Re: [PATCH v2 1/2] media: qcom: camss: Remove use_count guard in stop_streaming

[Johan Hovold <johan@kernel.org>]

On Tue, Jul 16, 2024 at 11:13:24PM +0100, Bryan O'Donoghue wrote:
> The use_count check was introduced so that multiple concurrent Raw Data
> Interfaces RDIs could be driven by different virtual channels VCs on the
> CSIPHY input driving the video pipeline.
> 
> This is an invalid use of use_count though as use_count pertains to the
> number of times a video entity has been opened by user-space not the number
> of active streams.
> 
> If use_count and stream-on count don't agree then stop_streaming() will
> break as is currently the case and has become apparent when using CAMSS
> with libcamera's released softisp 0.3.
> 
> The use of use_count like this is a bit hacky and right now breaks regular
> usage of CAMSS for a single stream case. As an example the "qcam"
> application in libcamera will fail with an -EBUSY result on stream stop and
> cannot then subsequently be restarted.

No, stopping qcam results in the splat below, and then it cannot be
started again and any attempts to do so fails with -EBUSY.

> The kernel log for this fault looks like this:
> 
> [ 1265.509831] WARNING: CPU: 5 PID: 919 at drivers/media/common/videobuf2/videobuf2-core.c:2183 __vb2_queue_cancel+0x230/0x2c8 [videobuf2_common]
> ...
> [ 1265.510630] Call trace:
> [ 1265.510636]  __vb2_queue_cancel+0x230/0x2c8 [videobuf2_common]
> [ 1265.510648]  vb2_core_streamoff+0x24/0xcc [videobuf2_common]
> [ 1265.510660]  vb2_ioctl_streamoff+0x5c/0xa8 [videobuf2_v4l2]
> [ 1265.510673]  v4l_streamoff+0x24/0x30 [videodev]
> [ 1265.510707]  __video_do_ioctl+0x190/0x3f4 [videodev]
> [ 1265.510732]  video_usercopy+0x304/0x8c4 [videodev]
> [ 1265.510757]  video_ioctl2+0x18/0x34 [videodev]
> [ 1265.510782]  v4l2_ioctl+0x40/0x60 [videodev]
> ...
> [ 1265.510944] videobuf2_common: driver bug: stop_streaming operation is leaving buffer 0 in active state
> [ 1265.511175] videobuf2_common: driver bug: stop_streaming operation is leaving buffer 1 in active state
> [ 1265.511398] videobuf2_common: driver bug: stop_streaming operation is leaving buffer 2 in active st

Johan