Message-ID: <4E637DEC.5080507@infradead.org>
Date:	Sun, 04 Sep 2011 10:32:28 -0300
From:	Mauro Carvalho Chehab <mchehab@...radead.org>
To:	Laurent Pinchart <laurent.pinchart@...asonboard.com>
CC:	"Hiremath, Vaibhav" <hvaibhav@...com>,
	"Ravi, Deepthy" <deepthy.ravi@...com>,
	"linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>
Subject: Re: [PATCHv2] ISP:BUILD:FIX: Move media_entity_init() and

Em 04-09-2011 06:01, Laurent Pinchart escreveu:
> Hi Mauro,
> 
> On Sunday 04 September 2011 00:21:38 Mauro Carvalho Chehab wrote:
>> Em 24-08-2011 10:25, Laurent Pinchart escreveu:
>>> On Wednesday 24 August 2011 14:19:01 Hiremath, Vaibhav wrote:
>>>> On Wednesday, August 24, 2011 5:00 PM Laurent Pinchart wrote:
>>>>> On Wednesday 24 August 2011 13:21:27 Ravi, Deepthy wrote:
>>>>>> On Wed, Aug 24, 2011 at 4:47 PM, Laurent Pinchart wrote:
>>>>>>> On Friday 19 August 2011 15:48:45 Deepthy Ravi wrote:
>>>>>>>> From: Vaibhav Hiremath <hvaibhav@...com>
>>>>>>>>
>>>>>>>> Fix the build break caused when CONFIG_MEDIA_CONTROLLER
>>>>>>>> option is disabled and if any sensor driver has to be used
>>>>>>>> between MC and non MC framework compatible devices.
>>>>>>>>
>>>>>>>> For example,if tvp514x video decoder driver migrated to
>>>>>>>> MC framework is being built without CONFIG_MEDIA_CONTROLLER
>>>>>>>> option enabled, the following error messages will result.
>>>>>>>> drivers/built-in.o: In function `tvp514x_remove':
>>>>>>>> drivers/media/video/tvp514x.c:1285: undefined reference to
>>>>>>>> `media_entity_cleanup'
>>>>>>>> drivers/built-in.o: In function `tvp514x_probe':
>>>>>>>> drivers/media/video/tvp514x.c:1237: undefined reference to
>>>>>>>> `media_entity_init'
>>>>>>>
>>>>>>> If the tvp514x is migrated to the MC framework, its Kconfig option
>>>>>>> should depend on MEDIA_CONTROLLER.
>>>>>>
>>>>>> The same TVP514x driver is being used for both MC and non MC
>>>>>> compatible devices, for example OMAP3 and AM35x. So if it is made
>>>>>> dependent on MEDIA CONTROLLER, we cannot enable the driver for MC
>>>>>> independent devices.
>>>>>
>>>>> Then you should use conditional compilation in the tvp514x driver
>>>>> itself. Or
>>>>
>>>> No. I am not in favor of conditional compilation in driver code.
>>>
>>> Actually, thinking some more about this, you should make the tvp514x
>>> driver depend on CONFIG_MEDIA_CONTROLLER unconditionally. This doesn't
>>> mean that the driver will become unusable by applications that are not
>>> MC-aware. Hosts/bridges don't have to export subdev nodes, they can just
>>> call subdev pad-level operations internally and let applications control
>>> the whole device through a single V4L2 video node.
>>>
>>>>> better, port the AM35x driver to the MC API.
>>>>
>>>> Why should we use MC if I have very simple device (like AM35x) which
>>>> only supports single path? I can very well use simple V4L2 sub-dev
>>>> based approach (master - slave), isn't it?
>>>
>>> The AM35x driver should use the in-kernel MC and V4L2 subdev APIs, but it
>>> doesn't have to expose them to userspace.
>>
>> I don't agree. If AM35x doesn't expose the MC API to userspace,
>> CONFIG_MEDIA_CONTROLLER should not be required at all.
>>
>> Also, according with the Linux best practices, when  #if tests for config
>> symbols are required, developers should put it into the header files, and
>> not inside the code, as it helps to improve code readability. From
>> Documentation/SubmittingPatches:
>>
>> 	2) #ifdefs are ugly
>>
>> 	Code cluttered with ifdefs is difficult to read and maintain.  Don't do
>> 	it.  Instead, put your ifdefs in a header, and conditionally define
>> 	'static inline' functions, or macros, which are used in the code.
>> 	Let the compiler optimize away the "no-op" case.
>>
>> So, this patch is perfectly fine on my eyes.
> 
> I'm sorry, but I don't agree.
> 
> Regarding the V4L2 subdev pad-level API, the goal is to convert all host and 
> subdev drivers to it, so that's definitely the way to go. This does *not* mean 
> that subdevs must expose a subdev device node. That's entirely optional. What 
> I'm talking about is switching from video::*_mbus_fmt operations to pad::*_fmt 
> operations. The pad-level format operations are very similar to video-level 
> format operations, and more generic. Drivers shouldn't implement both.

I agree that implementing two ways for doing the same thing is a bad idea, 
but especially since your idea is to convert all subdevs to it, this type 
of conversion should not require enabling CONFIG_MEDIA_CONTROLLER, as this
feature is used to enable the MC userspace API.

> Regarding the MC API, drivers are not required to register a media_device 
> instance. I have no issue with that. However, drivers should initialized the 
> subdev's embedded media_entity, as that's required by subdev pad-level 
> operations to get the number of pads for a subdev.

There are two solutions:

1) add some "fallback" method at the core to use the video::*_mbus_fmt way, when
MC is disabled;

2) split the config options into two: one configurable by the user to enable
the userspace MC API, and another, used internally that would select the MC
internal API when drivers need it.

As your plan is to convert all drivers to the new way, (2) doesn't make much
sense in long term, as, at the end, all drivers will be selecting it.

Also, I don't like the idea of increasing drivers complexity for the existing
drivers that work properly without MC. All those core conversions that were
done in the last two years caused already too much instability to them.

We should really avoid touching on them again for something that won't be
adding any new feature nor fixing any known bug.
> 
> This will result in no modification to the userspace.
> 

Regards,
Mauro
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