linux-kernel - [PATCH 17/27] xen/swiotlb: improve comment on gfp flags in xen_alloc

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Date:	Fri, 13 Mar 2009 10:00:02 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	David Airlie <airlied@...ux.ie>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
Subject: [PATCH 17/27] xen/swiotlb: improve comment on gfp flags in xen_alloc_coherent()

From: Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>

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@...rix.com>
---
 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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