[libcamera-devel] Concurrent camera usage in libcamera

[Naushir Patuck]

Hi all,

I wonder if I could get some thoughts regarding a problem we are seeing
with running multiple cameras in separate libcamera processes on Raspberry
Pi (this probably affects any/all platforms though).

In my scenario, I have libcamera process 1 running "cam -c 1 -C" and
streaming from imx477 using the Raspberry Pi pipeline handler.  In process
2, I have "cam -c 2 -C" running streaming from a USB camera using the
uvcvideo pipeline handler.  If I exit process 2, I get the following error
messages:

[0:01:33.155169911] [1392] ERROR V4L2 v4l2_videodevice.cpp:1241
/dev/video2[16:cap]: Unable to request 0 buffers: Device or resource busy
[0:01:33.155359910] [1392] ERROR V4L2 v4l2_videodevice.cpp:1241
/dev/video3[17:cap]: Unable to request 0 buffers: Device or resource busy
[0:01:33.155460595] [1392] ERROR V4L2 v4l2_videodevice.cpp:1241
/dev/video13[18:out]: Unable to request 0 buffers: Device or resource busy
[0:01:33.155564354] [1392] ERROR V4L2 v4l2_videodevice.cpp:1241
/dev/video14[19:cap]: Unable to request 0 buffers: Device or resource busy
[0:01:33.155664150] [1392] ERROR V4L2 v4l2_videodevice.cpp:1241
/dev/video15[20:cap]: Unable to request 0 buffers: Device or resource busy
[0:01:33.155763483] [1392] ERROR V4L2 v4l2_videodevice.cpp:1241
/dev/video16[21:cap]: Unable to request 0 buffers: Device or resource busy

These errors come up because the V4L2VideoDevice destructor in process 2 is
indirectly calling ioctl(REQBUFS, 0) on all the Unicam and ISP device nodes
while the device driver buffer queues are actively owned and used by
process 1. This does not actively interfere with process 1 running, it
continues happily enough.

The error here stems from the fact that the RPi pipeline handler
PipelineHandler::match() opens all the device nodes for every Camera object
registered through PipelineHandler::registerCamera().  So even if the
Camera was never used by a process, when terminating the V4L2VideoDevice
destructor will call ioctl(REQBUFS, 0) for every single device.  This
behavior is exactly the same for the ipu3 and rkisp1 pipeline handlers.

So the question is how to fix this and avoid the error messages?

1) The V4L2VideoDevice destructor should not call ioctl(REQBUFS, 0) if it
has not allocated buffers for the device:

diff --git a/src/libcamera/v4l2_videodevice.cpp
b/src/libcamera/v4l2_videodevice.cpp
index e30858c9fa02..108e60f035ab 100644
--- a/src/libcamera/v4l2_videodevice.cpp
+++ b/src/libcamera/v4l2_videodevice.cpp
@@ -758,7 +758,8 @@ void V4L2VideoDevice::close()
        if (!isOpen())
                return;

-       releaseBuffers();
+       if (cache_)
+               releaseBuffers();
        delete fdBufferNotifier_;

2) All pipeline handlers need to change and avoid calling device open() in
PipelineHandler::match(), and defer this call to when configure() gets
called.

Option 2 is not trivial, as (at least in the RPi pipeline handler) a bunch
of validation occurs in PipelineHandler::match() that needs the device
nodes to be open. In my opinion, option 1 seems to be the correct thing to
do.

Any other suggestions or thoughts?

Regards,
Naush
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