sysfs for my chips

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

Hi Greg !

(random CC list of clueful people)

On some new powerpc platforms (non-hypervisor or rather linux is the
hypervisor), I want to expose a bunch of stuff per "chip", the chips
being currently the processor chips and the "centaurs" (think of them as
the bottom half of the memory controllers).

Among other, I want a sysfs file in there to access "xscom" on the chip
which is a sideband bus used for low level stuff (think jtag on steroid)
which we can use, among others, for chip health monitoring, general
debugging and diagnostics, etc...

I might add more such as VPD, model information, etc... later or at
least a link to corresponding device-tree node.

How do you suggest I expose that ? So far I've been thinking about
something like

	/sys/chips/{processor,centaur}/chip#/files

or to avoid namespace clashes

	/sys/firmware/chips/{processor,centaur}/chip#/files

Or maybe just

	/sys/firmware/chips/chip#/files

(the chip type can be inferred from the chip#, they use the same space
at least as far my firmware exposes them to Linux)

(the actual access to xscom goes via firmware tho it makes *some* sense)

But I could instead create platform devices corresponding to the
device-tree representation of each of those chips ... and have the
platform devices contain the magic attributes. That's a bit more
convoluted though.

What's the current trend of the day for that sort of thing ? I'd rather
avoid yet-another-chardev-with-ioctl's here ...

Cheers,
Ben.