Re: [RFC 00/19] Async sub-notifiers and how to use them

Hans Verkuil
2017-08-23T09:09:23Z

On 08/04/17 20:25, Sakari Ailus wrote:
> Niklas Söderlund wrote:
>> Hi Sakari,
>>
>> On 2017-07-20 19:14:01 +0300, Sakari Ailus wrote:
>>> Hi Hans,
>>>
>>> Thanks for the review.
>>>
>>> On Wed, Jul 19, 2017 at 01:42:55PM +0200, Hans Verkuil wrote:
>>>> On 18/07/17 21:03, Sakari Ailus wrote:
>>>>> Hi folks,
>>>>>
>>>>> This RFC patchset achieves a number of things which I've put to the same
>>>>> patchset for they need to be show together to demonstrate the use cases.
>>>>>
>>>>> I don't really intend this to compete with Niklas's patchset but much of
>>>>> the problem area addressed by the two is the same.
>>>>>
>>>>> Comments would be welcome.
>>>>>
>>>>> - Add AS3645A LED flash class driver.
>>>>>
>>>>> - Add async notifiers (by Niklas).
>>>>>
>>>>> - V4L2 sub-device node registration is moved to take place at the same time
>>>>> with the registration of the sub-device itself. With this change,
>>>>> sub-device node registration behaviour is aligned with video node
>>>>> registration.
>>>>>
>>>>> - The former is made possible by moving the bound() callback after
>>>>> sub-device registration.
>>>>>
>>>>> - As all the device node registration and link creation is done as the
>>>>> respective devices are probed, there is no longer dependency to the
>>>>> notifier complete callback which as itself is seen problematic. The
>>>>> complete callback still exists but there's no need to use it, pending
>>>>> changes in individual drivers.
>>>>>
>>>>> See:
>>>>> <URL:http://www.spinics.net/lists/linux-media/msg118323.html>
>>>>>
>>>>> As a result, if a part of the media device fails to initialise because it
>>>>> is e.g. physically broken, it will be possible to use what works.
>>>>
>>>> I've got major problems with this from a userspace point of view. In the vast
>>>> majority of cases you just want to bail out if one or more subdevs fail.
>>>
>>> I admit it's easier for the user space if the device becomes available only
>>> when all its component drivers have registered.
>>>
>>> Also remember that video nodes are registered in the file system right on
>>> device probe time. It's only sub-device and media device node registration
>>> that has taken place in the notifier's complete handler.
>>
>> Is this always the case? In the R-Car VIN driver I register the video
>> devices using video_register_device() in the complete handler. Am I
>> doing things wrong in that driver? I had a patch where I moved the
>> video_register_device() call to probe time but it got shoot down in
>> review and was dropped.
>
> I don't think the current implementation is wrong, it's just different
> from other drivers; there's really no requirement regarding this AFAIU.
> It's one of the things where no attention has been paid I presume.

It actually is a requirement: when a device node appears applications can
reasonably expect to have a fully functioning device. True for any device
node. You don't want to have to wait until some unspecified time before
the full functionality is there.

I try to pay attention to this when reviewing code, since not following this
rule basically introduces a race condition which is hard to test.

> However doing anything that can fail earlier on would be nicer since
> there's no reasonable way to signal an error from complete callback either.

Right.

Adding support for cases where devices may not be present is very desirable,
but this should go through an RFC process first to hammer out all the details.

Today we do not support this and we have to review code with that in mind.

So the first async subnotifiers implementation should NOT support this (although
it can of course be designed with this in mind). Once it is in we can start
on an RFC on how to support partial pipelines. I have a lot of questions about
that that need to be answered first.

One thing at a time. Trying to do everything at once never works.

Regards,

Hans