Re: [RFC v2 4/4] v4l: events: Don't sleep in dequeue if none are subscribed

[Sakari Ailus <sakari.ailus@linux.intel.com>]

Hans Verkuil wrote:
...
>>>> +        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.
>
>> -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").

Thinking about this some more, -ENOENT is probably what we should 
return. *But* when there are no events to dequeue, we should instead 
return -EAGAIN (i.e. EWOULDBLOCK) which VIDIOC_DQBUF also uses.

However I'm not sure if anything depends on -ENOENT currently (probably 
not really) so changing this might require some consideration. No error 
codes are currently defined for VIDIOC_DQEVENT; was planning to fix that 
while we're at this.

-- 
Cheers,

Sakari Ailus
sakari.ailus@linux.intel.com