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

[Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>]

On Thu, 2015-06-25 at 02:08 +0200, Luis R. Rodriguez wrote:
> 
> OK thanks I'll proceed with these patches then.
> 
> > As for user mappings,
> 
> Which APIs were you considering in this regard BTW?

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

> > 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?).
> 
> > 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--



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