linux-kernel - Re: [PATCH 3/3] i2c: nomadik: Add Device Tree support to the Nomadik I2C driver

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Date:	Mon, 3 Sep 2012 14:28:50 +0100
From:	Lee Jones <lee.jones@...aro.org>
To:	Linus Walleij <linus.walleij@...aro.org>
Cc:	Rob Herring <rob.herring@...xeda.com>,
	devicetree-discuss@...ts.ozlabs.org,
	Stephen Warren <swarren@...dia.com>,
	Wolfram Sang <w.sang@...gutronix.de>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	STEricsson_nomadik_linux@...t.st.com, linus.walleij@...ricsson.com,
	arnd@...db.de, linux-i2c@...r.kernel.org
Subject: Re: [PATCH 3/3] i2c: nomadik: Add Device Tree support to the Nomadik
 I2C driver

On Mon, Sep 03, 2012 at 03:19:13PM +0200, Linus Walleij wrote:
> On Mon, Sep 3, 2012 at 2:34 PM, Lee Jones <lee.jones@...aro.org> wrote:
> 
> > When booting DT booting take a different path and no platform data
> > is passed. We can't boot DT AND register devices with platform data
> > or else we will double probe every device. The only way to pass
> > pdata when booting with DT is with AUX_DATA() and that's a hack to
> > get around things we don't have support for yet. Up until now that
> > has been DMA bindings, clock and pinctrl names and call-backs.
> 
> So if we pass some augmented platform data using AUX_DATA()
> that appears as pdata in this case, and gets discarded.
> 
> Thus we cannot use AUX_DATA() to override a broken, as in
> "the interrupt number is wrong" device tree.

No, you cannot to that. You'd have to fix it in DT (which is easier).

> > If DT is corrupt or missing the kernel will boot using platform
> > data, but np will always be NULL, so we don't have the problem you
> > were alluding to above.
> 
> That was not the problem I had in mind.
> 
> I had a valid, but incorrect device tree in mind. I.e the device
> is there, but with wrong base address, or wrong IRQ number.
> 
> If pdata takes precedence, we can use AUX_DATA() to
> override such errors from the platform, since drivers/of/platform.c
> helpfully pokes in the auxdata as the platform data.

Yes, that's what happens.

> I thought this was one of the reasons why auxdata exist
> at all.

I don't think so. I've been told that AUXDATA is just a hack. My
aim is to rid the boardfile of all AUXDATA entries.

> Or is the proper solution to runtime-patch the device tree
> per se in such cases? How is that actually done then?

No, you can't do that either. DT is only read once at boot-time.

The correct solution would be to fix the broken DT.

-- 
Lee Jones
Linaro ST-Ericsson Landing Team Lead
Linaro.org │ Open source software for ARM SoCs
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