DW SPI: add option to use external gpio for chip select

DW SPI internal chip select management has limitation:
it hold CS line in active state only when the FIFO is not
empty. If the FIFO freed before we add new data the SPI transaction will
be broken.

So add option to use external gpio for chip select. Gpio can be added
via device tree using standard gpio bindings.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>

drivers/spi/designware_spi.c

@@ -10,6 +10,7 @@
  * SPDX-License-Identifier:	GPL-2.0
  */
 
+#include <asm-generic/gpio.h>
 #include <common.h>
 #include <clk.h>
 #include <dm.h>
@@ -98,6 +99,8 @@ struct dw_spi_priv {
 	struct clk clk;
 	unsigned long bus_clk_rate;
 
+	struct gpio_desc cs_gpio;	/* External chip-select gpio */
+
 	int bits_per_word;
 	u8 cs;			/* chip select pin */
 	u8 tmode;		/* TR/TO/RO/EEPROM */
@@ -131,6 +134,32 @@ static inline void dw_writew(struct dw_spi_priv *priv, u32 offset, u16 val)
 	__raw_writew(val, priv->regs + offset);
 }
 
+static int request_gpio_cs(struct udevice *bus)
+{
+#if defined(CONFIG_DM_GPIO) && !defined(CONFIG_SPL_BUILD)
+	struct dw_spi_priv *priv = dev_get_priv(bus);
+	int ret;
+
+	/* External chip select gpio line is optional */
+	ret = gpio_request_by_name(bus, "cs-gpio", 0, &priv->cs_gpio, 0);
+	if (ret == -ENOENT)
+		return 0;
+
+	if (ret < 0) {
+		printf("Error: %d: Can't get %s gpio!\n", ret, bus->name);
+		return ret;
+	}
+
+	if (dm_gpio_is_valid(&priv->cs_gpio)) {
+		dm_gpio_set_dir_flags(&priv->cs_gpio,
+				      GPIOD_IS_OUT | GPIOD_IS_OUT_ACTIVE);
+	}
+
+	debug("%s: used external gpio for CS management\n", __func__);
+#endif
+	return 0;
+}
+
 static int dw_spi_ofdata_to_platdata(struct udevice *bus)
 {
 	struct dw_spi_platdata *plat = bus->platdata;
@@ -145,7 +174,7 @@ static int dw_spi_ofdata_to_platdata(struct udevice *bus)
 	debug("%s: regs=%p max-frequency=%d\n", __func__, plat->regs,
 	      plat->frequency);
 
-	return 0;
+	return request_gpio_cs(bus);
 }
 
 static inline void spi_enable_chip(struct dw_spi_priv *priv, int enable)
@@ -316,6 +345,18 @@ static int poll_transfer(struct dw_spi_priv *priv)
 	return 0;
 }
 
+static void external_cs_manage(struct udevice *dev, bool on)
+{
+#if defined(CONFIG_DM_GPIO) && !defined(CONFIG_SPL_BUILD)
+	struct dw_spi_priv *priv = dev_get_priv(dev->parent);
+
+	if (!dm_gpio_is_valid(&priv->cs_gpio))
+		return;
+
+	dm_gpio_set_value(&priv->cs_gpio, on ? 1 : 0);
+#endif
+}
+
 static int dw_spi_xfer(struct udevice *dev, unsigned int bitlen,
 		       const void *dout, void *din, unsigned long flags)
 {
@@ -334,6 +375,10 @@ static int dw_spi_xfer(struct udevice *dev, unsigned int bitlen,
 		return -1;
 	}
 
+	/* Start the transaction if necessary. */
+	if (flags & SPI_XFER_BEGIN)
+		external_cs_manage(dev, false);
+
 	cr0 = (priv->bits_per_word - 1) | (priv->type << SPI_FRF_OFFSET) |
 		(priv->mode << SPI_MODE_OFFSET) |
 		(priv->tmode << SPI_TMOD_OFFSET);
@@ -395,6 +440,10 @@ static int dw_spi_xfer(struct udevice *dev, unsigned int bitlen,
 		ret = -ETIMEDOUT;
 	}
 
+	/* Stop the transaction if necessary */
+	if (flags & SPI_XFER_END)
+		external_cs_manage(dev, true);
+
 	return ret;
 }