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

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

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.

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

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.

Yeah, kind of awkward. Multi-Function Device (MFD) drivers have this kind of problem a lot. A given I2C device might have a few dozen child devices -- including regulators and clock generators, RTCs, GPIOs, input devices, etc -- that can't be set up until the parent I2C device is probed.

It's solved easily enough by making sure the associations and their devices get set up after the core of the MFD comes up ... in the probe() routine. And by making sure the MFD and its subsystem initializes early enough that those other devices don't fail too many hidden assumptions.

This is a special case of a general problem: init sequencing. The initcall levels don't provide sufficient control, so you need to work around them.

Static device allocation is a bit problematic, and some folk really dislike it ... I'm not that averse to it, but since the cost of a device keeps growing I'd rather avoid that in most cases. A couple dozen bytes of board_info, especially if it's __initdata, versus several hundred bytes of "struct foo_device", multiplied by even just a dozen devices that won't always be present, adds up.

That leaves a dynamic solution for setting up linkage between various devices. In the past I've done that using callbacks to board-specific code that knows what linkages to establish.

You can see that with the GPIO controllers that use I2C or SPI
communications: they have setup callbacks that receive the device and GPIO information, which can continue to set the stage for the main event ... and teardown callbacks to break it all down after the show is done.

So for example those callbacks can be used to configure and then register the devices which rely on those GPIOs ... like LED banks, MMC/SD card sense switches, voltage regulator enables, and so on.

- Dave

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