Re: [RFC PATCH] ARM: pmu: add OF match support

On Wed, Feb 9, 2011 at 11:53 AM, Will Deacon [off-list ref] wrote:

Grant,

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Following on from Rob's update, it would be nice if you could specify that
the PMU is a CPU PMU (as opposed to L2-cache, bus, gpu etc) in the string.
That way adding different PMUs in the future seems more natural and it accounts
for your concerns above. Is that ok, or does the compatible string have to
match that used by the platform bus?

It does make sense to encode the specific implementation into the
compatible string.  A single device driver can bind against multiple
compatible strings.  ie. the match table could include {.compatible =
"arm,cortex-a9-pmu"},{.compatible = "arm,cortex-a9-l2cache-pmu"}...

Ok - that's great! Specifying the CPU is probably a little verbose, but
something like "arm,armv7pmu" would be really helpful when it comes to
multiple devices.

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As for versioning, the PMU detection is done dynamically at runtime,
so knowing that we're poking a CPU is enough.

Fair enough.  It is still good practice in the compatible list to
encode the specific PMU implementation (maybe arm,cortex-a9-pmu?)
instead of trying to define a 'generic' or wildcard compatible value.
Newer implementations can always claim compatibility with an older
implementation so that the kernel doesn't have to be modified to find
the new devices.  "arm,pmu" is probably too generic.

"arm,pmu" is definitely too generic. If it's good practice to specify
as much as possible, then go for it - I just feel a little uneasy having
to add lots of redundant kernel code but it sounds like you'll hopefully
prove me wrong on that :)

Shouldn't be any redundant kernel code.  It's just a list of match
values, and the driver code already knows how to handle it.  The
advantage of being specific is that if the driver ever does need
information about the specific implementation (say, because the
hardware reported value is buggy), then the data is available.

g.