[PATCH 04/32] spi: dw: Introduce polling device tree property

[Damien Le Moal <damien.lemoal@wdc.com>]

With boards that have slow interrupts context switch, and a fast device
connected to a spi master, e.g. an SD card through mmc-spi, using
dw_spi_poll_transfer() intead of the regular interrupt based
dw_spi_transfer_handler() function is more efficient and can avoid a lot
of RX FIFO overflow errors while keeping the device SPI frequency
reasonnably high (for speed). Introduce the "polling" device tree
property to allow requesting polled processing of transfer depending on
the connected device while keeping the spi master interrupts property
unschanged. E.g. device trees such as:

Generic soc.dtsi dts:

spi0: spi@53000000 {
	#address-cells = <1>;
	#size-cells = <0>;
	compatible = "snps,dw-apb-ssi";
	reg = <0x53000000 0x100>;
	interrupts = <2>;
	...
}

Board specific dts:

...
&spi0 {
	polling;
	status = "okay";

	slot@0 {
		compatible = "mmc-spi-slot";
		reg = <0>;
		voltage-ranges = <3300 3300>;
		spi-max-frequency = <4000000>;
	};
}

will result in using polled transfers for the SD card while other boards
using spi0 for different peripherals can use interrupt based transfers
without needing to change the generic base soc dts.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
---
 drivers/spi/spi-dw-mmio.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/drivers/spi/spi-dw-mmio.c b/drivers/spi/spi-dw-mmio.c
index d0cc5bf4fa4e..3f1bc384cb45 100644
--- a/drivers/spi/spi-dw-mmio.c
+++ b/drivers/spi/spi-dw-mmio.c
@@ -20,6 +20,7 @@
 #include <linux/property.h>
 #include <linux/regmap.h>
 #include <linux/reset.h>
+#include <linux/interrupt.h>
 
 #include "spi-dw.h"
 
@@ -246,9 +247,13 @@ static int dw_spi_mmio_probe(struct platform_device *pdev)
 
 	dws->paddr = mem->start;
 
-	dws->irq = platform_get_irq(pdev, 0);
-	if (dws->irq < 0)
-		return dws->irq; /* -ENXIO */
+	if (device_property_read_bool(&pdev->dev, "polling")) {
+		dws->irq = IRQ_NOTCONNECTED;
+	} else {
+		dws->irq = platform_get_irq(pdev, 0);
+		if (dws->irq < 0)
+			return dws->irq; /* -ENXIO */
+	}
 
 	dwsmmio->clk = devm_clk_get(&pdev->dev, NULL);
 	if (IS_ERR(dwsmmio->clk))
-- 
2.28.0