[Xen-devel] [PATCH 17/27] xen/swiotlb: improve comment on gfp flags in xen_alloc_coherent()

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

From: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>

Clarify why we don't care about the kernel's pseudo-phys restrictions,
so long as the underlying pages are in the right place.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>
---
 drivers/pci/xen-iommu.c |   10 ++++++----
 1 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/drivers/pci/xen-iommu.c b/drivers/pci/xen-iommu.c
index 7b6fd3e..13cfe0c 100644
--- a/drivers/pci/xen-iommu.c
+++ b/drivers/pci/xen-iommu.c
@@ -202,15 +202,17 @@ static void *xen_alloc_coherent(struct device *dev, 
size_t size,
        unsigned long vstart;
        u64 mask;
 
-       /* ignore region specifiers */
+       /*
+        * Ignore region specifiers - the kernel's ideas of
+        * pseudo-phys memory layout has nothing to do with the
+        * machine physical layout.  We can't allocate highmem
+        * because we can't return a pointer to it.
+        */
        gfp &= ~(__GFP_DMA | __GFP_HIGHMEM);
 
        if (dma_alloc_from_coherent(dev, size, dma_handle, &ret))
                return ret;
 
-       if (dev == NULL || (dev->coherent_dma_mask < 0xffffffff))
-               gfp |= GFP_DMA;
-
        vstart = __get_free_pages(gfp, order);
        ret = (void *)vstart;
 
-- 
1.6.0.6


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