mmKconfig: add an option to disable bounce

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@gmail.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Russell King <rmk@arm.linux.org.uk>
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/mm/Kconfig b/mm/Kconfig
index 3bea74f1ccfe..e742d06285b7 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