Re: PAGE_CACHE_WC strikes again

[Jesse Barnes <jbarnes@virtuousgeek.org>]

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.