[POWERPC] Use 4kB iommu pages even on 64kB-page systems

The 10Gigabit ethernet device drivers appear to be able to chew
up all 256MB of TCE mappings on pSeries systems, as evidenced by
numerous error messages:

 iommu_alloc failed, tbl c0000000010d5c48 vaddr c0000000d875eff0 npages 1

Some experimentation indicates that this is essentially because
one 1500 byte ethernet MTU gets mapped as a 64K DMA region when
the large 64K pages are enabled. Thus, it doesn't take much to
exhaust all of the available DMA mappings for a high-speed card.

This patch changes the iommu allocator to work with its own
unique, distinct page size. Although the patch is long, its
actually quite simple: it just #defines a distinct IOMMU_PAGE_SIZE
and then uses this in all the places that matter.

As a side effect, it also dramatically improves network performance
on platforms with H-calls on iommu translation inserts/removes (since
we no longer call it 16 times for a 1500 bytes packet when the iommu HW
is still 4k).

In the future, we might want to make the IOMMU_PAGE_SIZE a variable
in the iommu_table instance, thus allowing support for different HW
page sizes in the iommu itself.

Signed-off-by: Linas Vepstas <linas@austin.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Olof Johansson <olof@lixom.net>
Acked-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>

1 files changed, 2 insertions, 2 deletions

diff --git a/arch/powerpc/kernel/vio.c b/arch/powerpc/kernel/vio.c
index cb87e71eec66..ed007878d1bf 100644
--- a/arch/powerpc/kernel/vio.c
+++ b/arch/powerpc/kernel/vio.c
@@ -92,9 +92,9 @@ static struct iommu_table *vio_build_iommu_table(struct vio_dev *dev)
                                 &tbl->it_index, &offset, &size);
 
                 /* TCE table size - measured in tce entries */
-                tbl->it_size = size >> PAGE_SHIFT;
+                tbl->it_size = size >> IOMMU_PAGE_SHIFT;
                 /* offset for VIO should always be 0 */
-                tbl->it_offset = offset >> PAGE_SHIFT;
+                tbl->it_offset = offset >> IOMMU_PAGE_SHIFT;
                 tbl->it_busno = 0;
                 tbl->it_type = TCE_VB;