Re: [PATCH v3] drm/i915/dp: Use max params for panels < eDP 1.4

[Ville Syrjälä]

On Fri, Aug 20, 2021 at 03:52:59PM +0800, Kai-Heng Feng wrote:
> Users reported that after commit 2bbd6dba84d4 ("drm/i915: Try to use
> fast+narrow link on eDP again and fall back to the old max strategy on
> failure"), the screen starts to have wobbly effect.
> 
> Commit a5c936add6a2 ("drm/i915/dp: Use slow and wide link training for
> everything") doesn't help either, that means the affected eDP 1.2 panels
> only work with max params.
> 
> So use max params for panels < eDP 1.4 as Windows does to solve the
> issue.
> 
> v3:
>  - Do the eDP rev check in intel_edp_init_dpcd()
> 
> v2:
>  - Check eDP 1.4 instead of DPCD 1.1 to apply max params
> 
> Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/3714
> Fixes: 2bbd6dba84d4 ("drm/i915: Try to use fast+narrow link on eDP again and fall back to the old max strategy on failure")
> Fixes: a5c936add6a2 ("drm/i915/dp: Use slow and wide link training for everything")
> Suggested-by: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
> Signed-off-by: Kai-Heng Feng <kai.heng.feng@xxxxxxxxxxxxx>

Slapped a cc:stable on it and pushed to drm-intel-next. Thanks.

> ---
>  drivers/gpu/drm/i915/display/intel_dp.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
> index 75d4ebc669411..e0dbd35ae7bc0 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp.c
> @@ -2445,11 +2445,14 @@ intel_edp_init_dpcd(struct intel_dp *intel_dp)
>  	 */
>  	if (drm_dp_dpcd_read(&intel_dp->aux, DP_EDP_DPCD_REV,
>  			     intel_dp->edp_dpcd, sizeof(intel_dp->edp_dpcd)) ==
> -			     sizeof(intel_dp->edp_dpcd))
> +			     sizeof(intel_dp->edp_dpcd)) {
>  		drm_dbg_kms(&dev_priv->drm, "eDP DPCD: %*ph\n",
>  			    (int)sizeof(intel_dp->edp_dpcd),
>  			    intel_dp->edp_dpcd);
>  
> +		intel_dp->use_max_params = intel_dp->edp_dpcd[0] < DP_EDP_14;
> +	}
> +
>  	/*
>  	 * This has to be called after intel_dp->edp_dpcd is filled, PSR checks
>  	 * for SET_POWER_CAPABLE bit in intel_dp->edp_dpcd[1]
> -- 
> 2.32.0

-- 
Ville Syrjälä
Intel