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.
Ben Herrenschmidt deserves the credit for this patch. He pointed out the
solution and what code would do the job.
v2:
Add a comment in the code.
Signed-off-by: Thadeu Lima de Souza Cascardo <casca...@linux.vnet.ibm.com>
Cc: Benjamin Herrenschmidt <b...@kernel.crashing.org>
Cc: Paul Mackerras <pau...@samba.org>
Cc: linuxppc-dev@lists.ozlabs.org
---
Resending, since it was recommended to add a comment about this
particular code.
---
arch/powerpc/kernel/iommu.c | 8 ++++++++
1 files changed, 8 insertions(+), 0 deletions(-)
diff --git a/arch/powerpc/kernel/iommu.c b/arch/powerpc/kernel/iommu.c
index 961bb03..8395301 100644
--- 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);
--
1.7.4.4
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