[PATCH v2] drm/i915/dp: Use max params for panels < eDP 1.4

[Ville Syrjälä]

On Thu, Aug 19, 2021 at 01:14:55AM +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.
> 
> 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 at linux.intel.com>
> Signed-off-by: Kai-Heng Feng <kai.heng.feng at canonical.com>
> ---
>  drivers/gpu/drm/i915/display/intel_dp.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
> index 75d4ebc669411..f87fad78f1a9f 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp.c
> @@ -1330,14 +1330,16 @@ intel_dp_compute_link_config(struct intel_encoder *encoder,
>  	limits.min_bpp = intel_dp_min_bpp(pipe_config->output_format);
>  	limits.max_bpp = intel_dp_max_bpp(intel_dp, pipe_config);
>  
> -	if (intel_dp->use_max_params) {
> +	if (intel_dp->use_max_params ||
> +	    intel_dp->edp_dpcd[0] < DP_EDP_14) {

I think we're going to need a is_edp check here or else we'll always
take this path on extenral DP when edp_dpcd[] is zeroed.

Hmm. Maybe just stick
intel_dp->use_max_params = intel_dp->edp_dpcd[0] < DP_EDP_14;
into intel_edp_init_dpcd()?

>  		/*
>  		 * Use the maximum clock and number of lanes the eDP panel
>  		 * advertizes being capable of in case the initial fast
> -		 * optimal params failed us. The panels are generally
> -		 * designed to support only a single clock and lane
> -		 * configuration, and typically on older panels these
> -		 * values correspond to the native resolution of the panel.
> +		 * optimal params failed us or the EDP rev is earlier than 1.4.
> +		 * The panels are generally designed to support only a single
> +		 * clock and lane configuration, and typically on older panels
> +		 * these values correspond to the native resolution of the
> +		 * panel.
>  		 */
>  		limits.min_lane_count = limits.max_lane_count;
>  		limits.min_clock = limits.max_clock;
> -- 
> 2.32.0

-- 
Ville Syrjälä
Intel