Re: [Intel-gfx] [PATCH] drm/i915: Reenable LTTPR non-transparent LT mode for DPCD_REV<1.4

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Tested on latest drm-tip with DPCD=1.2 Sink and LTTPR set to non-
transparent mode:

[  706.966375] i915 0000:00:02.0: [drm:drm_dp_dpcd_read] AUX USBC2/DDI
TC2/PHY TC2: 0xf0000 AUX -> (ret=  8) 14 1e 80 55 04 00 00 00
[  706.966383] i915 0000:00:02.0:
[drm:intel_dp_init_lttpr_and_dprx_caps [i915]] LTTPR common
capabilities: 14 1e 80 55 04 00 00 00
[  706.966900] i915 0000:00:02.0: [drm:drm_dp_dpcd_write] AUX USBC2/DDI
TC2/PHY TC2: 0xf0003 AUX <- (ret=  1) 55
[  706.967397] i915 0000:00:02.0: [drm:drm_dp_dpcd_write] AUX USBC2/DDI
TC2/PHY TC2: 0xf0003 AUX <- (ret=  1) aa
[  706.968384] i915 0000:00:02.0: [drm:drm_dp_dpcd_read] AUX USBC2/DDI
TC2/PHY TC2: 0xf0020 AUX -> (ret=  3) 03 00 00
[  706.968388] i915 0000:00:02.0:
[drm:intel_dp_init_lttpr_and_dprx_caps [i915]] LTTPR 1 PHY
capabilities: 03 00 00
[  706.969913] i915 0000:00:02.0: [drm:drm_dp_dpcd_read] AUX USBC2/DDI
TC2/PHY TC2: 0x00000 AUX -> (ret= 15) 12 14 c4 01 01 00 01 00 02 02 06
00 00 00 00
[  706.969917] i915 0000:00:02.0: [drm:drm_dp_read_dpcd_caps] AUX
USBC2/DDI TC2/PHY TC2: DPCD: 12 14 c4 01 01 00 01 00 02 02 06 00 00 00
00

Tested-by: Khaled Almahallawy <khaled.almahallawy@xxxxxxxxx>
Reviewed-by: Khaled Almahallawy <khaled.almahallawy@xxxxxxxxx>

