spi: imx: add module parameter to control DMA use

Add the boolean module parameter "use_dma" to control the use of DMA by
the driver.  There are about two dozen other drivers with a "use_dma"
parameter of some sort.

DMA may allow faster and more efficient transfers than using PIO, but it
also adds overhead for small transfers.

High speed receive operations may be less likely to have issues with
FIFO overflow when using DMA than when using PIO.

The eCSPI appears to insert a 4 bit pause after each word in DMA mode,
not done in PIO mode, which can make DMA transfers 50% slower than PIO.

In some cases DMA may be a net win while in others PIO might be.  It
depends on the application.  So allow DMA to be enabled or disabled at
the driver level.  The default will be to have it enabled when possible.
Signed-off-by: Trent Piepho <tpiepho@impinj.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-imx.c

@@ -28,6 +28,10 @@
 
 #define DRIVER_NAME "spi_imx"
 
+static bool use_dma = true;
+module_param(use_dma, bool, 0644);
+MODULE_PARM_DESC(use_dma, "Enable usage of DMA when available (default)");
+
 #define MXC_CSPIRXDATA		0x00
 #define MXC_CSPITXDATA		0x04
 #define MXC_CSPICTRL		0x08
@@ -219,6 +223,9 @@ static bool spi_imx_can_dma(struct spi_master *master, struct spi_device *spi,
 {
 	struct spi_imx_data *spi_imx = spi_master_get_devdata(master);
 
+	if (!use_dma)
+		return false;
+
 	if (!master->dma_rx)
 		return false;