On Mon, Aug 31, 2020 at 03:18:27PM +0000, Clint Sbisa wrote: > Using write-combine is crucial for performance of PCI devices where > significant amounts of transactions go over PCI BARs. Write-combine is an x86ism that means nothing on ARM64 platforms so this should be rewritten to say what you actually mean, namely, you want to allow prefetchable resources to be mapped with "write combine" semantics (which means normal non-cacheable memory on arm64) through proc/sysfs. This is an outright can of worms and the PCI specs don't help in this respect, since we may end up mapping resources that have read side-effects with normal NC mappings (ie that's what "write combine" is in arm64 - pgprot_writecombine() and that's speculative memory). I am referring to "Additional Guidance on the Prefetchable Bit in Memory Space BARs" in the PCI specifications - it does not make any sense and must be removed because people use it to design endpoints. True - this is a problem even in kernel drivers but at least there the ioremap_ semantics is in the driver and can be vetted. This patch would make it user space ABI so I am a little nervous about merging this code TBH. > arm64 supports write-combine PCI mappings, so the appropriate define > has been added which will expose write-combine mappings under sysfs > for prefetchable PCI resources. > > Signed-off-by: Clint Sbisa <csbisa@xxxxxxxxxx> > --- > arch/arm64/include/asm/pci.h | 1 + > 1 file changed, 1 insertion(+) > > diff --git a/arch/arm64/include/asm/pci.h b/arch/arm64/include/asm/pci.h > index 70b323cf8300..b33ca260e3c9 100644 > --- a/arch/arm64/include/asm/pci.h > +++ b/arch/arm64/include/asm/pci.h > @@ -17,6 +17,7 @@ > #define pcibios_assign_all_busses() \ > (pci_has_flag(PCI_REASSIGN_ALL_BUS)) > > +#define arch_can_pci_mmap_wc() 1 I am not comfortable with this blanket enable. Some existing drivers, eg: drivers/infiniband/hw/mlx5 use this macro to detect WC capability which again, it is x86 specific, on arm64 it means nothing and can have consequences on the driver operations. Thanks, Lorenzo > #define ARCH_GENERIC_PCI_MMAP_RESOURCE 1 > > extern int isa_dma_bridge_buggy; > -- > 2.23.3 >
