Hi,
This series is an attempt to scratch an old itch, it's problematic to
support unbind and then a second call to complete in v4l2-async. When
the second complete call happens a lot of things can go wrong.
When v4l2-async complete callback is called multiple video devices are
registered with video_register_device(). Then if a v4l2-async unbind
happens they are unregistered with video_unregister_device().
Their are multiple problems with this, specially for R-Car VIN.
1. Defending on which subdevice is unbound parts of the video pipeline
can still function. There are for example two CSI-2 receivers
connected to two different CSI-2 buses in the pipeline. If one of the
receivers are unbound the other can still function perfectly well.
But with the current setup everything goes away, this is bad for
operational safety.
2. The struct video_device contains a static struct device, which in
turn contains a static struct kref. When the kref is release by
calling video_unregister_device() and then later trying to
re-register the video device video_register_device() the kref life
time management kicks in and produces warnings in later kernels or
OOPS in older ones.
It has been discussed in the past at various conferences that it could
be OK to not video_unregister_device() if a v4l2-async unbind happens.
The argument against it was that user-space needed a way to check if a
pipeline was completely probed or not. And this used to be that the
video devices where only present if everything was available.
It was agreed in principle that if an alternate way for media controller
centric devices could be found to inform user-space of this fact could
be found it would be OK to not unregister video devices in case of an
unbind or even allow registering the video devices at probe time instead
of at v4l2-async complete time.
This series aims to provide such a mechanism using the media device
itself to (optionally) report if the media graph is complete or not.
Patch 1/5, 2/5 and 3/5 adds the plumbing inside the core to carry such
information from a driver to user-space. Patch 4/5 and 5/5 implements
the new media device callback to report graph status and stops
registering the video devices in case an v4l2-async unbind happens.
A complementary series to v4l2-utils is posted as [1] which demonstrates
the usage of this series from user-space.
1. [PATCH 0/2] v4l-utils: media-ctl: Print media graph completes if
available
Niklas Söderlund (5):
uapi/linux/media.h: add flag field to struct media_device_info
media-device: Add a graph_complete callback to struct media_device_ops
mc-device.c: If graph completes status is available report it to
user-space
rcar-vin: Report the completeness of the media graph
rcar-vin: Do not unregister video device when a subdevice is unbound
drivers/media/mc/mc-device.c | 4 ++++
drivers/media/platform/rcar-vin/rcar-core.c | 18 +++++++++++++-----
drivers/media/platform/rcar-vin/rcar-vin.h | 4 ++++
include/media/media-device.h | 3 +++
include/uapi/linux/media.h | 9 ++++++++-
5 files changed, 32 insertions(+), 6 deletions(-)
--
2.25.1
