[Intel-gfx] [RFC][PATCH 3/3] drm/i915: Make RPS enable immediate

[Daniel Vetter]

On Mon, May 23, 2016 at 05:09:43PM +0300, ville.syrjala at linux.intel.com wrote:
> From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> 
> On SNB (at least) it's dangeruopus to hang the GPU with an infinite
> batch buffer loop while RPS is disabled. The only thing that keeps
> the system going in such circumstances are the internal RPS timers,
> so we should never feed the GPU with RPS disabled unless we want to
> risk a total system hang.
> 
> Previously we didn't fully disable RPS, but that changes in
> commit 2030d684f7b3 ("drm/i915/bxt: Explicitly clear the Turbo control register")
> so we probably didn't see the problem so often previously. But
> even before that we were at the mercy of the BIOS for the initial
> RPS state, so if the BIOS didn't enable RPS a GPU hang immediately
> upon boot could have been fatal.
> 
> To avoid the problems let's just make the RPS enable immediate.
> This renders the delayed_resume_work useless actually. We could perhaps
> just move the ring freq table initialization to the work and do the
> other stull synchronously?
> 
> Fixes: 2030d684f7b3 ("drm/i915/bxt: Explicitly clear the Turbo control register")
> Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
> ---
> This is more and RFC at this point. Perhaps we might want to keep the delayed
> work but just for the ring freq table update (which is the main reson this whole
> thing exists in the first place). Another factor is that wait_for() is not
> exactly optiomal currently, so it makes the operation slower than it needs to
> be. Would need some hard numbers to see if it's worth keeping the delayed work
> or not I suppose.

Loading the ring freq tables takes forever, so definitely want to keep
that. I'd just split rps setup in two parts and push the schedule_work
down a bit. Long term we can go overboard with async (and maybey only
stall for all the GT setup in the very first batchbuffer).
-Daniel

> 
>  drivers/gpu/drm/i915/intel_pm.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
> index 817c84c22782..e65c5c2b9b4e 100644
> --- a/drivers/gpu/drm/i915/intel_pm.c
> +++ b/drivers/gpu/drm/i915/intel_pm.c
> @@ -6505,6 +6505,7 @@ void intel_enable_gt_powersave(struct drm_i915_private *dev_priv)
>  		if (schedule_delayed_work(&dev_priv->rps.delayed_resume_work,
>  					   round_jiffies_up_relative(HZ)))
>  			intel_runtime_pm_get_noresume(dev_priv);
> +		flush_delayed_work(&dev_priv->rps.delayed_resume_work);
>  	}
>  }
>  
> -- 
> 2.7.4
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch