On Mon, 28 Dec 2020, Sergio M. Ammirata, Ph.D. wrote:
Hello Nicolas,
We already had a file descriptor signaling method on a
private branch. Here is the link:
https://code.videolan.org/rist/librist/-/commits/descriptor_method/
I assume this would work?
Is the idea to add our file handle to a larger array on a
master select loop inside ffmpeg?
Just to make it clear, this is only a plan for the replacement of
the current polling based interrupt logic, and it probably won't happen
anytime soon.
After the select is triggered, is it a requirement that the
data also be read with a standard "read" call on the file
descriptor or is calling an API acceptable? In the solution
above, we use an API to retrieve the data.
Sending the data within the file descriptor context would
require serialization of the rist_data_block structure and
having it deserialized by the calling application.
I'd say it is too early to say which method will fit ffmpeg's approach
best, on the other hand ffmpeg will have to support many protocols with
different polling capabilities, so I'd expect ffmpeg should be able to
adapt to both.
And maybe my comment made the impression that fd based polling is needed
for proper ffmpeg support, but as far as I see it that is not really the
case, at least not for the current ffmpeg code. We are just used to having
a poll() call and a read() call, because the read() call does not usually
provide timeout capability, so we always had to use poll() first. But
in librist the read() call already have a timeout parameter, so we can
simply use that instead of poll()+read().
Regards,
Marton
Regards,
Sergio
---------- Forwarded message ---------
Date: Mon, 28 Dec 2020, 12:24
Subject: Re: [FFmpeg-devel] [PATCH] libavformat: add
librist protocol
To: FFmpeg development discussions and patches <
ffmpeg-devel@ffmpeg.org>
Marton Balint (12020-12-25):
Actually it should be POLLING_TIME as defined in
libavformat/network.h for
blocking mode if you want behaviour to be consistent with
other protocols.
The function cannot block indefinitely even in blocking
mode to be able
allow generic code in
libavformat/avio.c:retry_transfer_wrapper to abort a
blocking wait.
You are right, I forgot about it. It should be 0 in non-
blocking mode
and POLLING_TIME in blocking mode. Definitely not 5.
Other protocols are directly using poll/select to wait
for data with
POLLING_TIME timeout, but since librist does not provide
a way to poll for
Note that it is far from a perfect solution. In particular,
it makes our
network protocols unsuitable for idle connections with
embedded
applications, because these timeouts prevent the device
from sleeping.
At some point, we will need to redesign our protocols API
around an
event loop. In fact, all our I/O and threading probably
need to be
merged into a single event loop and scheduler.
data, this is the best we can do (and return EAGAIN even
in blocking mode if
the POLLING_TIME elapses, because retry_transfer_wrapper
will retry in that
case anyway)
It looks to me that librist is not ready for prime time. We
could
integrate it as experimental, to help it getting there. But
I do not
know if the people involved would be receptive.
Regards,
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