Re: RFC: Working around dynamic device allocation in i2c. Interactionwith other subystems.

From: Jonathan Cameron
Date: Thu Jan 22 2009 - 07:55:20 EST

Added spi mailing list (+ likely interested people) as the same approach
is used for device registration as with i2c.

> Dear All,
> Within board configuration files, i2c devices are currently allocated using
> i2c_board_info structures. The only element of these that retains it's
> memory address once the struct device elements are allocated (upon adapter
> initialization) is the platform data pointer.
> Several subsystems (regulator and clock for example) use an association
> method based upon a device specific string associated with a pointer to
> a device structure. Unfortunately as things currently stand there is no
> means of obtaining a suitable device for i2c devices at the point when
> it is required (in the board config).
> So the question is, how to overcome this problem?
> Options that I can come up with are:
> 1) Relax the constraints that the token used for device identification
> in subsystems using the regulators approach to a void * and use
> the platform data pointer of an i2c device. Note this requires
> every device which may need a regulator to have platform data.
> Whilst this would work, it is far from ideal.
As Mark Brown pointed out:
This would also remove the ability of the APIs using this to use the
struct device for other things like showing what they're doing in sysfs
or use dev_printk.
> 2) Allow more static allocation of struct i2c_client. The way of doing
> this with minimal disruption would be to add a pointer to i2c_board_info
> to a preallocated i2c_client structure and if this is supplied do not
> allocate another. A flag can then be used to indicated whether the
> structure has been statically allocated or not (thus preventing it
> being inadvertently freed.
> 3) Allow static allocation of i2c_client structures as a direct alternative
> to having any i2c_board_info structures at all. As the majority if not
> all of i2c_board_info's elements are simply copied into the i2c_client
> structure anyway, there is considerable overhead in option 2. Clearly
> it would not be simple or necessarily advisable to remove i2c_board_info
> structures so I would propose providing an alternative set of registration
> functions which would only be used when people cared about the problem
> we are addressing here.
> What do people think? In particular can anyone come up with any other /
> better way round this issue. (or am I missing something?)
> In particular, are there any similar cases already in kernel that would
> suggest a particular approach to solving this issue?
> I have an implementation of option 2 that works fine and is relatively simple.
> Thanks,
> ---
> Jonathan Cameron
> --
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