On 1/24/19 8:11 AM, xinwu wrote:
> Hi Sakari,
>
> Thanks for your response.
>
>
> On 2019年01月22日 18:03, Sakari Ailus wrote:
>> Hi Xinwu,
>>
>> On Tue, Jan 22, 2019 at 04:34:44PM +0800, Liu, Xinwu wrote:
>>> device_register exposes the device to userspace.
>>>
>>> Therefore, while the register process is ongoing, the userspace program
>>> will fail to open the device (ENODEV will be set to errno currently).
>>> The program in userspace must re-open the device to cover this case.
>>>
>>> It is more reasonable to expose the device after everythings ready.
>>>
>>> Signed-off-by: Liu, Xinwu <xinwu.liu@xxxxxxxxx>
>>> ---
>>> drivers/media/v4l2-core/v4l2-dev.c | 14 ++++++++------
>>> 1 file changed, 8 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/drivers/media/v4l2-core/v4l2-dev.c b/drivers/media/v4l2-core/v4l2-dev.c
>>> index d7528f8..01e5cc9 100644
>>> --- a/drivers/media/v4l2-core/v4l2-dev.c
>>> +++ b/drivers/media/v4l2-core/v4l2-dev.c
>>> @@ -993,16 +993,11 @@ int __video_register_device(struct video_device *vdev,
>>> goto cleanup;
>>> }
>>> - /* Part 4: register the device with sysfs */
>>> + /* Part 4: Prepare to register the device */
>>> vdev->dev.class = &video_class;
>>> vdev->dev.devt = MKDEV(VIDEO_MAJOR, vdev->minor);
>>> vdev->dev.parent = vdev->dev_parent;
>>> dev_set_name(&vdev->dev, "%s%d", name_base, vdev->num);
>>> - ret = device_register(&vdev->dev);
>>> - if (ret < 0) {
>>> - pr_err("%s: device_register failed\n", __func__);
>>> - goto cleanup;
>>> - }
>>> /* Register the release callback that will be called when the last
>>> reference to the device goes away. */
>>> vdev->dev.release = v4l2_device_release;
>>> @@ -1020,6 +1015,13 @@ int __video_register_device(struct video_device *vdev,
>>> /* Part 6: Activate this minor. The char device can now be used. */
>>> set_bit(V4L2_FL_REGISTERED, &vdev->flags);
>>> + /* Part 7: Register the device with sysfs */
>>> + ret = device_register(&vdev->dev);
>>> + if (ret < 0) {
>>> + pr_err("%s: device_register failed\n", __func__);
>>> + goto cleanup;
>> The error handling needs to reflect the change as well.
>
> Yes, I think it need to clear the V4L2_FL_REGISTERED also.
>>
>> Speaking of which, the error handling appears to be somewhat broken here.
>> It should be fixed first before making changes to the registration order.
>
> I guess you mean to call "put_device()" in this error handling.
>>
>> The problem the patch addresses is related to another problem: how to tell
>> the user space all devices in the media hardware complex have been
>> registered successfully. Order generally has been the node first, and only
>> then the entity. That suggests the order should probably be:
>>
>> 1. video_register_media_controller
>> 2. set_bit(V4L2_FL_REGISTERED)
>> 3. device_register
>>
>> I wonder what Hans thinks.
Sorry for the late reply, I missed this question.
The core problem is that video_register_device sets up all the internal
data structures *and* creates the device node. This should really be split up
(as is done in other subsystems) into an allocate that does everything except
create the device node, and a register that actually does that last step.
The lack of this separation means that a device that has to register multiple
devices can't have a consistent valid state before it starts registering
devices.
Compare it to drivers/media/cec/cec-core.c where this is done a lot better.
That said, it would be a huge job reworking this. Although it can be done
by creating new functions separate from the existing ones. E.g.:
int v4l2_dev_initialize(struct video_device *vdev);
int v4l2_dev_register(struct video_device *vdev);
v4l2_dev_initialize does everything except registering the device node. That's
done by v4l2_dev_register. This way drivers can initialize everything first,
ensuring a fully consistent state before actually registering the device nodes.
I think this would be very nice to have.
That said, for the current function I agree that the order should indeed be
changed according to the proposal. It's just doesn't address the full problem.
Regards,
Hans
>
> Yes, that's my suggestion, thanks for the highlight. I also want to know if it's worth to make this change.
>>
>>> + }
>>> +
>>> return 0;
>>> cleanup:
>
