CMA: document cma=0

It isn't obvious that CMA can be disabled on the kernel's command line, so
document it.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Chuck Ebbert <cebbert.lkml@gmail.com>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index a126a31..809e880 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -656,7 +656,8 @@
 			Sets the size of kernel global memory area for
 			contiguous memory allocations and optionally the
 			placement constraint by the physical address range of
-			memory allocations. For more information, see
+			memory allocations. A value of 0 disables CMA
+			altogether. For more information, see
 			include/linux/dma-contiguous.h
 
 	cmo_free_hint=	[PPC] Format: { yes | no }