Re: [PATCH] drm/i915: don't set unpin_work if vblank_get fails

From: Keith Packard <keithp@keithp.com>
To: Jesse Barnes <jbarnes@virtuousgeek.org>, intel-gfx@lists.freedesktop.org
Subject: Re: [PATCH] drm/i915: don't set unpin_work if vblank_get fails
Date: Mon, 22 Aug 2011 11:45:05 -0700	[thread overview]
Message-ID: <yunei0d5lqm.fsf@aiko.keithp.com> (raw)
In-Reply-To: <20110822110531.57d85c32@jbarnes-desktop>

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On Mon, 22 Aug 2011 11:05:31 -0700, Jesse Barnes <jbarnes@virtuousgeek.org> wrote:
> This fixes a race where we may try to finish a page flip and decrement
> the refcount even if our vblank_get failed and we ended up with a
> spurious flip pending interrupt.
> 
> Fixes https://bugs.freedesktop.org/show_bug.cgi?id=34211.
> 
> Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
> 
> diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
> index 2319f62..0910537 100644
> --- a/drivers/gpu/drm/i915/intel_display.c
> +++ b/drivers/gpu/drm/i915/intel_display.c
> @@ -6896,6 +6896,10 @@ static int intel_crtc_page_flip(struct drm_crtc *crtc,
>  	work->old_fb_obj = intel_fb->obj;
>  	INIT_WORK(&work->work, intel_unpin_work_fn);
>  
> +	ret = drm_vblank_get(dev, intel_crtc->pipe);
> +	if (ret)
> +		goto free_work;
> +
>  	/* We borrow the event spin lock for protecting unpin_work */
>  	spin_lock_irqsave(&dev->event_lock, flags);
>  	if (intel_crtc->unpin_work) {
> @@ -6906,6 +6910,11 @@ static int intel_crtc_page_flip(struct drm_crtc *crtc,
>  		return -EBUSY;

You'll need a drm_vblank_put above this return.

>  	}
>  	intel_crtc->unpin_work = work;
> +	/*
> +	 * Past this point, if we fail we'll let the flip completion code
> +	 * clean up the vblank refcount and pin work.  It'll be a spurious
> +	 * completion, but we handle that case.
> +	 */

I don't see how this is going to happen reliably; the hardware will have
to generate a suitable interrupt, which on IRL and later will have to be
an actual page flip interrupt.

-- 
keith.packard@intel.com

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