[PATCH v2] mm: add an option to disable bounce

[vinayakm.list at gmail.com]

From: Vinayak Menon <vinayakm.list at gmail.com>

There are times when HIGHMEM is enabled, but
we don't prefer CONFIG_BOUNCE to be enabled.
CONFIG_BOUNCE can reduce the block device
throughput, and this is not ideal for machines
where we don't gain much by enabling it. So
provide an option to deselect CONFIG_BOUNCE. The
observation was made while measuring eMMC throughput
using iozone on an ARM device with 1GB RAM.

Signed-off-by: Vinayak Menon <vinayakm.list at gmail.com>
---
 mm/Kconfig |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/mm/Kconfig b/mm/Kconfig
index 3bea74f..e742d06 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -263,8 +263,14 @@ config ZONE_DMA_FLAG
 	default "1"
 
 config BOUNCE
-	def_bool y
+	bool "Enable bounce buffers"
+	default y
 	depends on BLOCK && MMU && (ZONE_DMA || HIGHMEM)
+	help
+	  Enable bounce buffers for devices that cannot access
+	  the full range of memory available to the CPU. Enabled
+	  by default when ZONE_DMA or HIGHMEM is selected, but you
+	  may say n to override this.
 
 # On the 'tile' arch, USB OHCI needs the bounce pool since tilegx will often
 # have more than 4GB of memory, but we don't currently use the IOTLB to present
-- 
1.7.10.4