[PATCH v2] [media] BZ#84401: Revert "[media] v4l: vb2: Don't return POLLERR during transient buffer underruns"

[Mauro Carvalho Chehab <mchehab@osg.samsung.com>]

This reverts commit 9241650d62f79a3da01f1d5e8ebd195083330b75.

The commit 9241650d62f7 was meant to solve a race issue that
affects Gstreamer version 0.10, when streaming for a long time.

It does that by returning POLERR if VB2 is not streaming.

However, it broke VBI userspace support on alevt and mtt (and maybe
other VBI apps), as they rely on the old behavior.

Due to that, we need to roll back and restore the previous behavior.

For more details, see:
	https://bugzilla.kernel.org/show_bug.cgi?id=84401

So, let's rollback the change for now, and work on some other
fix for it.

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Hans Verkuil <hans.verkuil@cisco.com>
Cc: Pawel Osciak <pawel@osciak.com>
Cc: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>


--

v2: Just changed the description.

This is a regression that broke a core feature with most (all) VBI
apps. We should hurry sending a fix for it.

So, let's just revert the patch while we're discussing/testing a
solution that would solve Laurent's usecase scenario without breaking
VBI.

PS.: this patch should, of course, be c/c to 3.16 too, but I think we'll
need to do a manual backport, as the check for q->error is likely newer
than 3.16.

diff --git a/drivers/media/v4l2-core/videobuf2-core.c b/drivers/media/v4l2-core/videobuf2-core.c
index 7e6aff673a5a..da2d0adcc992 100644
--- a/drivers/media/v4l2-core/videobuf2-core.c
+++ b/drivers/media/v4l2-core/videobuf2-core.c
@@ -2583,10 +2583,10 @@ unsigned int vb2_poll(struct vb2_queue *q, struct file *file, poll_table *wait)
 	}
 
 	/*
-	 * There is nothing to wait for if no buffer has been queued and the
-	 * queue isn't streaming, or if the error flag is set.
+	 * There is nothing to wait for if no buffer has been queued
+	 * or if the error flag is set.
 	 */
-	if ((list_empty(&q->queued_list) && !vb2_is_streaming(q)) || q->error)
+	if (list_empty(&q->queued_list) || q->error)
 		return res | POLLERR;
 
 	/*