Re: [PATCH v3] drm/vgem: Attach sw fences to exported vGEM dma-buf (ioctl)

[Chris Wilson <chris@chris-wilson.co.uk>]

On Thu, Jul 14, 2016 at 10:59:04AM +0100, Chris Wilson wrote:
> On Thu, Jul 14, 2016 at 10:12:17AM +0200, Daniel Vetter wrote:
> > On Thu, Jul 14, 2016 at 08:04:19AM +0100, Chris Wilson wrote:
> > > vGEM buffers are useful for passing data between software clients and
> > > hardware renders. By allowing the user to create and attach fences to
> > > the exported vGEM buffers (on the dma-buf), the user can implement a
> > > deferred renderer and queue hardware operations like flipping and then
> > > signal the buffer readiness (i.e. this allows the user to schedule
> > > operations out-of-order, but have them complete in-order).
> > > 
> > > This also makes it much easier to write tightly controlled testcases for
> > > dma-buf fencing and signaling between hardware drivers.
> > > 
> > > v2: Don't pretend the fences exist in an ordered timeline, but allocate
> > > a separate fence-context for each fence so that the fences are
> > > unordered.
> > > v3: Make the debug output more interesting, and so the signaled status.
> > > 
> > > Testcase: igt/vgem_basic/dmabuf-fence
> > > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > > Cc: Sean Paul <seanpaul@chromium.org>
> > > Cc: Zach Reizner <zachr@google.com>
> > > Cc: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
> > > Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> > > Acked-by: Zach Reizner <zachr@google.com>
> > 
> > One thing I completely forgotten: This allows userspace to hang kernel
> > drivers. i915 (and other gpu drivers) can recover using hangcheck, but
> > dumber drivers (v4l, if that ever happens) probably never except such a
> > case. We've had a similar discusion with the userspace fences exposed in
> > sw_fence, and decided to move all those ioctl into debugfs. I think we
> > should do the same for this vgem-based debugging of implicit sync. Sorry
> > for realizing this this late.
> 
> One of the very tests I make is to ensure that we recover from such a
> hang. I don't see the difference between this any of the other ways
> userspace can shoot itself (and others) in the foot.

So one solution would be to make vgem fences automatically timeout (with
a flag for root to override for the sake of testing hang detection).
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre
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