[PATCH] x86-64: improved iommu documentation

- add SWIOTLB config help text
- mention Documentation/x86_64/boot-options.txt in
  Documentation/kernel-parameters.txt
- remove the duplication of the iommu kernel parameter documentation.
- Better explanation of some of the iommu kernel parameter options.
- "32MB<<order" instead of "32MB^order".
- Mention the default "order" value.
- list the four existing PCI-DMA mapping implementations of arch x86_64
- group the iommu= option keywords by PCI-DMA mapping implementation.
- Distinguish iommu= option keywords from number arguments.
- Explain the meaning of DAC and SAC.

Signed-off-by: Karsten Weiss <knweiss@science-computing.de>
Signed-off-by: Andi Kleen <ak@suse.de>
Acked-by: Muli Ben-Yehuda <muli@il.ibm.com>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>

1 files changed, 8 insertions, 2 deletions

diff --git a/arch/x86_64/Kconfig b/arch/x86_64/Kconfig
index 02dd39457bcf..a55382a1bb46 100644
--- a/arch/x86_64/Kconfig
+++ b/arch/x86_64/Kconfig
@@ -458,8 +458,8 @@ config IOMMU
           on systems with more than 3GB. This is usually needed for USB,
           sound, many IDE/SATA chipsets and some other devices.
           Provides a driver for the AMD Athlon64/Opteron/Turion/Sempron GART
-          based IOMMU and a software bounce buffer based IOMMU used on Intel
-          systems and as fallback.
+          based hardware IOMMU and a software bounce buffer based IOMMU used
+          on Intel systems and as fallback.
           The code is only active when needed (enough memory and limited
           device) unless CONFIG_IOMMU_DEBUG or iommu=force is specified
           too.
@@ -496,6 +496,12 @@ config CALGARY_IOMMU_ENABLED_BY_DEFAULT
 # need this always selected by IOMMU for the VIA workaround
 config SWIOTLB
         bool
+        help
+          Support for software bounce buffers used on x86-64 systems
+          which don't have a hardware IOMMU (e.g. the current generation
+          of Intel's x86-64 CPUs). Using this PCI devices which can only
+          access 32-bits of memory can be used on systems with more than
+          3 GB of memory. If unsure, say Y.
 
 config X86_MCE
         bool "Machine check support" if EMBEDDED