Applied "spi: imx: add module parameter to control DMA use" to the spi tree

[Mark Brown <broonie@xxxxxxxxxx>]

The patch

   spi: imx: add module parameter to control DMA use

has been applied to the spi tree at

   https://git.kernel.org/pub/scm/linux/kernel/git/broonie/spi.git 

All being well this means that it will be integrated into the linux-next
tree (usually sometime in the next 24 hours) and sent to Linus during
the next merge window (or sooner if it is a bug fix), however if
problems are discovered then the patch may be dropped or reverted.  

You may get further e-mails resulting from automated or manual testing
and review of the tree, please engage with people reporting problems and
send followup patches addressing any issues that are reported if needed.

If any updates are required or you are submitting further changes they
should be sent as incremental updates against current git, existing
patches will not be replaced.

Please add any relevant lists and maintainers to the CCs when replying
to this mail.

Thanks,
Mark

>From 0a9c8998e75b69b3c347751a65ddd5bf7e72b2dd Mon Sep 17 00:00:00 2001
From: Trent Piepho <tpiepho@xxxxxxxxxx>
Date: Mon, 4 Mar 2019 23:02:36 +0000
Subject: [PATCH] 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@xxxxxxxxxx>
Signed-off-by: Mark Brown <broonie@xxxxxxxxxx>
---
 drivers/spi/spi-imx.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/spi/spi-imx.c b/drivers/spi/spi-imx.c
index 6ec647bbba77..e08646e715ba 100644
--- a/drivers/spi/spi-imx.c
+++ b/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;
 
-- 
2.20.1