Re: [PATCH 2/9] drm/i915: Reinstate an early latency==0 check for skl+

[Matt Roper <matthew.d.roper@intel.com>]

On Fri, Dec 21, 2018 at 07:14:29PM +0200, Ville Syrjala wrote:
> From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> 
> I thought we could remove all the early latency==0 checks
> and rely on skl_wm_method{1,2}() checking for it. But
> skl_compute_plane_wm() applies a bunch of workarounds to bump
> up the latency before calling those guys so clearly it won't
> end up doing the right thing. Also not sure if the calculations
> based on the method1/2 results are safe agaisnt overflows so
> it might not work all that well in any case. Let's put the
> early check back.
> 
> Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

Should we remove the tests from skl_wm_method{1,2}() now?  I suppose
someone could still use the debugfs interface to set a latency value of
exactly (UINT32_MAX - workaround amount) to make latency wrap around and
hit 0, but I'm not sure if that's really any worse than if they shoot
themselves in the foot by setting a too-low non-zero latency.  I don't
think we divide by latency anywhere.

Either way,

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>


> ---
>  drivers/gpu/drm/i915/intel_pm.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
> index d132ef10fa60..0aac7e7b660f 100644
> --- a/drivers/gpu/drm/i915/intel_pm.c
> +++ b/drivers/gpu/drm/i915/intel_pm.c
> @@ -4701,6 +4701,9 @@ static void skl_compute_plane_wm(const struct intel_crtc_state *cstate,
>  		to_intel_atomic_state(cstate->base.state);
>  	bool apply_memory_bw_wa = skl_needs_memory_bw_wa(state);
>  
> +	if (latency == 0)
> +		return;
> +
>  	/* Display WA #1141: kbl,cfl */
>  	if ((IS_KABYLAKE(dev_priv) || IS_COFFEELAKE(dev_priv) ||
>  	    IS_CNL_REVID(dev_priv, CNL_REVID_A0, CNL_REVID_B0)) &&
> -- 
> 2.19.2
> 
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-- 
Matt Roper
Graphics Software Engineer
IoTG Platform Enabling & Development
Intel Corporation
(916) 356-2795
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