Re: [PATCH v3] drm/i915: Disable fast link training if DP config changes

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Wed, Dec 16, 2015 at 02:26:58PM +0200, Mika Kahola wrote:
> Disable DP fast link training if DP link configuration
> changes. If one of the DP link parameters i.e. link
> bandwidth, lane count, rate selection, port clock or bpp
> changes the link training does no longer apply the
> previously computed voltage swing and pre-emphasis values.
> Instead, the link training is started with zero values.
> 
> v3: Remove cached old link parameters. Instead, disable
>     fast link training feature when link parameters are
>     set (Ville)
> 
> v2: Readout DPCD register to check if no aux handshaking is
>     required in link training (Ander)
> 
> Signed-off-by: Mika Kahola <mika.kahola@intel.com>
> ---
>  drivers/gpu/drm/i915/intel_dp.c               |  3 ++-
>  drivers/gpu/drm/i915/intel_dp_link_training.c | 16 ++++++++++++++++
>  drivers/gpu/drm/i915/intel_drv.h              |  1 +
>  3 files changed, 19 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
> index 31ba241..92adf21 100644
> --- a/drivers/gpu/drm/i915/intel_dp.c
> +++ b/drivers/gpu/drm/i915/intel_dp.c
> @@ -1679,6 +1679,7 @@ void intel_dp_set_link_params(struct intel_dp *intel_dp,
>  {
>  	intel_dp->link_rate = pipe_config->port_clock;
>  	intel_dp->lane_count = pipe_config->lane_count;
> +	intel_dp->train_set_valid = false;

This would disable the optimization every time we fire up the link.
You would have to check the new vs. current params first, and clear
the flag only if there's a difference.

>  }
>  
>  static void intel_dp_prepare(struct intel_encoder *encoder)
> @@ -3849,7 +3850,7 @@ intel_dp_link_down(struct intel_dp *intel_dp)
>  	intel_dp->DP = DP;
>  }
>  
> -static bool
> +bool
>  intel_dp_get_dpcd(struct intel_dp *intel_dp)
>  {
>  	struct intel_digital_port *dig_port = dp_to_dig_port(intel_dp);
> diff --git a/drivers/gpu/drm/i915/intel_dp_link_training.c b/drivers/gpu/drm/i915/intel_dp_link_training.c
> index 8888793..0e95a98 100644
> --- a/drivers/gpu/drm/i915/intel_dp_link_training.c
> +++ b/drivers/gpu/drm/i915/intel_dp_link_training.c
> @@ -85,6 +85,22 @@ static bool
>  intel_dp_reset_link_train(struct intel_dp *intel_dp,
>  			uint8_t dp_train_pat)
>  {
> +	bool has_dpcd;
> +	bool flt_supported = false;
> +
> +	has_dpcd = intel_dp_get_dpcd(intel_dp);
> +
> +	if (has_dpcd) {
> +		if (intel_dp->dpcd[DP_DPCD_REV] >= 0x11)
> +			flt_supported = (intel_dp->dpcd[DP_MAX_DOWNSPREAD] &
> +					 DP_NO_AUX_HANDSHAKE_LINK_TRAINING);
> +	}
> +
> +	intel_dp->train_set_valid &= flt_supported;

AFAIK what we do isn't really fast link training. Fast link training is
where you skip the AUX communication entirely, ie. just send the training
patterns for a shot predefined time, and afterwards checks if the link
is in good condition or not.

> +
> +	DRM_DEBUG_KMS("fast link training enabled: %s\n",
> +		      intel_dp->train_set_valid ? "true" : "false");
> +
>  	if (!intel_dp->train_set_valid)
>  		memset(intel_dp->train_set, 0, sizeof(intel_dp->train_set));
>  	intel_dp_set_signal_levels(intel_dp);
> diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
> index 798463e..94041fd 100644
> --- a/drivers/gpu/drm/i915/intel_drv.h
> +++ b/drivers/gpu/drm/i915/intel_drv.h
> @@ -1239,6 +1239,7 @@ int intel_dp_sink_crc(struct intel_dp *intel_dp, u8 *crc);
>  bool intel_dp_compute_config(struct intel_encoder *encoder,
>  			     struct intel_crtc_state *pipe_config);
>  bool intel_dp_is_edp(struct drm_device *dev, enum port port);
> +bool intel_dp_get_dpcd(struct intel_dp *intel_dp);
>  enum irqreturn intel_dp_hpd_pulse(struct intel_digital_port *intel_dig_port,
>  				  bool long_hpd);
>  void intel_edp_backlight_on(struct intel_dp *intel_dp);
> -- 
> 1.9.1
> 
> _______________________________________________
> Intel-gfx mailing list
> Intel-gfx@lists.freedesktop.org
> http://lists.freedesktop.org/mailman/listinfo/intel-gfx

-- 
Ville Syrjälä
Intel OTC
_______________________________________________
Intel-gfx mailing list
Intel-gfx@lists.freedesktop.org
http://lists.freedesktop.org/mailman/listinfo/intel-gfx