Re: [RFC] Support for events with a large payload

[Sylwester Nawrocki <sylvester.nawrocki@gmail.com>]

On 07/03/2013 09:34 PM, Laurent Pinchart wrote:
> On Wednesday 03 July 2013 02:01:59 Sakari Ailus wrote:
>> On Mon, Jun 24, 2013 at 03:40:14PM +0200, Hans Verkuil wrote:
>> ...
>>
>>> Since the payloads are larger I am less concerned about speed. There is
>>> one problem, though: if you dequeue the event and the buffer that should
>>> receive the payload is too small, then you have lost that payload. You
>>> can't allocate a new, larger, buffer and retry. So this approach can only
>>> work if you really know the maximum payload size.
>>>
>>> The advantage is also that you won't lose payloads.
>>
>> Forgot to answer this one --- I think it's fair to assume the user knows the
>> maximum size of the payload. What we also could do in such a case is to
>> return the error (e.g. ENOSPC) and put the required size to the large event
>> size field. But first someone must come up with a variable size event
>> without well defined maximum size for this to make much sense.
>
> And while we're discussing use cases, Hans, what are you current use cases for
> 64 bytes event payloads ?

One of the use cases could be face detection events. A face marker would
contain at least 4 rectangle data structures (face, left/right eye, 
mouth,...),
which is itself 64 bytes. Plus Euler angle information, confidence, 
smile/blink
level etc. We could add an object detection specific ioctl(s) (I'm not sure
if such won't be needed anyway), but the event API looks like a good
infrastructure to handle this kind of data.

--
Regards,
Sylwester