[Intel-gfx] [PATCH] drm/i915/dp: Give up link training clock recovery after 10 failed attempts

[Manasi Navare]

On Fri, Jul 13, 2018 at 03:23:41PM -0700, Dhinakaran Pandiyan wrote:
> On Fri, 2018-07-13 at 14:22 -0700, Rodrigo Vivi wrote:
> > On Fri, Jul 13, 2018 at 10:32:15AM -0700, Nathan Ciobanu wrote:
> > > 
> > > Limit the link training clock recovery loop to 10 failed attempts
> > > at
> > > LANEx_CR_DONE per DP 1.4 spec.
> > Where exactly in the spec?

I see that this is specified in 3.5.1.2.2 Clock Recovery Sequence section
in DP 1.4 spec. Might be a good idea to mention this expliitly in the commit message.

> > 
> > > 
> > > Some USB-C MST hubs cause us to get
> > > stuck in this loop on hot-plugging indefinitely as

This is likely to occur in case of USB Type C cases where only fewer lanes
might be connected whereas the max lane count returned is still 4.
If this is the case, explicitly mentioning this is also a good idea.

Doesnt this get recovered at all after the fallback when it will eventually
try with the lower lane count?

> 
> Also include the information (the vswing toggling part) about why it is
> stuck in the loop. 
> 
> > > drm_dp_clock_recovery_ok() never returns true and none of the
> > > other conditions occur.
> > Although it seems really bad situation that we need to avoid...
> > 
> > > 
> > > 
> > > Signed-off-by: Nathan Ciobanu <nathan.d.ciobanu at linux.intel.com>
> > > ---
> > >  drivers/gpu/drm/i915/intel_dp_link_training.c | 8 +++++++-
> > >  1 file changed, 7 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/gpu/drm/i915/intel_dp_link_training.c
> > > b/drivers/gpu/drm/i915/intel_dp_link_training.c
> > > index 4da6e33c7fa1..66c1a70343ba 100644
> > > --- a/drivers/gpu/drm/i915/intel_dp_link_training.c
> > > +++ b/drivers/gpu/drm/i915/intel_dp_link_training.c
> > > @@ -129,7 +129,7 @@ static bool
> > > intel_dp_link_max_vswing_reached(struct intel_dp *intel_dp)
> > >  intel_dp_link_training_clock_recovery(struct intel_dp *intel_dp)
> > >  {
> > >  	uint8_t voltage;
> > > -	int voltage_tries, max_vswing_tries;
> > > +	int voltage_tries, max_vswing_tries, cr_tries;
> > >  	uint8_t link_config[2];
> > >  	uint8_t link_bw, rate_select;
> > >  
> > > @@ -172,6 +172,7 @@ static bool
> > > intel_dp_link_max_vswing_reached(struct intel_dp *intel_dp)
> > >  
> > >  	voltage_tries = 1;
> > >  	max_vswing_tries = 0;
> > > +	cr_tries = 0;
> > >  	for (;;) {
> > >  		uint8_t link_status[DP_LINK_STATUS_SIZE];
> > >  
> > > @@ -215,6 +216,11 @@ static bool
> > > intel_dp_link_max_vswing_reached(struct intel_dp *intel_dp)
> > >  		if (intel_dp_link_max_vswing_reached(intel_dp))
> > >  			++max_vswing_tries;
> > >  
> > > +		if (cr_tries == 9) {
> > > +			DRM_ERROR("Failed clock recovery 10 times,
> > > giving up!\n");
> > > +			return false;
> > > +		}
> > > +		++cr_tries;
> > If I understood correctly this is a global thing for the for(;;)
> > right?
> > 
> > Shouldn't we make then like a:
> > 
> > - for(;;)
> > + for(cr_tries = 0; cr_tries < 10; cr_tries++)
> > 	{
> > 	}

I think incrementing it at the end of the loop makes sense because we
are checking for the 10 total read requests for lane_cr_done
and adjust_request_lane.
But if the max voltage swing is reached before that it should exit earlier.

Manasi

> > 
> > + DRM_ERROR("Failed clock recovery 10 times, giving up!\n"); 
> > + return false;
> > }
> > 
> > Thanks,
> > Rodrigo.
> > 
> > > 
> > >  	}
> > >  }
> > >  
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