RFC: Working around dynamic device allocation in i2c. Interaction with other subystems.

[Jonathan Cameron <jic23@cam.ac.uk>]

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.

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