Re: [PATCH 02/20] PCI: fix pci_remap_iospace() remap attribute

Bjorn Helgaas
2017-03-16T21:49:12Z

[+cc Luis]

On Mon, Feb 27, 2017 at 03:14:13PM +0000, Lorenzo Pieralisi wrote:
> According to the PCI local bus specifications (Revision 3.0, 3.2.5),
> I/O Address space transactions are non-posted. On architectures where
> I/O space is implemented through a chunk of memory mapped space mapped
> to PCI address space (ie IA64/ARM/ARM64) the memory mapping for the
> region backing I/O Address Space transactions determines the I/O
> transactions attributes (before the transactions actually reaches the
> PCI bus where it is handled according to the PCI specifications).
>
> Current pci_remap_iospace() interface, that is used to map the PCI I/O
> Address Space into virtual address space, use pgprot_device() as memory
> attribute for the virtual address mapping, that in some architectures
> (ie ARM64) provides non-cacheable but write bufferable mappings (ie
> posted writes), which clash with the non-posted write behaviour for I/O
> Address Space mandated by the PCI specifications.
>
> Update the prot ioremap_page_range() parameter in pci_remap_iospace()
> to pgprot_noncached to ensure that the virtual mapping backing
> I/O Address Space guarantee non-posted write transactions issued
> when addressing I/O Address Space through the MMIO mapping.
>
> Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Will Deacon <will.deacon@arm.com>
> Cc: Bjorn Helgaas <bhelgaas@google.com>
> Cc: Russell King <linux@armlinux.org.uk>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> ---
> drivers/pci/pci.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index bd98674..bfb3c6e 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -3375,7 +3375,7 @@ int pci_remap_iospace(const struct resource *res, phys_addr_t phys_addr)
> return -EINVAL;
>
> return ioremap_page_range(vaddr, vaddr + resource_size(res), phys_addr,
> - pgprot_device(PAGE_KERNEL));
> + pgprot_noncached(PAGE_KERNEL));

pgprot_device() is equivalent to pgprot_noncached() on all arches
except ARM64, and I trust you're doing the right thing on ARM64, so
I'm fine with this from a PCI perspective.

I do find this puzzling because I naively expected pgprot_noncached()
to match up with ioremap_nocache(), and apparently it doesn't.

For example, ARM64 ioremap_nocache() uses PROT_DEVICE_nGnRE, which
doesn't match the MT_DEVICE_nGnRnE in pgprot_noncached().

The point of these patches is to use non-posted mappings. Apparently
you can do that with pgprot_noncached() here, but ioremap_nocache()
isn't enough for the config space mappings?

I suppose that's a consequence of the pgprot_noncached() vs
ioremap_nocache() mismatch, but this is all extremely confusing.

> #else
> /* this architecture does not have memory mapped I/O space,
> so this function should never be called */
> --
> 2.10.0
>
>
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