On 06/05/2018 03:10 PM, Dave Stevenson wrote: > Hi Tomasz. > > Thanks for formalising this. > I'm working on a stateful V4L2 codec driver on the Raspberry Pi and > was having to deduce various implementation details from other > drivers. I know how much we all tend to hate having to write > documentation, but it is useful to have. > > On 5 June 2018 at 11:33, Tomasz Figa <tfiga@xxxxxxxxxxxx> wrote: >> Due to complexity of the video decoding process, the V4L2 drivers of >> stateful decoder hardware require specific sequencies of V4L2 API calls >> to be followed. These include capability enumeration, initialization, >> decoding, seek, pause, dynamic resolution change, flush and end of >> stream. >> >> Specifics of the above have been discussed during Media Workshops at >> LinuxCon Europe 2012 in Barcelona and then later Embedded Linux >> Conference Europe 2014 in Düsseldorf. The de facto Codec API that >> originated at those events was later implemented by the drivers we already >> have merged in mainline, such as s5p-mfc or mtk-vcodec. >> >> The only thing missing was the real specification included as a part of >> Linux Media documentation. Fix it now and document the decoder part of >> the Codec API. >> >> Signed-off-by: Tomasz Figa <tfiga@xxxxxxxxxxxx> >> --- >> Documentation/media/uapi/v4l/dev-codec.rst | 771 +++++++++++++++++++++ >> Documentation/media/uapi/v4l/v4l2.rst | 14 +- >> 2 files changed, 784 insertions(+), 1 deletion(-) >> >> diff --git a/Documentation/media/uapi/v4l/dev-codec.rst b/Documentation/media/uapi/v4l/dev-codec.rst >> index c61e938bd8dc..0483b10c205e 100644 >> --- a/Documentation/media/uapi/v4l/dev-codec.rst >> +++ b/Documentation/media/uapi/v4l/dev-codec.rst >> @@ -34,3 +34,774 @@ the codec and reprogram it whenever another file handler gets access. >> This is different from the usual video node behavior where the video >> properties are global to the device (i.e. changing something through one >> file handle is visible through another file handle). > > I know this isn't part of the changes, but raises a question in > v4l2-compliance (so probably one for Hans). > testUnlimitedOpens tries opening the device 100 times. On a normal > device this isn't a significant overhead, but when you're allocating > resources on a per instance basis it quickly adds up. > Internally I have state that has a limit of 64 codec instances (either > encode or decode), so either I allocate at start_streaming and fail on > the 65th one, or I fail on open. I generally take the view that > failing early is a good thing. > Opinions? Is 100 instances of an M2M device really sensible? Resources should not be allocated by the driver until needed (i.e. the queue_setup op is a good place for that). It is perfectly legal to open a video node just to call QUERYCAP to see what it is, and I don't expect that to allocate any hardware resources. And if I want to open it 100 times, then that should just work. It is *always* wrong to limit the number of open arbitrarily. Regards, Hans
