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

From: Javier Martinez Canillas <javierm@redhat.com>
To: Bing Bu Cao <bingbu.cao@linux.intel.com>, linux-kernel@vger.kernel.org
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>,
	Tian Shu Qiu <tian.shu.qiu@intel.com>,
	Jian Xu Zheng <jian.xu.zheng@intel.com>,
	Sakari Ailus <sakari.ailus@linux.intel.com>,
	Yong Zhi <yong.zhi@intel.com>, Bingbu Cao <bingbu.cao@intel.com>,
	linux-media@vger.kernel.org
Subject: Re: [PATCH] media: intel-ipu3: cio2: register the mdev on v4l2 async notifier complete
Date: Mon, 3 Sep 2018 10:52:04 +0200	[thread overview]
Message-ID: <44eb94a8-3712-155b-b3ab-35538f5b6b38@redhat.com> (raw)
In-Reply-To: <b15b236e-e0a7-8b2f-1e1f-196c9dc04f4d@linux.intel.com>

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