Currently, when a child platform device (sometimes referred to as a
sub-device) is registered via the Multi-Functional Device (MFD) API,
the framework attempts to match the newly registered platform device
with its associated Device Tree (OF) node.  Until now, the device has
been allocated the first node found with an identical OF compatible
string.  Unfortunately, if there are, say for example '3' devices
which are to be handled by the same driver and therefore have the same
compatible string, each of them will be allocated a pointer to the
*first* node.

An example Device Tree entry might look like this:

  mfd_of_test {
          compatible = "mfd,of-test-parent";
          #address-cells = <0x02>;
          #size-cells = <0x02>;

          child@aaaaaaaaaaaaaaaa {
                  compatible = "mfd,of-test-child";
                  reg = <0xaaaaaaaa 0xaaaaaaaa 0 0x11>,
                        <0xbbbbbbbb 0xbbbbbbbb 0 0x22>;
          };

          child@cccccccc {
                  compatible = "mfd,of-test-child";
                  reg = <0x00000000 0xcccccccc 0 0x33>;
          };

          child@dddddddd00000000 {
                  compatible = "mfd,of-test-child";
                  reg = <0xdddddddd 0x00000000 0 0x44>;
          };
  };

When used with example sub-device registration like this:

  static const struct mfd_cell mfd_of_test_cell[] = {
        OF_MFD_CELL("mfd-of-test-child", NULL, NULL, 0, 0, "mfd,of-test-child"),
        OF_MFD_CELL("mfd-of-test-child", NULL, NULL, 0, 1, "mfd,of-test-child"),
        OF_MFD_CELL("mfd-of-test-child", NULL, NULL, 0, 2, "mfd,of-test-child")
  };

... the current implementation will result in all devices being allocated
the first OF node found containing a matching compatible string:

  [0.712511] mfd-of-test-child mfd-of-test-child.0: Probing platform device: 0
  [0.712710] mfd-of-test-child mfd-of-test-child.0: Using OF node: child@aaaaaaaaaaaaaaaa
  [0.713033] mfd-of-test-child mfd-of-test-child.1: Probing platform device: 1
  [0.713381] mfd-of-test-child mfd-of-test-child.1: Using OF node: child@aaaaaaaaaaaaaaaa
  [0.713691] mfd-of-test-child mfd-of-test-child.2: Probing platform device: 2
  [0.713889] mfd-of-test-child mfd-of-test-child.2: Using OF node: child@aaaaaaaaaaaaaaaa

After this patch each device will be allocated a unique OF node:

  [0.712511] mfd-of-test-child mfd-of-test-child.0: Probing platform device: 0
  [0.712710] mfd-of-test-child mfd-of-test-child.0: Using OF node: child@aaaaaaaaaaaaaaaa
  [0.713033] mfd-of-test-child mfd-of-test-child.1: Probing platform device: 1
  [0.713381] mfd-of-test-child mfd-of-test-child.1: Using OF node: child@cccccccc
  [0.713691] mfd-of-test-child mfd-of-test-child.2: Probing platform device: 2
  [0.713889] mfd-of-test-child mfd-of-test-child.2: Using OF node: child@dddddddd00000000

Which is fine if all OF nodes are identical.  However if we wish to
apply an attribute to particular device, we really need to ensure the
correct OF node will be associated with the device containing the
correct address.  We accomplish this by matching the device's address
expressed in DT with one provided during sub-device registration.
Like this:

  static const struct mfd_cell mfd_of_test_cell[] = {
        OF_MFD_CELL_REG("mfd-of-test-child", NULL, NULL, 0, 1, "mfd,of-test-child", 0xdddddddd00000000),
        OF_MFD_CELL_REG("mfd-of-test-child", NULL, NULL, 0, 2, "mfd,of-test-child", 0xaaaaaaaaaaaaaaaa),
        OF_MFD_CELL_REG("mfd-of-test-child", NULL, NULL, 0, 3, "mfd,of-test-child", 0x00000000cccccccc)
  };

This will ensure a specific device (designated here using the
platform_ids; 1, 2 and 3) is matched with a particular OF node:

  [0.712511] mfd-of-test-child mfd-of-test-child.0: Probing platform device: 0
  [0.712710] mfd-of-test-child mfd-of-test-child.0: Using OF node: child@dddddddd00000000
  [0.713033] mfd-of-test-child mfd-of-test-child.1: Probing platform device: 1
  [0.713381] mfd-of-test-child mfd-of-test-child.1: Using OF node: child@aaaaaaaaaaaaaaaa
  [0.713691] mfd-of-test-child mfd-of-test-child.2: Probing platform device: 2
  [0.713889] mfd-of-test-child mfd-of-test-child.2: Using OF node: child@cccccccc

This implementation is still not infallible, hence the mention of
"best effort" in the commit subject.  Since we have not *insisted* on
the existence of 'reg' properties (in some scenarios they just do not
make sense) and no device currently uses the new 'of_reg' attribute,
we have to make an on-the-fly judgement call whether to associate the
OF node anyway.  Which we do in cases where parent drivers haven't
specified a particular OF node to match to.  So there is a *slight*
possibility of the following result (note: the implementation here is
convoluted, but it shows you one means by which this process can
still break):

  /*
   * First entry will match to the first OF node with matching compatible
   * Second will fail, since the first took its OF node and is no longer available
   * Third will succeed
   */
  static const struct mfd_cell mfd_of_test_cell[] = {
        OF_MFD_CELL("mfd-of-test-child", NULL, NULL, 0, 1, "mfd,of-test-child"),
	OF_MFD_CELL_REG("mfd-of-test-child", NULL, NULL, 0, 2, "mfd,of-test-child", 0xaaaaaaaaaaaaaaaa),
        OF_MFD_CELL_REG("mfd-of-test-child", NULL, NULL, 0, 3, "mfd,of-test-child", 0x00000000cccccccc)
  };

The result:

  [0.753869] mfd-of-test-parent mfd_of_test: Registering 3 devices
  [0.756597] mfd-of-test-child: Failed to locate of_node [id: 2]
  [0.759999] mfd-of-test-child mfd-of-test-child.1: Probing platform device: 1
  [0.760314] mfd-of-test-child mfd-of-test-child.1: Using OF node: child@aaaaaaaaaaaaaaaa
  [0.760908] mfd-of-test-child mfd-of-test-child.2: Probing platform device: 2
  [0.761183] mfd-of-test-child mfd-of-test-child.2: No OF node associated with this device
  [0.761621] mfd-of-test-child mfd-of-test-child.3: Probing platform device: 3
  [0.761899] mfd-of-test-child mfd-of-test-child.3: Using OF node: child@cccccccc

We could code around this with some pre-parsing semantics, but the
added complexity required to cover each and every corner-case is not
justified.  Merely patching the current failing (via this patch) is
already working with some pretty small corner-cases.  Other issues
should be patched in the parent drivers which can be achieved simply
by implementing OF_MFD_CELL_REG().
Signed-off-by: Lee Jones <lee.jones@linaro.org>