Re: [PATCH 1/3] vb2: fix VBI/poll regression

[Hans Verkuil <hverkuil@xs4all.nl>]

On 09/20/2014 11:08 AM, James Harper wrote:
>>
>> The recent conversion of saa7134 to vb2 unconvered a poll() bug that
>> broke the teletext applications alevt and mtt. These applications
>> expect that calling poll() without having called VIDIOC_STREAMON will
>> cause poll() to return POLLERR. That did not happen in vb2.
>>
>> This patch fixes that behavior. It also fixes what should happen when
>> poll() is called when STREAMON is called but no buffers have been
>> queued. In that case poll() will also return POLLERR, but only for
>> capture queues since output queues will always return POLLOUT
>> anyway in that situation.
>>
>> This brings the vb2 behavior in line with the old videobuf behavior.
>>
> 
> What (mis)behaviour would this cause in userspace application?

If an app would rely on poll to return POLLERR to do the initial STREAMON
(seen in e.g. alevt) or to do the initial QBUF (I'm not aware of any apps
that do that, but they may exist), then that will currently fail with vb2
because poll() will just wait indefinitely in those cases.

Regards,

	Hans