Re: RFC: poll behavior

[Mauro Carvalho Chehab <mchehab@redhat.com>]

Em 01-07-2011 06:45, Hans Verkuil escreveu:
> On Thursday, June 30, 2011 22:35:15 Mauro Carvalho Chehab wrote:

>>> This also leads to another ambiguity with poll(): what should poll do if 
>>> another filehandle started streaming? So fh1 called STREAMON (and so becomes 
>>> the 'owner' of the stream), and you poll on fh2. If a frame becomes available, 
>>> should fh2 wake up? Is fh2 allowed to call DQBUF?
>>
>> IMO, both fh's should get the same results. This is what happens if you're
>> writing into a file and two or more processes are selecting at the EOF.
> 
> Yes, but multiple filehandles are allowed to write/read from a file at the
> same time. That's not true for V4L2. Only one filehandle can do I/O at a time.

Actually, this is not quite true currently, as you could, for example use one fd
for QBUF, and another for DQBUF, with the current behavior, but, with luck,
no applications are doing weird things like that. Yet, tests are needed to avoid
breaking something, if we're willing to change it.

> I'm going to look into changing fs/select.c so that the poll driver function
> can actually see the event mask provided by the application.

Why? A POLLERR should be notified, whatever mask is there, as the application
may need to abort (for example, in cases like hardware removal).

Mauro.