Re: [PATCH v2] arm64: pci: add support for pci_mmap_page_range

[Will Deacon <will.deacon@arm.com>]

On Thu, Mar 16, 2017 at 12:17:27PM +0000, David Woodhouse wrote:
> On Fri, 2016-04-15 at 14:09 +0100, Will Deacon wrote:
> > 
> > > +     if (write_combine)
> > > +             vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);
> > > +     else
> > > +             vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
> > 
> > For consistency with ioremap, this should be pgprot_device.
> 
> What's the difference?

The different between ioremap (which used pgprot_device) and a mapping
created using pgprot_noncached is that the former allows for early
acknowledgement of writes (e.g. at a bridge). See this recent series from
Lorenzo that is also trying to clean this up:

http://lkml.kernel.org/r/20170227151436.18698-1-lorenzo.pieralisi@arm.com

> I note that VFIO is using pgprot_noncached() too, in vfio_pci_mmap() —
> where it open-codes an entirely arch-agnostic version of
> pci_mmap_page_range() all for itself. Should that be changed to
> pgprot_device() too?

I think so. At least, on arm64, pgprot_noncached is only really needed
for PCI config space and "I don't know that this is, but I'm going to map
it anyway" regions in /dev/mem.

> Let me see if I can get this straight...
> 
> We have the legacy interface through /proc/bus/pci, where the user
> passes a "user-visible" bus address not necessarily (on platforms with
> HAVE_PCI_RESOURCE_TO_USER) a host physical address.
> 
> The arch-specific pci_mmap_page_range() exists to work around that
> translation, on the two platforms which need it. It *also* has (on
> about three platforms) support for a write-combining mapping.
> 
> The sysfs interface theough /sys/bus/pci/devices/*/resource* probably
> doesn't need to use pci_mmap_page_range() at all, *except* for the
> 'resourceX_wc' variant which has write-combining support.
> 
> How about we do the following (probably not in this order):
>  • Kill pci_mmap_page_range() entirely.
>  • Implement a generic version which has (arch-assisted) WC support
>    but no knowledge of the horrid pci_resource_to_user() mapping.
>  • Require pci_user_to_resource() to be provided by platforms with
>    HAVE_ARCH_PCI_RESOURCE_TO_USER, and call that from *generic* code,
>    for the legacy procfs interface, before invoking the generic
>    replacement for pci_mmap_page_range().
> 
> (Yes, we still need to support mmap of I/O resources on... is it only
> powerpc? And there are a few inconsistencies, like powerpc forcing WC
> even on the sysfs files that *don't* have _wc in their name, that
> probably want to be cleaned up as we consolidate...)

Happy to review patches :)

Will