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 _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev