linux-kernel - Re: [PATCH 7/7] OF/ACPI/I2C: Add generic match function for the aforementioned systems

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Message-ID: <CACxGe6uV7G7WVd84XacA6LwFHaCm-2gUYzdTsjRgy3_UVx8Yxg@mail.gmail.com>
Date:	Thu, 5 Jun 2014 18:37:45 +0100
From:	Grant Likely <grant.likely@...aro.org>
To:	Lee Jones <lee.jones@...aro.org>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Wolfram Sang <wsa@...-dreams.de>,
	Linux I2C <linux-i2c@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Linus Walleij <linus.walleij@...aro.org>
Subject: Re: [PATCH 7/7] OF/ACPI/I2C: Add generic match function for the
 aforementioned systems

On Thu, Jun 5, 2014 at 4:55 PM, Lee Jones <lee.jones@...aro.org> wrote:
> On Thu, 05 Jun 2014, Grant Likely wrote:
>> On Thu, Jun 5, 2014 at 11:37 AM, Lee Jones <lee.jones@...aro.org> wrote:
>> > On Thu, 05 Jun 2014, Grant Likely wrote:
>> >
>> >> On Wed,  4 Jun 2014 13:09:56 +0100, Lee Jones <lee.jones@...aro.org> wrote:
>> >> > Currently this is a helper function for the I2C subsystem to aid the
>> >> > matching of non-standard compatible strings and devices which use DT
>> >> > and/or ACPI, but do not supply any nodes (see: [1] Method 4).  However,
>> >> > it has been made more generic as it can be used to only make one call
>> >> > for drivers which support any mixture of OF, ACPI and/or I2C matching.
>> >> >
>> >> > The initial aim is for of_match_device() to be replaced by this call
>> >> > in all I2C device drivers.
>> >> >
>> >> > [1] Documentation/i2c/instantiating-devices
>> >> >
>> >> > Signed-off-by: Lee Jones <lee.jones@...aro.org>
>> >>
>> >> I don't like this. It drops all type safety on the match entry
>> >> and the caller has no idea what it got back.
>> >
>> > Okay, so what's the best way forward?
>> >
>> > Introduce a i2c_of_match_device() call instead?
>>
>> I still think the way to do it is to emulate the missing i2c_device_id
>> when calling the drivers .probe() hook by having a temporary copy on
>> the stack and filling it with data from the OF or ACPI table....
>
> That's the opposite of what I'm trying to achieve.  I'm trying to get
> rid of unused i2c_device_id tables, rather than reinforce their
> mandatory existence.  I think an i2c_of_match_device() with knowledge
> of how to match via pure DT principles (of_node/compatible) and a
> fall-back, which is able to match on a provided of_device_id table
> alone i.e. without the requirement of an existing of_node.

What I'm suggesting does allow all those tables to be removed, but
without a painful API breakage... having said that, it is only the
drivers that drop their i2c_device_id tables that need to support the
fallback behaviour, so it would be fine to pass null into the
i2c_device_id argument and make the driver call the new function that
returns an of_device_id regardless of whether a node is attached.

> I've also been mulling over the idea of removing the second probe()
> parameter, as suggested by Wolfram.  However, this has quite deep
> ramifications which would require a great deal of driver adaptions.

That is of course the ideal. It would be a lot of work (I count 633
users), but it would get i2c exactly where you want it to be. I did
that kind of work when I merge platform_bus_type with
of_platform_bus_type.

You can mitigate it though by creating a .probe2 hook that doesn't
have the parameter and then change over all the users in separate
commits, finally removing the old hook when safe to do so.

g.
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