[RFC 0/2] Match i2c_device_id when using DT ids through ACPI

[Crestez Dan Leonard <leonard.crestez@intel.com>]

Linux supports instantiating devices using devicetree ids from ACPI by setting
the id to PRP0001 and adding the devicetree compatible string in _DSD
properties. This is described in Documentation/acpi/enumeration.txt.

I've tried to use this feature using custom ACPI tables and one issue I
encountered is that a lot of i2c device drivers are written for DT only and
expect a valid i2c_device_id *id parameter to their probe function but this is
always NULL for the PRP0001 case. Others call of_match_device in order to
determine their exact model number and that will also fail.

Drivers normally do this in order to differentiate between minor model numbers
supported by the same driver, for example hmc5883 versus hmc5983.

For ACPI it is common to add calls to acpi_match_device in order to do such
differentiation. It might make sense to call some form of acpi_of_match_device
except no such function is currently exported. I think it makes a lot of sense
to refactor the current acpi_of_match_device to return the of_device_id and
export it.

This is in PATCH 1. After exposing this function it could be called from each
driver that needs to support multiple models through PRP0001. I guess the
alternative to exposing acpi_of_match_device would be for driver code to poke
at acpi_device->data.of_compatible?

Going further it might make sense to attempt to fetch the i2c_device_id when
instantiating through PRP0001. This already happens automatically for
devicetree and it makes a lot of sense it would work through ACPI with DT ids.
Patch 2 hacks an additional search in i2c register code. This makes some
drivers "just work" without manually handling the "ACPI with DT ids" case.

For P1 it might make sense to change the parameters around so that
acpi_of_match_device will have the same signature as of_match_device.

The purpose of these changes would be to make more drivers easier to
instantiate through ACPI firmware. It's possible that I misunderstood and this
problem already has a different solution.

Crestez Dan Leonard (2):
  acpi: Expose acpi_of_match_device
  i2c: Pass i2c_device_id to probe func when using DT ids through ACPI

 drivers/acpi/bus.c     | 13 +++++++------
 drivers/i2c/i2c-core.c | 26 +++++++++++++++++++++++---
 include/linux/acpi.h   |  8 ++++++++
 3 files changed, 38 insertions(+), 9 deletions(-)

-- 
2.5.5