cma: make default CMA area size zero for x86

This makes CMA memory area size zero for x86 in default configuration
(doesn't change on the other architectures).  If default CMA size is
zero, DMA_CMA is disabled.  It can be enabled by passing cma= to the
kernel.

This makes less impact on x86.  Because there is no mainline driver that
requires it for x86, and Peter Hurley reported the performance
regression, as this is trying to drive _all_ dma mapping allocations
through a _very_ small window.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Reported-by: Peter Hurley <peter@hurleysoftware.com>
Cc: Peter Hurley <peter@hurleysoftware.com>
Cc: Chuck Ebbert <cebbert.lkml@gmail.com>
Cc: Jean Delvare <jdelvare@suse.de>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Don Dutile <ddutile@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 7 insertions, 1 deletions

diff --git a/drivers/base/Kconfig b/drivers/base/Kconfig
index df04227d00cf..98504ec99c7d 100644
--- a/drivers/base/Kconfig
+++ b/drivers/base/Kconfig
@@ -267,18 +267,24 @@ comment "Default contiguous memory area size:"
 config CMA_SIZE_MBYTES
         int "Size in Mega Bytes"
         depends on !CMA_SIZE_SEL_PERCENTAGE
+        default 0 if X86
         default 16
         help
           Defines the size (in MiB) of the default memory area for Contiguous
-          Memory Allocator.
+          Memory Allocator.  If the size of 0 is selected, CMA is disabled by
+          default, but it can be enabled by passing cma=size[MG] to the kernel.
+
 
 config CMA_SIZE_PERCENTAGE
         int "Percentage of total memory"
         depends on !CMA_SIZE_SEL_MBYTES
+        default 0 if X86
         default 10
         help
           Defines the size of the default memory area for Contiguous Memory
           Allocator as a percentage of the total memory in the system.
+          If 0 percent is selected, CMA is disabled by default, but it can be
+          enabled by passing cma=size[MG] to the kernel.
 
 choice
         prompt "Selected region size"