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 <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Akinobu Mita <[email protected]>
Cc: Chuck Ebbert <[email protected]>
Cc: Marek Szyprowski <[email protected]>
Cc: Konrad Rzeszutek Wilk <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Documentation/kernel-parameters.txt

@@ -656,7 +656,8 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			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 }

drivers/base/Kconfig

@@ -252,6 +252,9 @@ config DMA_CMA
 	  to allocate big physically-contiguous blocks of memory for use with
 	  hardware components that do not support I/O map nor scatter-gather.
 
+	  You can disable CMA by specifying "cma=0" on the kernel's command
+	  line.
+
 	  For more information see <include/linux/dma-contiguous.h>.
 	  If unsure, say "n".