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    SubjectRe: PAGE_CACHE_WC strikes again
    On Tue, 31 Mar 2009 17:03:10 -0800
    Suresh Siddha <suresh.b.siddha@intel.com> wrote:

    > On Tue, 2009-03-31 at 17:29 -0700, Pallipadi, Venkatesh wrote:
    > > The key point here is
    > >
    > > > setting PAGE_CACHE_WC disables the WC effect of the
    > > > MTRR on my non-PAT (disabled due to CPU errata)
    > >
    > > When PAT is disabled, the default setting in PAT MSR is
    > > 00 - WB
    > > 01 - WT
    > > 10 - UC_MINUS
    > > 11 - UC
    > >
    > > There is no way to set WC with PAT. By hardcoding _PAGE_CACHE_WC
    > > (which is 01) the driver is basically selecting write-through!
    > >
    > > And when MTRR says WC and PAT says WT, effective type is UC.
    > >
    > > Basically, no one should be hard-coding the memory type. Please use
    > > pgprot_writecombine() which does the right thing by using WC
    > > (when PAT is enabled) or UC_MINUS (when PAT is disabled).
    >
    > And the driver should use right API to track the underlying page frame
    > thats getting mapped by this vma, with the corresponding attribute.
    > API's like remap_pfn_range(), vm_insert_pfn() will setup the PTE's
    > aswell as track the pfn's attributes.
    >
    > API's like set_memory_uc/wc() will explicitly setup the page
    > attributes.
    >
    > Jesse, As far as I see, the drm GEM fault handler routines don't seem
    > to do any of this. Am I missing something? We need to fix this so
    > that we can avoid potential aliasing issues.

    Ok so we use set_memory_wc/wb in the physical mapping stuff, which I
    think is fine. We also use io_mapping_create_wc on the whole aperture
    at load time. That will take care of any pages we map there right? So
    along with this fix I think we'll be ok?

    --
    Jesse Barnes, Intel Open Source Technology Center

    diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
    index c1173d8..fa828a8 100644
    --- a/drivers/gpu/drm/drm_gem.c
    +++ b/drivers/gpu/drm/drm_gem.c
    @@ -538,11 +538,7 @@ int drm_gem_mmap(struct file *filp, struct vm_area_struct *vma)
    vma->vm_ops = obj->dev->driver->gem_vm_ops;
    vma->vm_private_data = map->handle;
    /* FIXME: use pgprot_writecombine when available */
    - prot = pgprot_val(vma->vm_page_prot);
    -#ifdef CONFIG_X86
    - prot |= _PAGE_CACHE_WC;
    -#endif
    - vma->vm_page_prot = __pgprot(prot);
    + vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);

    /* Take a ref for this mapping of the object, so that the fault
    * handler can dereference the mmap offset's pointer to the object.




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