Thank you for the patch
~Khaled

On Thu, 2021-05-13 at 00:28 +0300, Imre Deak wrote:
> The driver currently disables the LTTPR non-transparent link training
> mode for sinks with a DPCD_REV<1.4, based on the following
> description
> of the LTTPR DPCD register range in DP standard 2.0 (at the 0xF0000
> register description):
> 
> ""
> LTTPR-related registers at DPCD Addresses F0000h through F02FFh are
> valid
> only for DPCD r1.4 (or higher).
> """
> 
> The transparent link training mode should still work fine, however
> the
> implementation for this in some retimer FWs seems to be broken, see
> the
> References: link below.
> 
> After discussions with DP standard authors the above "DPCD r1.4" does
> not refer to the DPCD revision (stored in the DPCD_REV reg at
> 0x00000),
> rather to the "LTTPR field data structure revision" stored in the
> 0xF0000 reg. An update request has been filed at vesa.org (see
> wg/Link/documentComment/3746) for the upcoming v2.1 specification to
> clarify the above description along the following lines:
> 
> """
> LTTPR-related registers at DPCD Addresses F0000h through F02FFh are
> valid only for LT_TUNABLE_PHY_REPEATER_FIELD_DATA_STRUCTURE_REV 1.4
> (or
> higher)
> """
> 
> Based on my tests Windows uses the non-transparent link training mode
> for DPCD_REV==1.2 sinks as well (so presumably for all DPCD_REVs),
> and
> forcing it to use transparent mode on ICL/TGL platforms leads to the
> same LT failure as reported at the References: link.
> 
> Based on the above let's assume that the transparent link training
> mode
> is not well tested/supported and align the code to the correct
> interpretation of what the r1.4 version refers to.
> 
> References: https://gitlab.freedesktop.org/drm/intel/-/issues/3415
> Fixes: 264613b406eb ("drm/i915: Disable LTTPR support when the DPCD
> rev < 1.4")
> Cc: <stable@xxxxxxxxxxxxxxx> # v5.11+
> Signed-off-by: Imre Deak <imre.deak@xxxxxxxxx>
> ---
>  .../drm/i915/display/intel_dp_link_training.c | 71 +++++++++------
> ----
>  1 file changed, 33 insertions(+), 38 deletions(-)
> 
> 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 6bf6f1ec13ed8..08bceae40aa8d 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> @@ -128,50 +128,14 @@ intel_dp_set_lttpr_transparent_mode(struct
> intel_dp *intel_dp, bool enable)
>  	return drm_dp_dpcd_write(&intel_dp->aux, DP_PHY_REPEATER_MODE,
> &val, 1) == 1;
>  }
>  
> -/**
> - * intel_dp_init_lttpr_and_dprx_caps - detect LTTPR and DPRX caps,
> init the LTTPR link training mode
> - * @intel_dp: Intel DP struct
> - *
> - * Read the LTTPR common and DPRX capabilities and switch to non-
> transparent
> - * link training mode if any is detected and read the PHY
> capabilities for all
> - * detected LTTPRs. In case of an LTTPR detection error or if the
> number of
> - * LTTPRs is more than is supported (8), fall back to the no-LTTPR,
> - * transparent mode link training mode.
> - *
> - * Returns:
> - *   >0  if LTTPRs were detected and the non-transparent LT mode was
> set. The
> - *       DPRX capabilities are read out.
> - *    0  if no LTTPRs or more than 8 LTTPRs were detected or in case
> of a
> - *       detection failure and the transparent LT mode was set. The
> DPRX
> - *       capabilities are read out.
> - *   <0  Reading out the DPRX capabilities failed.
> - */
> -int intel_dp_init_lttpr_and_dprx_caps(struct intel_dp *intel_dp)
> +static int intel_dp_init_lttpr(struct intel_dp *intel_dp)
>  {
>  	int lttpr_count;
> -	bool ret;
>  	int i;
>  
> -	ret = intel_dp_read_lttpr_common_caps(intel_dp);
> -
> -	/* The DPTX shall read the DPRX caps after LTTPR detection. */
> -	if (drm_dp_read_dpcd_caps(&intel_dp->aux, intel_dp->dpcd)) {
> -		intel_dp_reset_lttpr_common_caps(intel_dp);
> -		return -EIO;
> -	}
> -
> -	if (!ret)
> +	if (!intel_dp_read_lttpr_common_caps(intel_dp))
>  		return 0;
>  
> -	/*
> -	 * The 0xF0000-0xF02FF range is only valid if the DPCD revision
> is
> -	 * at least 1.4.
> -	 */
> -	if (intel_dp->dpcd[DP_DPCD_REV] < 0x14) {
> -		intel_dp_reset_lttpr_common_caps(intel_dp);
> -		return 0;
> -	}
> -
>  	lttpr_count = drm_dp_lttpr_count(intel_dp->lttpr_common_caps);
>  	/*
>  	 * Prevent setting LTTPR transparent mode explicitly if no
> LTTPRs are
> @@ -211,6 +175,37 @@ int intel_dp_init_lttpr_and_dprx_caps(struct
> intel_dp *intel_dp)
>  
>  	return lttpr_count;
>  }
> +
> +/**
> + * intel_dp_init_lttpr_and_dprx_caps - detect LTTPR and DPRX caps,
> init the LTTPR link training mode
> + * @intel_dp: Intel DP struct
> + *
> + * Read the LTTPR common and DPRX capabilities and switch to non-
> transparent
> + * link training mode if any is detected and read the PHY
> capabilities for all
> + * detected LTTPRs. In case of an LTTPR detection error or if the
> number of
> + * LTTPRs is more than is supported (8), fall back to the no-LTTPR,
> + * transparent mode link training mode.
> + *
> + * Returns:
> + *   >0  if LTTPRs were detected and the non-transparent LT mode was
> set. The
> + *       DPRX capabilities are read out.
> + *    0  if no LTTPRs or more than 8 LTTPRs were detected or in case
> of a
> + *       detection failure and the transparent LT mode was set. The
> DPRX
> + *       capabilities are read out.
> + *   <0  Reading out the DPRX capabilities failed.
> + */
> +int intel_dp_init_lttpr_and_dprx_caps(struct intel_dp *intel_dp)
> +{
> +	int lttpr_count = intel_dp_init_lttpr(intel_dp);
> +
> +	/* The DPTX shall read the DPRX caps after LTTPR detection. */
> +	if (drm_dp_read_dpcd_caps(&intel_dp->aux, intel_dp->dpcd)) {
> +		intel_dp_reset_lttpr_common_caps(intel_dp);
> +		return -EIO;
> +	}
> +
> +	return lttpr_count;
> +}
>  EXPORT_SYMBOL(intel_dp_init_lttpr_and_dprx_caps);
>  
>  static u8 dp_voltage_max(u8 preemph)




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