[Intel-gfx] [PATCH] drm/i915/dp: Improve clock recovery loop limit comment

[Marc Herbert]

Reviewed-by: Marc Herbert <marc.herbert at intel.com>

Thx Nathan, I think this helps. I'm still curious how training normally
converges much faster than the total number of possibilities but - unlike
this latest clarification below - I expect the spec(s) to document that.


On 24/07/2018 15:33, Nathan Ciobanu wrote:
> Clarifies the clock recovery loop limit comment that 80
> max_cr_tries for pre-DP1.4 devices was chosen as a very
> tolerant upper bound.
> Assumptions made:
> - DP1.4 syncs should be smarter so they won't need more
> than 10 tries
> - pre-DP1.4 syncs should be compliant enough to not need
> that many tries (80) but we should tolerate any that may
> trigger this corner case
> 
> Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan at intel.com>
> Cc: Rodrigo Vivi <rodrigo.vivi at intel.com>
> Cc: Marc Herbert <marc.herbert at intel.com>
> Suggested-by: Marc Herbert <marc.herbert at intel.com>
> Signed-off-by: Nathan Ciobanu <nathan.d.ciobanu at linux.intel.com>
> 
> diff --git a/drivers/gpu/drm/i915/intel_dp_link_training.c b/drivers/gpu/drm/i915/intel_dp_link_training.c
> index 07e128c7443c..a9f40985a621 100644
> --- a/drivers/gpu/drm/i915/intel_dp_link_training.c
> +++ b/drivers/gpu/drm/i915/intel_dp_link_training.c
> @@ -172,10 +172,12 @@ static bool intel_dp_link_max_vswing_reached(struct intel_dp *intel_dp)
>  	}
>  
>  	/*
> -	 * DP 1.4 spec clock recovery retries defined but
> -	 * for devices pre-DP 1.4 we set the retry limit
> -	 * to 4 (voltage levels) x 4 (preemphasis levels) x
> -	 * x 5 (same voltage retries) = 80 (max iterations)
> +	 * The DP 1.4 spec defines the max clock recovery retries value
> +	 * as 10 but for pre-DP 1.4 devices we set a very tolerant
> +	 * retry limit of 80 (4 voltage levels x 4 preemphasis levels x
> +	 * x 5 identical voltage retries). Since the previous specs didn't
> +	 * define a limit and created the possibility of an infinite loop
> +	 * we want to prevent any sync from triggering that corner case.
>  	 */
>  	if (intel_dp->dpcd[DP_DPCD_REV] >= DP_DPCD_REV_14)
>  		max_cr_tries = 10;
>