Re: [PATCH] V4L: Drop meaningless video_is_registered() call in v4l2_open()

[Hans Verkuil <hverkuil@xs4all.nl>]

On 09/09/2013 12:24 PM, Laurent Pinchart wrote:
> Hi Hans,
> 
> On Monday 09 September 2013 12:17:34 Hans Verkuil wrote:
>> On 09/09/2013 12:10 PM, Laurent Pinchart wrote:
>>> On Monday 09 September 2013 12:07:18 Hans Verkuil wrote:
>>>> On 09/09/2013 12:00 PM, Laurent Pinchart wrote:
>>>>> On Monday 09 September 2013 11:07:43 Hans Verkuil wrote:
>>>>>> On 09/06/2013 12:33 AM, Sylwester Nawrocki wrote:
>>>>> [snip]
>>>>>
>>>>>>> The main issue as I see it is that we need to track both driver
>>>>>>> remove() and struct device .release() calls and free resources only
>>>>>>> when last of them executes. Data structures which are referenced in
>>>>>>> fops must not be freed in remove() and we cannot use dev_get_drvdata()
>>>>>>> in fops, e.g. not protected with device_lock().
>>>>>>
>>>>>> You can do all that by returning 0 if probe() was partially successful
>>>>>> (i.e. one or more, but not all, nodes were created successfully) by
>>>>>> doing what I described above. I don't see another way that doesn't
>>>>>> introduce a race condition.
>>>>>
>>>>> But isn't this just plain wrong ? If probing fails, I don't see how
>>>>> returning success could be a good idea.
>>>>
>>>> Well, the nodes that are created are working fine. So it's partially OK
>>>> :-)
>>>>
>>>> That said, yes it would be better if it could safely clean up and return
>>>> an error. But it is better than returning an error and introducing a race
>>>> condition.
>>>>
>>>>>> That doesn't mean that there isn't one, it's just that I don't know of
>>>>>> a better way of doing this.
>>>>>
>>>>> We might need support from the device core.
>>>>
>>>> I do come back to my main question: has anyone actually experienced this
>>>> error in a realistic scenario? Other than in very low-memory situations I
>>>> cannot imagine this happening.
>>>
>>> What about running out of minors, which could very well happen with subdev
>>> nodes in complex SoCs ?
>>
>> Is that really realistic? What's the worst-case SoC we have in terms of
>> device nodes? Frankly, if this might happen then we should allow for more
>> minors or make the minor allocation completely dynamic.
> 
> For the 4 VSP1 instances on the R-Car H2, I need 33 video nodes and 65 (if I'm 
> not mistaken) subdev nodes. That doesn't include the camera interface.

So that leaves 158 free minors. That's a lot of webcams that can be attached :-)

> On a side note, this seems to indicate that the subdev API should probably 
> move to the /dev/media device node. That's something else to discuss.

I have 70 tty nodes in /dev. Just because there are a lot of them doesn't mean that
we have to merge them somehow. There may be other arguments for changing how we
handle them, but 'there are a lot of them' isn't a good argument, IMHO.

Regards,

	Hans

>> If you run into this situation then you have bigger problems than a
>> potential race condition.
>