Re: [PATCH 1/2] drm/i915: Refuse modes with hdisplay==4096 on pre-HSW DP

[Manasi Navare <manasi.d.navare@intel.com>]

On Thu, Jul 18, 2019 at 05:43:39PM +0300, Ville Syrjala wrote:
> From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> 
> The DP port/pipe goes wonky if we try to use timings with
> hdisplay==4096 on pre-HSW platforms. The link fails to train
> and the pipe may not signal vblank interrupts. On HDMI such at
> mode works just fine (tested on ELK/SNB/CHV). So let's refuse
> such modes on DP on older platforms.
> 
> Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

Makes sense if the link is not able to get trained that we reject this mode

> ---
>  drivers/gpu/drm/i915/display/intel_dp.c | 25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
> index 0eb5d66f87a7..c96030761eae 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp.c
> @@ -493,6 +493,25 @@ int intel_dp_get_link_train_fallback_values(struct intel_dp *intel_dp,
>  	return 0;
>  }
>  
> +static bool intel_dp_hdisplay_bad(struct drm_i915_private *dev_priv,
> +				  int hdisplay)
> +{
> +	/*
> +	 * Older platforms don't like hdisplay==4096 with DP.
> +	 *
> +	 * On ILK/SNB/IVB the pipe seems to be somewhat running (scanline
> +	 * and frame counter increment), but we don't get vblank interrupts,
> +	 * and the pipe underruns immediately. The link also doesn't seem
> +	 * to get trained properly.
> +	 *
> +	 * On CHV the vblank interrupts don't seem to disappear but
> +	 * otherwise the symptoms are similar.
> +	 *
> +	 * TODO: confirm the behaviour on HSW+
> +	 */
> +	return hdisplay == 4096 && !HAS_DDI(dev_priv);
> +}
> +
>  static enum drm_mode_status
>  intel_dp_mode_valid(struct drm_connector *connector,
>  		    struct drm_display_mode *mode)
> @@ -528,6 +547,9 @@ intel_dp_mode_valid(struct drm_connector *connector,
>  	max_rate = intel_dp_max_data_rate(max_link_clock, max_lanes);
>  	mode_rate = intel_dp_link_required(target_clock, 18);
>  
> +	if (intel_dp_hdisplay_bad(dev_priv, mode->hdisplay))
> +		return MODE_H_ILLEGAL;
> +
>  	/*
>  	 * Output bpp is stored in 6.4 format so right shift by 4 to get the
>  	 * integer value since we support only integer values of bpp.
> @@ -2208,6 +2230,9 @@ intel_dp_compute_config(struct intel_encoder *encoder,
>  	if (adjusted_mode->flags & DRM_MODE_FLAG_DBLCLK)
>  		return -EINVAL;
>  
> +	if (intel_dp_hdisplay_bad(dev_priv, adjusted_mode->crtc_hdisplay))
> +		return -EINVAL;

Why would we need another check in compute_config() since the mode would have been
rejected already in intel_dp_mode_valid()

Manasi

> +
>  	ret = intel_dp_compute_link_config(encoder, pipe_config, conn_state);
>  	if (ret < 0)
>  		return ret;
> -- 
> 2.21.0
> 
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