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@xxxxxxx > 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
