linux-3.12.27: add linux-3.12-spi-atmel-dma-min.patch, build spi-atmel as module

Forces spi-atmel to always use DMA mode instead of switching between
PIO and DMA depending on the transfer size.  Switching between the two
can cause Rx buffer corruption with the 3.12 driver.

This comes at the cost of lower performance (15 to 20%) for small
transfers (1-15 bytes), but improves reliability for protocols that use small
and large transfers regularly.
For large transfers (1024 bytes), DMA mode is 300% faster than PIO.

https://lkml.org/lkml/2013/10/21/534

1 files changed, 18 insertions, 0 deletions

diff --git a/recipes-kernel/linux/linux-3.12.27/linux-3.12-spi-atmel-dma-min.patch b/recipes-kernel/linux/linux-3.12.27/linux-3.12-spi-atmel-dma-min.patch
new file mode 100644
index 0000000..4aba504
--- /dev/null
+++ b/recipes-kernel/linux/linux-3.12.27/linux-3.12-spi-atmel-dma-min.patch
@@ -0,0 +1,18 @@
+Index: linux-3.12.27/drivers/spi/spi-atmel.c
+===================================================================
+--- linux-3.12.27.orig/drivers/spi/spi-atmel.c
++++ linux-3.12.27/drivers/spi/spi-atmel.c
+@@ -187,7 +187,12 @@
+ /* use PIO for small transfers, avoiding DMA setup/teardown overhead and
+  * cache operations; better heuristics consider wordsize and bitrate.
+  */
+-#define DMA_MIN_BYTES	16
++
++/* MTS: set DMA_MIN_BYTES=0 to use DMA all the time due to Rx buffer issues 
++ * when switching between smaller PIO and larger DMA transfers 
++ * https://lkml.org/lkml/2013/10/21/534
++ */
++#define DMA_MIN_BYTES	0
+ 
+ struct atmel_spi_dma {
+ 	struct dma_chan			*chan_rx;