Documentation: DMA-API-HOWTO.txt: add multiple types of IOMMUs support

Signed-off-by: NFUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Documentation/DMA-API-HOWTO.txt

@@ -753,6 +753,16 @@ to "Closing".
    alignment constraints (e.g. the alignment constraints about 64-bit
    objects).
 
+3) Supporting multiple types of IOMMUs
+
+   If your architecture needs to support multiple types of IOMMUs, you
+   can use include/linux/asm-generic/dma-mapping-common.h. It's a
+   library to support the DMA API with multiple types of IOMMUs. Lots
+   of architectures (x86, powerpc, sh, alpha, ia64, microblaze and
+   sparc) use it. Choose one to see how it can be used. If you need to
+   support multiple types of IOMMUs in a single system, the example of
+   x86 or powerpc helps.
+
 			   Closing
 
 This document, and the API itself, would not be in its current