[PATCH 3/4] drm/i915: Find fallback link rate/lane count

[Manasi Navare]

On Thu, Dec 08, 2016 at 11:46:02PM +0200, Jani Nikula wrote:
> On Tue, 06 Dec 2016, Manasi Navare <manasi.d.navare at intel.com> wrote:
> > If link training fails, then we need to fallback to lower
> > link rate first and if link training fails at RBR, then
> > fallback to lower lane count.
> > This function finds the next lower link rate/lane count
> > value after link training failure and limits the max
> > link_rate and lane_count values to these fallback values.
> >
> > v6:
> > * Cap the max link rate and lane count to the max
> > values obtained during fallback link training (Daniel Vetter)
> > v5:
> > * Start the fallback at the lane count value passed not
> > the max lane count (Jani Nikula)
> > v4:
> > * Remove the redundant variable link_train_failed
> > v3:
> > * Remove fallback_link_rate_index variable, just obtain
> > that using the helper intel_dp_link_rate_index (Jani Nikula)
> > v2:
> > Squash the patch that returns the link rate index (Jani Nikula)
> >
> > Acked-by: Tony Cheng <tony.cheng at amd.com>
> > Acked-by: Harry Wentland <harry.wentland at amd.com>
> > Cc: Ville Syrjala <ville.syrjala at linux.intel.com>
> > Cc: Jani Nikula <jani.nikula at linux.intel.com>
> > Cc: Daniel Vetter <daniel.vetter at intel.com>
> > Signed-off-by: Manasi Navare <manasi.d.navare at intel.com>
> > ---
> >  drivers/gpu/drm/i915/intel_dp.c  | 40 ++++++++++++++++++++++++++++++++++++++++
> >  drivers/gpu/drm/i915/intel_drv.h |  2 ++
> >  2 files changed, 42 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
> > index 434dc7d..b5c7526f 100644
> > --- a/drivers/gpu/drm/i915/intel_dp.c
> > +++ b/drivers/gpu/drm/i915/intel_dp.c
> > @@ -278,6 +278,46 @@ static int intel_dp_common_rates(struct intel_dp *intel_dp,
> >  			       common_rates);
> >  }
> >  
> > +static int intel_dp_link_rate_index(struct intel_dp *intel_dp,
> > +				    int *common_rates, int link_rate)
> > +{
> > +	int common_len;
> > +	int index;
> > +
> > +	common_len = intel_dp_common_rates(intel_dp, common_rates);
> > +	for (index = 0; index < common_len; index++) {
> > +		if (link_rate == common_rates[common_len - index - 1])
> > +			return common_len - index - 1;
> 
> Probably somewhere in the history of the patch series there was a time
> when it was necessary to search for the rates in reverse order. What
> possible benefit could that offer at this point?
>

The advantage here is that the link rate is more likely to match quicker
if we search in reverse order.

 
> > +	}
> > +
> > +	return -1;
> > +}
> > +
> > +int intel_dp_get_link_train_fallback_values(struct intel_dp *intel_dp,
> > +					    int link_rate, uint8_t lane_count)
> > +{
> > +	int common_rates[DP_MAX_SUPPORTED_RATES] = {};
> 
> No need to initialize because you initialize it a couple of lines later.
>

Agreed
 
> > +	int common_len;
> > +	int link_rate_index = -1;
> 
> No need to initialize because you initialize it a couple of lines later.
> 
> > +
> > +	common_len = intel_dp_common_rates(intel_dp, common_rates);
> > +	link_rate_index = intel_dp_link_rate_index(intel_dp,
> > +						   common_rates,
> > +						   link_rate);
> 
> Please stop and think, and don't rush each new iteration of the patches.
> 
> What's wrong with the above lines? Please think about it. Answer at the
> end of the mail (*).
>

> > +	if (link_rate_index > 0) {
> > +		intel_dp->max_sink_link_bw = drm_dp_link_rate_to_bw_code(common_rates[link_rate_index - 1]);
> > +		intel_dp->max_sink_lane_count = lane_count;
> > +	} else if (lane_count > 1) {
> > +		intel_dp->max_sink_link_bw = intel_dp_max_link_bw(intel_dp);
> > +		intel_dp->max_sink_lane_count = lane_count >> 1;
> > +	} else {
> > +		DRM_ERROR("Link Training Unsuccessful\n");
> > +		return -1;
> > +	}
> > +
> > +	return 0;
> > +}
> > +
> >  static enum drm_mode_status
> >  intel_dp_mode_valid(struct drm_connector *connector,
> >  		    struct drm_display_mode *mode)
> > diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
> > index b6526ad..47e3671 100644
> > --- a/drivers/gpu/drm/i915/intel_drv.h
> > +++ b/drivers/gpu/drm/i915/intel_drv.h
> > @@ -1400,6 +1400,8 @@ bool intel_dp_init_connector(struct intel_digital_port *intel_dig_port,
> >  void intel_dp_set_link_params(struct intel_dp *intel_dp,
> >  			      int link_rate, uint8_t lane_count,
> >  			      bool link_mst);
> > +int intel_dp_get_link_train_fallback_values(struct intel_dp *intel_dp,
> > +					    int link_rate, uint8_t lane_count);
> >  void intel_dp_start_link_train(struct intel_dp *intel_dp);
> >  void intel_dp_stop_link_train(struct intel_dp *intel_dp);
> >  void intel_dp_sink_dpms(struct intel_dp *intel_dp, int mode);
> 
> 
> (*) You do intel_dp_common_rates(intel_dp, common_rates) twice, for no
> reason at all.
>

Actually the first call was to obtain the common_len which was needed earlier but
we no longer need it because of the simplified fallback logic modifying the
max sink link rate directly.
So yes I will remove the first call to intel_dp_common_rate()
Good catch! Thanks Jani.

Regards
Manasi 
> -- 
> Jani Nikula, Intel Open Source Technology Center