Message-ID: <9e4733910801120800g5b13c84u8720442fc8551339@mail.gmail.com>
Date:	Sat, 12 Jan 2008 11:00:31 -0500
From:	"Jon Smirl" <jonsmirl@...il.com>
To:	"Jean Delvare" <khali@...ux-fr.org>
Cc:	i2c@...sensors.org, linuxppc-dev@...abs.org,
	linux-kernel@...r.kernel.org
Subject: Re: [i2c] [PATCH 0/5] Version 17, series to add device tree naming to i2c

On 1/12/08, Jean Delvare <khali@...ux-fr.org> wrote:
> On Fri, 11 Jan 2008 15:16:57 -0500, Jon Smirl wrote:
> > On 1/11/08, Jean Delvare <khali@...ux-fr.org> wrote:
> > > Secondly, it promotes OF device names as acceptable aliases. This I
> > > don't think I agree with. While I see some value in moving the OF name
> > > -> Linux name translation to the drivers themselves (even though I
> > > don't see this as a mandatory move either), this doesn't imply that OF
> > > names should be used as aliases. I don't like the idea that different
> > > architectures will name the same device differently in a visible way.
> > > This could easily break user-space code that makes assumptions on the
> > > device names (libsensors comes to mind.) So, I think that this part
> > > will need some more discussion.
> >
> > They're aliases.  On the x86 my e1000 Ethernet driver loads using this
> > alias name:
> > "pci:v00008086d00001010sv*sd*bc*sc*i*"
> > In fact, the e1000 driver has 63 alias names in addition to "e1000"
> >
> > But it's still the e1000 driver after it is loaded.
> > jonsmirl@...ra:/home/linux/drivers/net/e1000$ lsmod | grep e1000
> > e1000                 115968  0
> >
> > Loading a I2C driver with an OF alias name is not going to change the
> > module name after it is loaded. In fact, once the module is in memory
> > there's no way to tell what name was used to load it.
>
> Of course. That's not what I was worried about... what I was worried
> about is something your patch set doesn't do but I misread the code and
> I thought it was doing. I'll read it again before I make more comments
> on this.
>
> > OF device names are set by the Open Firmware committee. It is not
> > reasonable to force the Linux names back into Open Firmware since this
> > would force the other operating systems using Open Firmware to adopt
> > the Linux names.
>
> I never meant to force the Linux names into Open Firmware. It wouldn't
> make sense especially when the Linux names are invented by random
> contributors with no specific rules, and can even change over time.
>
> What I meant is that the translation from Open Firmware device name to
> Linux device name could happen in different ways. Making module aliases
> out of the is one possibility but this is not the only one.
>
> I am curious why the translation could not happen "offline". As I
> understand it, you're getting the device names from these .dts files.
> However you're not parsing them in the kernel directly, are you? I
> presume that you have some tool that converts these files into C code
> that the kernel can use? This conversion tool could translate the names.

Those dts files are for embedded devices that were specifically
developed for Linux. All of the PowerPC Macs in the world have a
device tree in ROM that was developed by Apple following the Open
Firmware standard. Same thing for Sun boxes, but I'm not working on
those.

The kernel has an existing mechanism for handling translations like
these, the alias scheme.


> > This issue hasn't been visible before since there was a global table
> > in the PowerPC code mapping all known Open Firmware names into linux
> > names. Keeping this as a global table doesn't scale. The mapping needs
> > to be done by each device individually.
>
> Looking at your patch set, I see only 11 entries in the table (in
> arch/powerpc/sysdev/fsl_soc.c) that patch #2 deletes. Are there more in
> other files? I'm asking because 11 entries hardly qualifies as "doesn't
> scale". I sure hope that you're not doing all this for the sole purpose
> of getting rid of this 11-element table.

Currently developers add entries to the table in their private builds
for the i2c devices they are using. Another way to avoid adding a
table entry is to create a platform device in the platform code. But
this support is being extended to audio codecs too. There are hundreds
of audio codecs.

The whole purpose of this code is to dynamically load the correct i2c
and audio drivers by reading the device tree instead of having static
i2s/codec devices for every possible platform combination.


-- 
Jon Smirl
jonsmirl@...il.com
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