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

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Date:	Thu, 5 Jun 2014 16:55:09 +0100
From:	Lee Jones <lee.jones@...aro.org>
To:	Grant Likely <grant.likely@...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, 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.

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.

> Actually I would completely skip trying to get the data from ACPI.
> Since all of this is to continue supporting instantiating devices from
> sysfs, having a fallback to the OF table completely solves that use
> case.

> Anticipating an objection of: "what do we do with drivers that are
> ACPI only?"... That will be an incredibly rare case. If that ever does
> happen then we'll solve it by simply adding an of_device_id table.

I'm fine with leaving out ACPI support for now.

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