Re: [PATCH] usbtv: fix dependency

[Guennadi Liakhovetski <g.liakhovetski@gmx.de>]

Hi Hans, Mauro

On Fri, 28 Jun 2013, Hans Verkuil wrote:

> On Fri June 28 2013 14:42:46 Mauro Carvalho Chehab wrote:
> > Em Fri, 28 Jun 2013 13:18:44 +0200
> > Hans Verkuil <hverkuil@xs4all.nl> escreveu:
> > 
> > > On Fri June 28 2013 13:00:43 Mauro Carvalho Chehab wrote:
> > > > Em Fri, 28 Jun 2013 10:24:15 +0200
> > > > Hans Verkuil <hverkuil@xs4all.nl> escreveu:
> > > > 
> > > > > This fixes a dependency problem as found by Randy Dunlap:
> > > > > 
> > > > > https://lkml.org/lkml/2013/6/27/501
> > > > > 
> > > > > Mauro, is there any reason for any V4L2 driver to depend on VIDEO_DEV instead of
> > > > > just VIDEO_V4L2?
> > > > > 
> > > > > Some drivers depend on VIDEO_DEV, some on VIDEO_V4L2, some on both. It's all
> > > > > pretty chaotic.
> > > > 
> > > > It should be noticed that, despite its name, this config is actually a
> > > > joint dependency of VIDEO_DEV and I2C that will compile drivers as module
> > > > if either I2C or VIDEO_DEV is a module:
> > > > 
> > > > 	config VIDEO_V4L2
> > > > 		tristate
> > > > 		depends on (I2C || I2C=n) && VIDEO_DEV
> > > > 		default (I2C || I2C=n) && VIDEO_DEV
> > > > 
> > > > So, a V4L2 device that doesn't have any I2C device doesn't need to depend
> > > > on VIDEO_V4L2. That includes, for example, reversed-engineered webcam
> > > > drivers where the sensor code is inside the driver and a few capture-only
> > > > device drivers.
> > > 
> > > Yes, it does have to depend on it. That's exactly why usbtv is failing: like
> > > any other v4l2 driver usbtv needs the videodev.ko module. That is dependent
> > > on VIDEO_V4L2. What is happening here is that the dependency of usbtv on
> > > VIDEO_DEV allows it to be built as part of the kernel, but VIDEO_V4L2 is built
> > > as a module due to its I2C dependency with the result that usbtv can't link to
> > > the videodev functions.
> > > 
> > > The way things are today I do not believe any v4l2 driver should depend on
> > > VIDEO_DEV, instead they should all depend on VIDEO_V4L2. That would make a
> > > lot more sense.
> > 
> > Hmm...
> > 
> > $ git grep -l i2c drivers/media/v4l2-core/
> > drivers/media/v4l2-core/tuner-core.c		(not part of videodev.ko module)
> > drivers/media/v4l2-core/v4l2-async.c
> > drivers/media/v4l2-core/v4l2-common.c
> > drivers/media/v4l2-core/v4l2-ctrls.c		(actually, there's just a comment there)
> > drivers/media/v4l2-core/v4l2-device.c
> > 
> > $ git grep  CONFIG_I2C drivers/media/v4l2-core/
> > drivers/media/v4l2-core/v4l2-common.c:#if IS_ENABLED(CONFIG_I2C)
> > drivers/media/v4l2-core/v4l2-common.c:#endif /* defined(CONFIG_I2C) */
> > drivers/media/v4l2-core/v4l2-device.c:#if IS_ENABLED(CONFIG_I2C)
> > 
> > yes, there are some parts of videodev that are dependent on I2C.
> > 
> > That's basically why all V4L2 drivers should depend on VIDEO_V4L2.
> > 
> > That's said, before the addition of v4l2-async, it was safe to compile
> > the core without I2C, as the parts of the code that are I2C specific are
> > protected by a:
> > 	#if defined(CONFIG_I2C)
> > 
> > With its addition, I suspect that we'll still have Kbuild issues, if I2C
> > is disabled and a driver that doesn't depends on I2C is compiled.
> > 
> > So, 2 patches seem to be needed:
> > 
> > 1) a patch that replaces all driver dependencies from CONFIG_DEV to
> >    CONFIG_V4L2;
> > 
> > 2) a patch that fixes the issues with v4l2-async.
> > 
> > With regards to the last one, I can see 3 ways to fix it:
> > 	1) don't add v4l2-async at videodev.ko if I2C is not selected;
> > 	2) protect the I2C specific parts of v4l2-async with
> > 		#if defined(CONFIG_I2C)
> > 	3) put v4l2-async on a separate module.
> > 
> > (or some combination of the above)
> > 
> > As only very few drivers use v4l2-async, as this is more focused to
> > fix troubles with OT, I think that the better would be to do (3) and
> > to add an specific Kconfig entry for it.
> 
> No, #2 is the right choice here. That's necessary anyway since it is a valid
> use-case that I2C is disabled and you use it for platform devices only.
> 
> Guennadi, can you look at this? The only thing that is probably needed is
> that match_i2c returns false if CONFIG_I2C is undefined.
> 
> I prefer to keep this part of videodev, at least for now: I think there will
> be a fairly quick uptake of this API internally, certainly for subdevs. Note
> BTW that even x86 kernels come with CONFIG_OF enabled these days.

This patch

http://git.linuxtv.org/gliakhovetski/v4l-dvb.git/commitdiff/a92d0222c693db29a5d00eaedcdebf748789c38e

has been pushed 3 days ago:

https://patchwork.linuxtv.org/patch/19090/

Thanks
Guennadi
---
Guennadi Liakhovetski, Ph.D.
Freelance Open-Source Software Developer
http://www.open-technology.de/