On Fri, Mar 24, 2017 at 11:40:33AM +0000, David Woodhouse wrote: > That leaves IA64 as the last holdout, as the selection of vm_page_prot > there is rather complicated: > > prot = phys_mem_access_prot(NULL, vma->vm_pgoff, size, > vma->vm_page_prot); > > /* > * If the user requested WC, the kernel uses UC or WC for this region, > * and the chipset supports WC, we can use WC. Otherwise, we have to > * use the same attribute the kernel uses. > */ > if (write_combine && > ((pgprot_val(prot) & _PAGE_MA_MASK) == _PAGE_MA_UC || > (pgprot_val(prot) & _PAGE_MA_MASK) == _PAGE_MA_WC) && > efi_range_is_wc(vma->vm_start, vma->vm_end - vma->vm_start)) > vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot); > else > vma->vm_page_prot = prot; > > > But I suspect it's *overcomplicated*, as the kernel should only ever be > mapping PCI memory BARs as UC or WC in the first place, so the middle > two checks in the if (write_combine…) condition are redundant.
Agreed. > And if the efi_range_is_wc() check isn't gratuitous, perhaps that > should be in the generic code whenever CONFIG_EFI is set? Sounds dubious whether EFI could even get this right. The efi memory map table is static, but we could remap a BAR to a different spot. Does the efi map have entries for all the places that you could remap a BAR? Isn't it more likely a property of the device whether it supports WC? -Tony
