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

[Chris Wilson]

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 at chris-wilson.co.uk>
> > Cc: Sean Paul <seanpaul at chromium.org>
> > Cc: Zach Reizner <zachr at google.com>
> > Cc: Gustavo Padovan <gustavo.padovan at collabora.co.uk>
> > Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
> > Acked-by: Zach Reizner <zachr at 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.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre