Messages in this thread Patch in this message | | | Subject | Re: [RFC Patch] use MTRR for write combining if PAT is not available | From | Suresh Siddha <> | Date | Thu, 22 Oct 2009 21:58:15 -0700 |
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On Thu, 2009-10-22 at 21:31 -0700, Jesse Barnes wrote: > Here's a quick & dirty version, totally untested. A cleaner approach > would be to separate the WC mapping routines and hide the return > -EINVAL in arch specific code...
Jesse How about this patch? Doing this in x86 is cleaner.
I would like Acks/sign-offs-by Thomas, Eric and Jesse, if it is ok with this patch and works.
thanks, suresh ---
From: Suresh Siddha <suresh.b.siddha@intel.com> Subject: x86, pat: return EINVAL for pci mmap WC request for !pat_enabled
Thomas Schlichter reported: > X.org uses libpciaccess which tries to mmap with write combining enabled via > /sys/bus/pci/devices/*/resource0_wc. Currently, when PAT is not enabled, the > kernel does fall back to uncached mmap. Then libpciaccess thinks it succeeded > mapping with write combining enabled and does not set up suited MTRR entries. > ;-(
Instead of silently mapping pci mmap region as UC minus in the case of !pat_enabled and wc request, we can return error. Eric Anholt mentioned that caller (like X) typically follows up with UC minus pci mmap request and if there is a free mtrr slot, caller will manage adding WC mtrr.
Jesse Barnes says: > Older versions of libpciaccess will behave better if we do it that way > (iirc it only allocates an MTRR if the resource_wc file doesn't exist or > fails to get mapped).
Reported-by: Thomas Schlichter <thomas.schlichter@web.de> Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com> ---
diff --git a/arch/x86/pci/i386.c b/arch/x86/pci/i386.c index b22d13b..a672f12 100644 --- a/arch/x86/pci/i386.c +++ b/arch/x86/pci/i386.c @@ -282,6 +282,15 @@ int pci_mmap_page_range(struct pci_dev *dev, struct vm_area_struct *vma, return -EINVAL; prot = pgprot_val(vma->vm_page_prot); + + /* + * Return error if pat is not enabled and write_combine is requested. + * Caller can followup with UC MINUS request and add a WC mtrr if there + * is a free mtrr slot. + */ + if (!pat_enabled && write_combine) + return -EINVAL; + if (pat_enabled && write_combine) prot |= _PAGE_CACHE_WC; else if (pat_enabled || boot_cpu_data.x86 > 3)
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