linux-kernel - Re: [PATCH v4 5/5] dma-buf/sync_file: only enable fence signalling on poll()

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Date:	Wed, 3 Aug 2016 12:43:28 +0100
From:	Chris Wilson <chris@...is-wilson.co.uk>
To:	Gustavo Padovan <gustavo@...ovan.org>
Cc:	dri-devel@...ts.freedesktop.org, marcheu@...gle.com,
	Daniel Stone <daniels@...labora.com>, seanpaul@...gle.com,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	linux-kernel@...r.kernel.org, laurent.pinchart@...asonboard.com,
	Gustavo Padovan <gustavo.padovan@...labora.co.uk>,
	John Harrison <John.C.Harrison@...el.com>, m.chehab@...sung.com
Subject: Re: [PATCH v4 5/5] dma-buf/sync_file: only enable fence signalling
 on poll()

On Tue, Jul 12, 2016 at 03:08:45PM -0300, Gustavo Padovan wrote:
> From: Gustavo Padovan <gustavo.padovan@...labora.co.uk>
> 
> Signalling doesn't need to be enabled at sync_file creation, it is only
> required if userspace waiting the fence to signal through poll().
> 
> Thus we delay fence_add_callback() until poll is called. It only adds the
> callback the first time poll() is called. This avoid re-adding the same
> callback multiple times.
> 
> v2: rebase and update to work with new fence support for sync_file
> 
> v3: use atomic operation to set enabled and protect fence_add_callback()

There's actually a spare bit in fence->flags you can use for this.

#define POLL_ENABLED FENCE_FLAG_USER_BITS

if (test_bit(POLL_ENABLED, &sync_file->fence->flags))
	fence_remove_callback(sync_file->fence, &sync_file->cb);

...

if (!test_and_set_bit(POLL_ENABLED, &sync_file->fence->flags)) {
	if (fence_add_callback(sync_file->fence, &sync_file->cb,
			       fence_check_cb_func) < 0)
		wake_up_all(&sync_file->wq);
}

Saves adding a raw atomic.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre