Re: [PATCH v7 5/9] PCI: Add pci_iomap_wc() variants

On Thu, 2015-06-25 at 02:08 +0200, Luis R. Rodriguez wrote:

OK thanks I'll proceed with these patches then.

quoted

As for user mappings,

Which APIs were you considering in this regard BTW?

mmap of the generic /sys/bus/pci/.../resource*

quoted

maybe the right thing to do is to let us do what we do by
default with a quirk that can set a flag in pci_dev to disable that
behaviour (maybe on a per BAR basis ?).

That might mean it could restrict userspace WC to require devices
to have WC parts on a full PCI BAR. Although this is restrictive
having reviewed most WC uses in the kernel I'd think this would be
a fair compromise to make, but again, if things are still murky
perhaps best we kiss this idea good bye for now and hope for it
to come in on future buses or ammendments (if that's even possible?).

quoted

I think the common case is that WC works.

If WC does not I will note one hack which migh be worth mentioning --
just for
the record, this was devised as a shortcoming of a device where they
failed to
split things properly and that *without* WC performance suffered quite
a bit so
they made one full PCI BAR WC and as a work around this:

http://lkml.kernel.org/r/20150416041837.GA5712@hykim-PC

That is for registers that needed it:

write; wmb;

Then if they wanted to wait till the NIC has seen the write, they did:

write; wmb; read;

Right, and as I mentioned, on some archs like powerpc (and possibly
more), writel() and co contains an implicit mb()

  Luis--