On 10/02/13 16:45, Sakari Ailus wrote:
> Hi Hans,
>
> Hans Verkuil wrote:
>> On 10/02/13 16:18, Sakari Ailus wrote:
>>> Hi Hans,
>>>
>>> Thanks for the comments!
>>>
>>> Hans Verkuil wrote:
>>>> On 10/02/13 15:45, Sakari Ailus wrote:
>>>>> Dequeueing events was is entirely possible even if none are subscribed,
>>>>> leading to sleeping indefinitely. Fix this by returning -ENOENT when no
>>>>> events are subscribed.
>>>>>
>>>>> Signed-off-by: Sakari Ailus <sakari.ailus@xxxxxxxxxxxxxxx>
>>>>> ---
>>>>> drivers/media/v4l2-core/v4l2-event.c | 11 +++++++++--
>>>>> 1 file changed, 9 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/drivers/media/v4l2-core/v4l2-event.c
>>>>> b/drivers/media/v4l2-core/v4l2-event.c
>>>>> index b53897e..553a800 100644
>>>>> --- a/drivers/media/v4l2-core/v4l2-event.c
>>>>> +++ b/drivers/media/v4l2-core/v4l2-event.c
>>>>> @@ -77,10 +77,17 @@ int v4l2_event_dequeue(struct v4l2_fh *fh, struct
>>>>> v4l2_event *event,
>>>>> mutex_unlock(fh->vdev->lock);
>>>>>
>>>>> do {
>>>>> - ret = wait_event_interruptible(fh->wait,
>>>>> - fh->navailable != 0);
>>>>> + bool subscribed;
>>>>
>>>> Can you add an empty line here?
>>>
>>> Sure.
>>>
>>>>> + ret = wait_event_interruptible(
>>>>> + fh->wait,
>>>>> + fh->navailable != 0 ||
>>>>> + !(subscribed = v4l2_event_has_subscribed(fh)));
>>>>> if (ret < 0)
>>>>> break;
>>>>> + if (!subscribed) {
>>>>> + ret = -EIO;
>>>>
>>>> Shouldn't this be -ENOENT?
>>>
>>> If I use -ENOENT, having no events subscribed is indistinguishable
>>> form no events pending condition. Combine that with using select(2),
>>> and you can no longer distinguish having no events subscribed from
>>> the case where you got an event but someone else (another thread or
>>> process) dequeued it.
>>
>> OK, but then your commit message is out of sync with the actual patch since
>> the commit log says ENOENT.
>
> Right. The error code was the last thing I changed before sending the
> patch, and I ignored it was also present in the commit message. :-P
>
>>> -EIO makes that explicit --- this also mirrors the behaviour of
>>> VIDIOC_DQBUF. (And it must be documented as well, which is missing
>>> from the patch currently.)
>>
>> I don't like using EIO for this. EIO generally is returned if a hardware
>> error or an unexpected hardware condition occurs. How about -ENOMSG? Or
>> perhaps EPIPE? (As in: "the pipe containing events is gone").
>
> There is no pipe (or at least wasn't; it's a queue or rather is
> implemented as a fifo :)) so of the two I prefer -ENOMSG. What would
> you think of -ENODATA or -EPERM (which is used e.g. when writing
> read-only controls)?
>
I don't like ENODATA, mostly because it is so close in meaning to ENOENT.
EPERM would work for me. It's probably a bit better than ENOMSG.
Regards,
Hans
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