Re: [PATCH 08/10 v2] v4l2-subdev: add a v4l2_i2c_subdev_board() function

From: Eduardo Valentin <eduardo.valentin@nokia.com>
To: ext Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Cc: Hans Verkuil <hverkuil@xs4all.nl>,
	Linux Media Mailing List <linux-media@vger.kernel.org>,
	Magnus Damm <magnus.damm@gmail.com>,
	Robert Jarzmik <robert.jarzmik@free.fr>,
	Darius Augulis <augulis.darius@gmail.com>,
	Paul Mundt <lethal@linux-sh.org>
Subject: Re: [PATCH 08/10 v2] v4l2-subdev: add a v4l2_i2c_subdev_board() function
Date: Fri, 22 May 2009 11:58:27 +0300	[thread overview]
Message-ID: <20090522085827.GA1964@esdhcp037198.research.nokia.com> (raw)
In-Reply-To: <Pine.LNX.4.64.0905211728420.6271@axis700.grange>

Hi Hans and Guennadi,

On Thu, May 21, 2009 at 05:33:48PM +0200, ext Guennadi Liakhovetski wrote:
> Hi Hans,
> 
> On Thu, 21 May 2009, Hans Verkuil wrote:
> 
> > On Friday 15 May 2009 19:20:10 Guennadi Liakhovetski wrote:
> > > Introduce a function similar to v4l2_i2c_new_subdev() but taking a
> > > pointer to a struct i2c_board_info as a parameter instead of a client
> > > type and an I2C address, and make v4l2_i2c_new_subdev() a wrapper around
> > > it.
> > >
> > > Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
> > > ---
> > >
> > > Hans, renamed as you requested and updated to a (more) current state.
> > 
> > NAK. Not because it is a bad idea, but because you need to patch against the 
> > version in the v4l-dvb repo. The version in the kernel is missing a lot of 
> > the compatibility code which we unfortunately need to keep.
> > 
> > Any function passing the board_info will be valid for kernels >= 2.6.26 
> > only.
> 
> Here's a quote from your earlier email.
> 
> On Tue, 21 Apr 2009, Hans Verkuil wrote:
> 
> > The board_info struct didn't appear until 2.6.22, so that's certainly a
> > cut-off point. Since the probe version of this call does not work on
> > kernels < 2.6.26 the autoprobing mechanism is still used for those older
> > kernels. I think it makes life much easier to require that everything that
> > uses board_info needs kernel 2.6.26 at the minimum. I don't think that is
> > an issue anyway for soc-camera. Unless there is a need to use soc-camera
> > from v4l-dvb with kernels <2.6.26?
> 
> So, will this my patch build and work with >= 2.6.22 or not? I really 
> would not like to consciously make code uglier now because of 
> compatibility with < 2.6.26 to make it better some time later again.

I've to agree with Guennadi, I believe newer code should not suffer because
of compatibility code, at least if it is possible. I also agree with you that
we must keep compatibility with older drivers.

What I propose it to have the mechanism of .s_config available only for
LINUX_VERSION_CODE < KERNEL_VERSION(2, 6, 26). Newer version can take advance
of the new i2c api features.

This is slightly different from what Hans proposed. The difference here
is that we do not force newer drivers to use a callback only because
of backward compatibility.

Well, this is what I think of this problem, you may have a different point
of view. What do you think?
> 
> Thanks
> Guennadi
> ---
> Guennadi Liakhovetski, Ph.D.
> Freelance Open-Source Software Developer
> http://www.open-technology.de/
> --
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-- 
Eduardo Valentin