linux-kernel - Re: more intel drm issues (was Re: [git pull] drm intel only fixes)

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Message-ID: <AANLkTimsf+nCtZrO8f9E0b3=_Y2FikbAHioeZk-Vp=Lm@mail.gmail.com>
Date:	Thu, 20 Jan 2011 09:51:00 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Chris Wilson <chris@...is-wilson.co.uk>
Cc:	Jeff Chua <jeff.chua.linux@...il.com>,
	Len Brown <len.brown@...el.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Dave Airlie <airlied@...ux.ie>, linux-kernel@...r.kernel.org,
	DRI mailing list <dri-devel@...ts.freedesktop.org>
Subject: Re: more intel drm issues (was Re: [git pull] drm intel only fixes)

On Thu, Jan 20, 2011 at 9:38 AM, Chris Wilson <chris@...is-wilson.co.uk> wrote:
>
>> because from looking at the code, I get the notion that
>> "intel_read_status_page()" may not be exact. But what happens if that
>> inexact value matches our cached ring->actual_head, so we never even
>> try to read the exact case? Does it _stay_ inexact for arbitrarily
>> long times? If so, we might wait for the ring to empty forever (well,
>> until the timeout - the behavior I see), even though the ring really
>> _is_ empty. No?
>
> Ah. Your analysis is spot on and this will cause a hang whilst polling if
> we enter the loop with the last known head the same as the reported value.

So how about just doing this in the loop? It will mean that the
_first_ read uses the fast cached one (the common case, hopefully),
but then if we loop, we'll use the slow exact one.

(cut-and-paste, so whitespace isn't good):

  diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c
b/drivers/gpu/drm/i915/intel_ringbuffer.c
  index 03e3370..11bbfb5 100644
  --- a/drivers/gpu/drm/i915/intel_ringbuffer.c
  +++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
  @@ -961,6 +961,8 @@ int intel_wait_ring_buffer(struct
intel_ring_buffer *ring, int n)
                  msleep(1);
                  if (atomic_read(&dev_priv->mm.wedged))
                          return -EAGAIN;
  +               /* Force a re-read. FIXME: what if read_status_page
says 0 too */
  +               ring->actual_head = 0;
          } while (!time_after(jiffies, end));
          trace_i915_ring_wait_end (dev);
          return -EBUSY;

but to get rid of the FIXME you should probably get rid of
"actual_head" entirely, and just use a local variable as a simple
boolean flag instead.

Hmm?

           Linus
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