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    SubjectRe: [PATCH, RFC] Earlier I2C initialization
    Jean Delvare wrote:
    > On Tue, 10 Jun 2008 09:27:34 +1200, Ryan Mallon wrote:
    >
    >> David Brownell wrote:
    >>
    >>> On Monday 09 June 2008, Ryan Mallon wrote:
    >>>
    >>>
    >>>>> Talk to i2c and framebuffer people about changing the link order.
    >>>>>
    >>>>> i2c should really be initialised before framebuffer devices because
    >>>>> framebuffer devices tend to want to read DDC from monitors, which is
    >>>>> basically a I2C EEPROM in the monitor.
    >>>>>
    >
    > This is already the case. i2c-core is initialized with
    > subsys_initcall(), so it's available to all drivers initialized with
    > module_init().
    >
    >
    >>>>> ... but there's probably some reason why it's done the way it is today,
    >>>>> and changing it could well cause stuff to break.
    >>>>>
    >>>>>
    >>>> We have made i2c the first driver subsystem to come up in our 2.6.20
    >>>> kernel since we use i2c io expanders for power domain control. All we
    >>>> did was change drivers/Makefile so that obj-$(CONFIG_I2C) += i2c/ is at
    >>>> the very top of the file. We didn't have any problems with doing this.
    >>>> YMMV of course.
    >>>>
    >
    > Why don't you simply initialize the drivers in question with
    > subsys_initcall()? That's what i2c-pnx, i2c-omap, i2c-davinci and
    > tps65010 are doing at the moment.
    >
    >
    How does this work for embedded devices where the same architecture is
    used in many different configurations? For example, we have a PXA270
    setup where we need i2c early, but many other PXA setups do not, so
    making i2c-pxa subsys_initcall to support a single board is maybe the
    wrong way to go?

    >>> OMAP does much the same thing, for the same reason, and the I2C
    >>> adapter gets initialized earlier too (so power management chips
    >>> will be fully usable before driver_initcall code runs).
    >>>
    >>> Unless there's a downside on x86, I'd just suggest someone submit
    >>> a patch moving I2C init "early" so it merges in 2.6.27 ... cc to
    >>> LKML to scare out more potential problems, but I have a hard time
    >>> imagining there'd really be any.
    >>>
    >> Okay, heres the patch. Is untested though (other than our experience
    >> under 2.6.20), so it probably needs some people to test. I'm not
    >> subscribed to LKML, so can people CC me if necessary.
    >>
    >> Signed-off-by: Ryan Mallon <ryan@bluewatersys.com>
    >>
    >> diff --git a/drivers/Makefile b/drivers/Makefile
    >> index f65deda..9eaf236 100644
    >> --- a/drivers/Makefile
    >> +++ b/drivers/Makefile
    >> @@ -5,6 +5,7 @@
    >> # Rewritten to use lists instead of if-statements.
    >> #
    >>
    >> +obj-y += i2c/
    >> obj-$(CONFIG_HAVE_GPIO_LIB) += gpio/
    >>
    >
    > Some i2c bus drivers bit-bang GPIO pins...
    >
    >
    >> obj-$(CONFIG_PCI) += pci/
    >>
    >
    > ... and many are PCI devices, so will this work OK?
    >
    Probably not :-). I didn't have hardware to test, it was just easy
    to put together the patch. I figured a change like this would
    require extensive testing anyway, since it is bound to break
    some obscure setup at least.

    I still think that possibly a better solution is to allow the link
    order for the driver subsystems to be configured somehow. At least
    for the embedded space this is useful if a particular board has
    some dependency on i2c, spi or some other subsystem being available
    early on, then it can be configured on a per board basis, rather
    than per arch, or per driver.

    I'm not sure how to accomplish this though, I don't think Kconfig
    lends it self to this sort of thing very well, and I don't
    understand the kernel build process well enough to attempt it
    myself.

    ~Ryan



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