Re: Using PCI config space to indicate config location

"Michael S. Tsirkin" [off-list ref] writes:

On Fri, Oct 12, 2012 at 08:21:50PM +1030, Rusty Russell wrote:

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"Michael S. Tsirkin" [off-list ref] writes:

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On Fri, Oct 12, 2012 at 08:59:36AM +1030, Rusty Russell wrote:

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For writes, the standard seems to be a commit latch.  We could abuse the
generation count for this: the driver writes to it to commit config
changes.

I think this will work. There are a couple of things that bother me:

This assumes read accesses have no side effects, and these are sometimes handy.
Also the semantics for write aren't very clear to me.
I guess device must buffer data until generation count write?
This assumes the device has a buffer to store writes,
and it must track each byte written. I kind of dislike this
tracking of accessed bytes. Also, device would need to resolve conflicts
if any in some device specific way.

It should be trivial to implement: you keep a scratch copy of the config
space, and copy it to the master copy when they hit the latch.

Implementation of this will show whether I've missed anything here, I
think.

What I refer to: what happens if driver does:
- write offset 1
- write offset 3
- hit commit latch

- nothing
- nothing
- effect of offset 1 and offset 3 writes

OK so this means that you also need to track which bytes where written
in order to know to skip byte 2.
This is what I referred to. If instead we ask driver to specify
offset/length explicitly device only needs to remember that.

I was assuming the implementation would keep two complete copies of the
config space: writes go to the scratch version, which gets copied to the
master version upon latch write.

But I do wonder if we should just skip this for now, since we don't
have any immediate need.

Cheers,
Rusty.