[PATCH] drm/i915/display: Read DP_ADAPTER_CAP to pass LinkLayer DPCD&EDID tests

[Cavitt, Jonathan]

-----Original Message-----
From: Intel-gfx <intel-gfx-bounces at lists.freedesktop.org> On Behalf Of Khaled Almahallawy
Sent: Tuesday, July 1, 2025 12:27 PM
To: intel-gfx at lists.freedesktop.org; intel-xe at lists.freedesktop.org
Cc: Almahallawy, Khaled <khaled.almahallawy at intel.com>; Shankar, Uma <uma.shankar at intel.com>; Nikula, Jani <jani.nikula at intel.com>
Subject: [PATCH] drm/i915/display: Read DP_ADAPTER_CAP to pass LinkLayer DPCD&EDID tests
> 
> 4.2.2.1 LinkLayer test states:
> "Verify that Source DUT does the following within 5 seconds after HPD
> being asserted:
> Reads the DPCD Receiver Capability field (DPCD: 00000h:0000Fh)"
> ...
> 
> Fail1:
> Source DUT failed to read the DPCD Receiver Capability field (DPCD:
> 00000h:0000Fh) through AUX_CH before link training."
> 
> Fix this by reading DP_ADAPTER_CAP(0x000F & 0x220F)
> 
> Cc: Uma Shankar <uma.shankar at intel.com>
> Cc: Jani Nikula <jani.nikula at intel.com>
> Signed-off-by: Khaled Almahallawy <khaled.almahallawy at intel.com>
> ---
>  drivers/gpu/drm/i915/display/intel_dp_link_training.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> 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 a479b63112ea..762dc073b824 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> @@ -252,6 +252,7 @@ int intel_dp_init_lttpr_and_dprx_caps(struct intel_dp *intel_dp)
>  {
>  	struct intel_display *display = to_intel_display(intel_dp);
>  	int lttpr_count = 0;
> +	u8 adapter_cap = 0;
>  
>  	/*
>  	 * Detecting LTTPRs must be avoided on platforms with an AUX timeout
> @@ -277,6 +278,12 @@ int intel_dp_init_lttpr_and_dprx_caps(struct intel_dp *intel_dp)
>  		return -EIO;
>  	}
>  
> +	/* Read DP_ADAPTET_CAP to pass LinkLayer CTS */
> +	drm_dp_dpcd_readb(&intel_dp->aux, DP_ADAPTER_CAP,
> +				   &adapter_cap);

I'm working under the assumption that we're reading these registers not to actually use
the values they hold but because reading them has some other effect during runtime.

Does reading that location clear the register field?  Or does it signal to the system to
perform some other process?

> +	drm_dp_dpcd_readb(&intel_dp->aux, 0x220f,
> +				   &adapter_cap);

0x220f should probably have a #define macro associated with it.  Could you please create one?
Maybe it needs to be added to drm_dp.h.  Or is there a better location for it?
-Jonathan Cavitt

> +
>  	return lttpr_count;
>  }
>  
> -- 
> 2.43.0
> 
>