linux-kernel - Re: [RFC 1/3] saa7115: Set saa7113 init to values from datasheet

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Message-ID: <20130531141420.GH2367@dell.arpanet.local>
Date:	Fri, 31 May 2013 16:14:20 +0200
From:	Jon Arne Jørgensen <jonarne@...arne.no>
To:	Mauro Carvalho Chehab <mchehab@...hat.com>
Cc:	Jon Arne Jørgensen <jonarne@...arne.no>,
	Timo Teras <timo.teras@....fi>,
	Andy Walls <awalls@...metrocast.net>,
	linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
	hans.verkuil@...co.com, prabhakar.csengg@...il.com,
	g.liakhovetski@....de, ezequiel.garcia@...e-electrons.com
Subject: Re: [RFC 1/3] saa7115: Set saa7113 init to values from datasheet

On Fri, May 31, 2013 at 10:08:27AM -0300, Mauro Carvalho Chehab wrote:
> Em Thu, 30 May 2013 21:00:01 +0200
> Jon Arne Jørgensen <jonarne@...arne.no> escreveu:
> 
> > On Thu, May 30, 2013 at 08:33:32AM +0300, Timo Teras wrote:
> > > On Thu, 30 May 2013 07:21:36 +0200
> > > Jon Arne Jørgensen <jonarne@...arne.no> wrote:
> > > 
> > > > On Wed, May 29, 2013 at 10:19:49PM -0400, Andy Walls wrote:
> > > > > Mauro Carvalho Chehab <mchehab@...hat.com> wrote:
> > > > > 
> > > > > >Em Wed, 29 May 2013 22:41:16 +0200
> > > > > >Jon Arne Jørgensen <jonarne@...arne.no> escreveu:
> > > > > >
> > > > > >> Change all default values in the initial setup table to match the
> > > > > >table
> > > > > >> in the datasheet.
> > > > > >
> > > > > >This is not a good idea, as it can produce undesired side effects
> > > > > >on the existing drivers that depend on it, and can't be easily
> > > > > >tested.
> > > > > >
> > > > > >Please, don't change the current "default". It is, of course, OK
> > > > > >to change them if needed via the information provided inside the
> > > > > >platform data.
> > > > >
> > > > > I was going to make a comment along the same line as Mauro.  
> > > > > Please leave the driver defaults alone.  It is almost impossible to
> > > > > regression test all the different devices with a SAA7113 chip, to
> > > > > ensure the change doesn't cause someone's device to not work
> > > > > properly.
> > > > >
> > > > 
> > > > You guys are totally right.
> > > > 
> > > > What if I clone the original saa7113_init table into a new one, and
> > > > make the driver use the new one if the calling driver sets
> > > > platform_data.
> > > > 
> > > > Something like this?
> > > > 
> > > >         switch (state->ident) {
> > > >         case V4L2_IDENT_SAA7111:
> > > >         case V4L2_IDENT_SAA7111A:
> > > >                 saa711x_writeregs(sd, saa7111_init);
> > > >                 break;
> > > >         case V4L2_IDENT_GM7113C:
> > > >         case V4L2_IDENT_SAA7113:
> > > > -		saa711x_writeregs(sd, saa7113_init);
> > > > +		if (client->dev.platform_data)
> > > > +			saa711x_writeregs(sd, saa7113_new_init);
> > > > +		else
> > > > +			saa711x_writeregs(sd, saa7113_init);
> > > 
> > > I would rather have the platform_data provide the new table. Or if you
> > > think bulk of the table will be the same for most users, then perhaps
> > > add there an enum saying which table to use - and name the tables
> > > according to the chip variant it applies to.
> > > 
> > 
> > I think the bulk of the table will be the same for all drivers.
> > It's one bit here and one bit there that needs changing.
> > As the driver didn't support platform data.
> > Changing to a new init table for the drivers that implement
> > platform_data shouldn't cause any regressions.
> 
> There are several things that are very bad on passing a table via
> platform data:
> 
> 	1) you're adding saa711x-specific data at the bridge driver,
> 	   so, the saa711x code is spread on several places at the
> 	   long term;
> 
> 	2) some part of the saa711x code may override the data there, 
> 	   as it is not aware about what bits should be preserved from
> 	   the new device;
> 
> 	3) due (2), latter changes on the code are more likely to
> 	   cause regressions;
> 
> 	4) also due to (2), some hacks can be needed, in order to warn
> 	   saa711x to handle some things differently.
> 
> That's why it is a way better to add meaningful parameters telling what
> bits are needed for the driver to work with the bridge. That's also
> why we do this with all other drivers.
> 

I agree with you here.
I never planned to pass the table via platform data.

I've just posted a new version of this patch set.
Please have a look.

Best regards
Jon Arne Jørgensen

> Regards,
> Mauro
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