Re: [PATCH] of: support an enumerated-bus compatible value

On Mon, Jul 2, 2012 at 9:59 AM, Stephen Warren [off-list ref] wrote:

On 07/01/2012 01:36 PM, Rob Herring wrote:

quoted

On 06/28/2012 06:05 PM, Stephen Warren wrote:

quoted

From: Stephen Warren <redacted>

An "enumerated" bus is one that is not memory-mapped, hence hence
typically has #address-cells=1, and #size-cells=0. Such buses would be
used to group related non-memory-mapped nodes together, often just under
the top-level of the device tree. The ability to group nodes into a non-
memory-mapped subnode of the root is important, since if nodes exist to
describe multiple entities of the same type, the nodes will have the
same name, and hence require a unit address to differentiate them. It
doesn't make sense to assign bogus unit addresses from the CPU's own
address space for this purpose. An example:

     regulators {
             compatible = "enumerated-bus";
             #address-cells = <1>;
             #size-cells = <0>;

             regulator@0 {
                     compatible = "regulator-fixed";
                     reg = <0>;
             };

             regulator@1 {
                     compatible = "regulator-fixed";
                     reg = <1>;
             };
     };

Finally, because such buses are not memory-mapped, we avoid creating
any IO/memory resources for the device.

This seems like a work-around to use reg instead of using cell-index
(which is discouraged). reg in this case is really not a hardware
description. Do you have an intended use or just trying to fix the error
messages?

I'm not familiar with cell-index; can you please describe it some more.
Looking at some existing files in arch/powerpc/boot/dts, it looks like
something that exists alongside reg rather than replacing it, so I don't
see how it'd solve the problem.

The portion of .dts file quoted above is the use-case. In more general
terms, I need to add a bunch of non-memory-mapped devices to DT. There
are multiple devices of a given type. The DT node names should be named
after the class of device not the instance, and hence all get named the
same. Hence, I need a unit address to differentiate the node names.
Hence I need to use the reg property in order that the unit address
matches the reg property. Is there some other way of solving these
requirements other than using a unit address to make the node names unique?

On 07/01/2012 04:03 PM, Grant Likely wrote:
...

quoted

Besides; if they are enumerated, non-memory mapped devices, then is it
really appropriate to use platform_{device,driver}? I don't think it
is.

Hmm, well /everything/ that gets instantiated from DT is a platform
device at present, at least for the platforms and bus types we're using
on Tegra and I believe all/most ARM platforms, except some small amounts
of AMBA.

Not true.  SPI devices beget spi_device, i2c devices i2c_client, etc.
The appropriate structure for the kind of device should always be
used.

g.

-- 
Grant Likely, B.Sc., P.Eng.
Secret Lab Technologies Ltd.