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>

diff --git a/mm/Kconfig b/mm/Kconfig
index 3bea74f1ccfe1f1f5c134fd5c98767d620553b60..e742d06285b780eea632efe643e18d695f766928 100644 (file)
--- 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