Re: [PATCH 1/5] drm/i915: Increase maximum polling time to 50ms for forcewake request/clear ack

Chris Wilson
2015-09-22T09:15:39Z

On Tue, Sep 22, 2015 at 10:06:47AM +0100, Tvrtko Ursulin wrote:
>
> On 08/26/2015 10:26 AM, Daniel Vetter wrote:
> >On Sun, Aug 23, 2015 at 05:52:47PM +0530, Sagar Arun Kamble wrote:
> >>On BXT, We Observe timeout for forcewake request completion with 2ms polling period as given here:
> >>[drm:fw_domains_get] ERROR render: timed out waiting for forcewake ack request.
> >>Polling for 50ms is recommended to avoid these timeouts.
> >>
> >>Change-Id: Ie715b0069a3049606e9602bc5e97a6511890864d
> >>Signed-off-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
> >
> >Probably needs cc: stable plus a pile of bugzilla links all over ...
>
> If the required wait is typically that long, shouldn't we be worried
> about busy polling?

With an exponential backoff (i.e. busyspin for the first ~jiffie/1ms)
then usleep_range 1, 2, 4, 8,..?

And also keeping the fw wakelock for longer than a jiffie to
optimistically avoid these multi-jiffie stalls. Whilst grumbling.
-Chris

--
Chris Wilson, Intel Open Source Technology Centre