powerpc: Reserve iommu page 0

Some devices have a dma-window that starts at the address 0. This allows
DMA addresses to be mapped to this address and returned to drivers as a
valid DMA address. Some drivers may not behave well in this case, since
the address 0 is considered an error or not allocated.

The solution to avoid this kind of error from happening is reserve the
page addressed as 0 so it cannot be allocated for a DMA mapping.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

diff --git a/arch/powerpc/kernel/iommu.c b/arch/powerpc/kernel/iommu.c
index 961bb03413f38cad337f7617b689e83f2185013b..0cfcf98aafca25e3e9cb1c210ad4f149601a9bd9 100644 (file)
--- a/arch/powerpc/kernel/iommu.c
+++ b/arch/powerpc/kernel/iommu.c
@@ -501,6 +501,14 @@ struct iommu_table *iommu_init_table(struct iommu_table *tbl, int nid)
        tbl->it_map = page_address(page);
        memset(tbl->it_map, 0, sz);
 
+       /*
+        * Reserve page 0 so it will not be used for any mappings.
+        * This avoids buggy drivers that consider page 0 to be invalid
+        * to crash the machine or even lose data.
+        */
+       if (tbl->it_offset == 0)
+               set_bit(0, tbl->it_map);
+
        tbl->it_hint = 0;
        tbl->it_largehint = tbl->it_halfpoint;
        spin_lock_init(&tbl->it_lock);