state can cause this. So try waking the sink device.
> Before reading the DPCD caps wake the sink for eDP and for DP after
> reading the lttpr caps if present and before reading the dpcd caps wake
> up the sink device.
> 
> v2: Use poll_timeout_us (Jani N)
>     Add the reason, why this change is required (Ville)
> 
> Closes: https://issues.redhat.com/browse/RHEL-120913

I wonder what should be the rule with non-public links like the above.
For instance, we do not put VLK-xxx links exactly because those are
non-public. Should/could we demand that RedHat opens a public ticket?
Jani?

> Signed-off-by: Arun R Murthy <arun.r.murthy@intel.com>
> ---
>  drivers/gpu/drm/i915/display/intel_dp.c       | 38 +++++++++++++++++++
>  drivers/gpu/drm/i915/display/intel_dp.h       |  1 +
>  .../drm/i915/display/intel_dp_link_training.c |  3 ++
>  3 files changed, 42 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
> index 025e906b63a9..fa0730f7b92a 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp.c
> @@ -4705,6 +4705,42 @@ intel_edp_set_sink_rates(struct intel_dp *intel_dp)
>  	intel_edp_set_data_override_rates(intel_dp);
>  }
>  
> +/* Spec says to try for 3 times, its doubled to add the software overhead */
> +#define AUX_CH_WAKE_RETRY	6
> +
> +void intel_dp_wake_sink(struct intel_dp *intel_dp)
> +{
> +	u8 value = 0;
> +	int ret = 0;
> +
> +	intel_dp_dpcd_set_probe(intel_dp, false);

Is there any particular reason to turn off/on the probing? I don't see
any reason why the DP_SET_POWER polling would need that. In any case
turning it off/on this way is not ok: reading the DPRX caps, which would
call this function, could happen at any time after a sink gets
connected, so turning probing back on at the end of this function would
re-enable it incorrectly for sinks where it's been already established
that the probing workaround is not needed and should stay disabled.

> +
> +	/*
> +	 * Wake the sink device
> +	 * Spec DP2.1 section 2.3.1.2 if AUX CH is powered down by writing 0x02
> +	 * to DP_SET_POWER dpcd reg, 1ms time would be required to wake it up
> +	 */
> +	ret = poll_timeout_us(ret = drm_dp_dpcd_readb(&intel_dp->aux, DP_SET_POWER, &value),
> +			      ret > 0,
> +			      1000, AUX_CH_WAKE_RETRY * 1000, true);
> +
> +	/*
> +	 * If sink is in D3 then it may not respond to the AUX tx so
> +	 * wake it up to D3_AUX_ON state
> +	 * If the above poll_timeout_us fails, try waking the sink.
> +	 */
> +	if (value == DP_SET_POWER_D3 || ret < 0) {
> +		/* After setting to D0 need a min of 1ms to wake(Spec DP2.1 sec 2.3.1.2) */
> +		drm_dp_dpcd_writeb(&intel_dp->aux, DP_SET_POWER,
> +				   DP_SET_POWER_D0);
> +		fsleep(1000);
> +		drm_dp_dpcd_writeb(&intel_dp->aux, DP_SET_POWER,
> +				   DP_SET_POWER_D3_AUX_ON);
> +	}
> +
> +	intel_dp_dpcd_set_probe(intel_dp, true);
> +}
> +
>  static bool
>  intel_edp_init_dpcd(struct intel_dp *intel_dp, struct intel_connector *connector)
>  {
> @@ -4713,6 +4749,8 @@ intel_edp_init_dpcd(struct intel_dp *intel_dp, struct intel_connector *connector
>  	/* this function is meant to be called only once */
>  	drm_WARN_ON(display->drm, intel_dp->dpcd[DP_DPCD_REV] != 0);
>  
> +	intel_dp_wake_sink(intel_dp);
> +
>  	if (drm_dp_read_dpcd_caps(&intel_dp->aux, intel_dp->dpcd) != 0)
>  		return false;
>  
> diff --git a/drivers/gpu/drm/i915/display/intel_dp.h b/drivers/gpu/drm/i915/display/intel_dp.h
> index b0bbd5981f57..3f16077c0cc7 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp.h
> +++ b/drivers/gpu/drm/i915/display/intel_dp.h
> @@ -232,6 +232,7 @@ bool intel_dp_dotclk_valid(struct intel_display *display,
>  bool intel_dp_joiner_candidate_valid(struct intel_connector *connector,
>  				     int hdisplay,
>  				     int num_joined_pipes);
> +void intel_dp_wake_sink(struct intel_dp *intel_dp);
>  
>  #define for_each_joiner_candidate(__connector, __mode, __num_joined_pipes) \
>  	for ((__num_joined_pipes) = 1; (__num_joined_pipes) <= (I915_MAX_PIPES); (__num_joined_pipes)++) \
> diff --git a/drivers/gpu/drm/i915/display/intel_dp_link_training.c b/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> index 54c585c59b90..cbb712ea9f60 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> @@ -270,6 +270,9 @@ int intel_dp_init_lttpr_and_dprx_caps(struct intel_dp *intel_dp)
>  		lttpr_count = intel_dp_init_lttpr(intel_dp, dpcd);
>  	}
>  
> +	/* After reading LTTPR wake up the sink before reading DPRX caps */
> +	intel_dp_wake_sink(intel_dp);
> +
>  	/*
>  	 * The DPTX shall read the DPRX caps after LTTPR detection, so re-read
>  	 * it here.
> -- 
> 2.25.1
>