linux-kernel - Re: [PATCH] media: intel-ipu3: cio2: register the mdev on v4l2 async notifier complete

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Message-ID: <44eb94a8-3712-155b-b3ab-35538f5b6b38@redhat.com>
Date:   Mon, 3 Sep 2018 10:52:04 +0200
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     Bing Bu Cao <bingbu.cao@...ux.intel.com>,
        linux-kernel@...r.kernel.org
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        Tian Shu Qiu <tian.shu.qiu@...el.com>,
        Jian Xu Zheng <jian.xu.zheng@...el.com>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Yong Zhi <yong.zhi@...el.com>,
        Bingbu Cao <bingbu.cao@...el.com>, linux-media@...r.kernel.org
Subject: Re: [PATCH] media: intel-ipu3: cio2: register the mdev on v4l2 async
 notifier complete

On 09/03/2018 10:49 AM, Bing Bu Cao wrote:
> 
> 
> On 09/03/2018 03:35 PM, Javier Martinez Canillas wrote:
>> Hi,
>>
>> Thanks a lot your feedback.
>>
>> On 09/03/2018 09:25 AM, Bing Bu Cao wrote:
>>>
>>> On 08/31/2018 11:20 PM, Javier Martinez Canillas wrote:
>>>> Commit 9832e155f1ed ("[media] media-device: split media initialization and
>>>> registration") split the media_device_register() function in two, to avoid
>>>> a race condition that can happen when the media device node is accessed by
>>>> userpace before the pending subdevices have been asynchronously registered.
>>>>
>>>> But the ipu3-cio2 driver calls the media_device_register() function right
>>>> after calling media_device_init() which defeats the purpose of having two
>>>> separate functions.
>>>>
>>>> In that case, userspace could have a partial view of the media device if
>>>> it opened the media device node before all the pending devices have been
>>>> bound. So instead, only register the media device once all pending v4l2
>>>> subdevices have been registered.
>>> Javier, Thanks for your patch.
>>> IMHO, there are no big differences for registering the cio2 before and after all the subdevices are ready.
>>> User may see a partial view of media graph but it presents what it really is then.
>>> It indicate that device is not available currently not it is not there.
>> I disagree that there are no differences. The media graph shouldn't be exposed
>> until its complete. That's the reason why we have a v4l2 async notifier .bound
>> and .complete callbacks (otherwise the .bound would be enough).
>>
>> It's also the reason why media register was split in _init and _register, as I
>> mentioned in the commit message.
> I revisit the commit 9832e155f1ed and understand and agree that.

Great. Does this mean the patch has your Reviewed-by / Acked-by ?

> Seems like there are some other drivers (such as rcar-vin and omap4iss) still have similar issues.

Yes it seems so. I don't have access to hardware with these IP blocks though,
I'm hesitant to attempt to fix them.

Best regards,
-- 
Javier Martinez Canillas
Software Engineer - Desktop Hardware Enablement
Red Hat