s390/pci: add s390_iommu_aperture kernel parameter

Some applications map the same memory area for DMA multiple times while
also mapping significant amounts of memory. With our current DMA code
these applications will run out of DMA addresses after mapping half of
the available memory because the number of DMA mappings is constrained
by the number of concurrently active DMA addresses we support which in
turn is limited by the minimum of hardware constraints and high_memory.

Limiting the number of active DMA addresses to high_memory is only
a heuristic to save memory used by the iommu_bitmap and DMA page tables
however. This was added under the assumption that it rarely makes sense
to DMA map more than system memory.

To accommodate special applications which insist on double mapping, which
works on other platforms, allow specifying a factor of how many times
installed memory is available as DMA address space. Use 0 as a special
value to apply no constraints beyond what hardware dictates at the
expense of significantly more memory use.

Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
Reviewed-by: Pierre Morel <pmorel@linux.ibm.com>
Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

1 files changed, 12 insertions, 0 deletions

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 91ba391f9b32..078a04890f69 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4982,6 +4982,18 @@
 			an IOTLB flush. Default is lazy flushing before reuse,
 			which is faster.
 
+	s390_iommu_aperture=	[KNL,S390]
+			Specifies the size of the per device DMA address space
+			accessible through the DMA and IOMMU APIs as a decimal
+			factor of the size of main memory.
+			The default is 1 meaning that one can concurrently use
+			as many DMA addresses as physical memory is installed,
+			if supported by hardware, and thus map all of memory
+			once. With a value of 2 one can map all of memory twice
+			and so on. As a special case a factor of 0 imposes no
+			restrictions other than those given by hardware at the
+			cost of significant additional memory use for tables.
+
 	sa1100ir	[NET]
 			See drivers/net/irda/sa1100_ir.c.