Re: [PATCH 3/3] drm/i915: Bump CHV PFI credits to 63 when cdclk>=czclk

From: Clint Taylor <clinton.a.taylor@intel.com>
To: ville.syrjala@linux.intel.com, intel-gfx@lists.freedesktop.org
Subject: Re: [PATCH 3/3] drm/i915: Bump CHV PFI credits to 63 when cdclk>=czclk
Date: Fri, 12 Jun 2015 09:24:24 -0700	[thread overview]
Message-ID: <557B07B8.4060107@intel.com> (raw)
In-Reply-To: <1432660960-8979-4-git-send-email-ville.syrjala@linux.intel.com>

On 05/26/2015 10:22 AM, ville.syrjala@linux.intel.com wrote:
> From: Ville Syrjälä <ville.syrjala@linux.intel.com>
>
> Switch from using 31 PFI credits to 63 PFI credits when cdclk>=czclk on
> CHV. The spec lists both 31 and 63 as "suggested" values, but based on
> feedback from hardware folks we should actually be using 63. Originally
> I picked the 31 basically by flipping a coin.
>
> Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> ---
>   drivers/gpu/drm/i915/intel_display.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
> index 067b1de..44b9c54 100644
> --- a/drivers/gpu/drm/i915/intel_display.c
> +++ b/drivers/gpu/drm/i915/intel_display.c
> @@ -5986,7 +5986,7 @@ static void vlv_program_pfi_credits(struct drm_i915_private *dev_priv)
>   	if (DIV_ROUND_CLOSEST(dev_priv->cdclk_freq, 1000) >= dev_priv->rps.cz_freq) {
>   		/* CHV suggested value is 31 or 63 */
>   		if (IS_CHERRYVIEW(dev_priv))
> -			credits = PFI_CREDIT_31;
> +			credits = PFI_CREDIT_63;
>   		else
>   			credits = PFI_CREDIT(15);
>   	} else {
>

Although not part of this review the else clause is setting PFI_CREDIT 
to 15 when the BPSEC states that the default of 8 should be used when 
cdclk/czclk < 1. According to the original patch, 15 is the optimal 
value as stated by another driver team.

Reviewed-by: Clint Taylor <Clinton.A.Taylor@intel.com>
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