spi: spi-fsl-spi: support use of the SPISEL_BOOT signal on MPC8309

The MPC8309 has a dedicated signal, SPISEL_BOOT, usually used as chip
select for the flash device from which the bootloader is loaded. It is
not an ordinary gpio, but is simply controlled via the SPI_CS register
in the system configuration.

To allow accessing such a spi slave, we need to teach
fsl_spi_cs_control() how to control the SPISEL_BOOT signal. To
distinguish the gpio-controlled slaves, continue to have those use
chip_select values of 0..ngpios-1, and use chip_select == ngpios for
the boot flash.

I'm not too happy with all the ifdeffery, but it seems to be necessary
for guarding the sysdev/fsl_soc.h and use of
get_immrbase() (spi-fsl-lib.c already contains similar ifdeffery).

Googling suggests that the MPC8306 is similar, with the SPI_CS
register at the same offset.
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/fsl-spi.txt

@@ -18,6 +18,10 @@ Optional properties:
 - gpios : specifies the gpio pins to be used for chipselects.
   The gpios will be referred to as reg = <index> in the SPI child nodes.
   If unspecified, a single SPI device without a chip select can be used.
+- fsl,spisel_boot : for the MPC8306 and MPC8309, specifies that the
+  SPISEL_BOOT signal is used as chip select for a slave device. Use
+  reg = <number of gpios> in the corresponding child node, i.e. 0 if
+  the gpios property is not present.
 
 Example:
 	spi@4c0 {

drivers/spi/spi-fsl-lib.h

@@ -95,8 +95,10 @@ static inline u32 mpc8xxx_spi_read_reg(__be32 __iomem *reg)
 
 struct mpc8xxx_spi_probe_info {
 	struct fsl_spi_platform_data pdata;
+	int ngpios;
 	int *gpios;
 	bool *alow_flags;
+	__be32 __iomem *immr_spi_cs;
 };
 
 extern u32 mpc8xxx_spi_tx_buf_u8(struct mpc8xxx_spi *mpc8xxx_spi);

drivers/spi/spi-fsl-spi.c

@@ -39,6 +39,14 @@
 #include <linux/spi/spi_bitbang.h>
 #include <linux/types.h>
 
+#ifdef CONFIG_FSL_SOC
+#include <sysdev/fsl_soc.h>
+#endif
+
+/* Specific to the MPC8306/MPC8309 */
+#define IMMR_SPI_CS_OFFSET 0x14c
+#define SPI_BOOT_SEL_BIT   0x80000000
+
 #include "spi-fsl-lib.h"
 #include "spi-fsl-cpm.h"
 #include "spi-fsl-spi.h"
@@ -701,10 +709,17 @@ static void fsl_spi_cs_control(struct spi_device *spi, bool on)
 	struct fsl_spi_platform_data *pdata = dev_get_platdata(dev);
 	struct mpc8xxx_spi_probe_info *pinfo = to_of_pinfo(pdata);
 	u16 cs = spi->chip_select;
+
+	if (cs < pinfo->ngpios) {
 		int gpio = pinfo->gpios[cs];
 		bool alow = pinfo->alow_flags[cs];
 
 		gpio_set_value(gpio, on ^ alow);
+	} else {
+		if (WARN_ON_ONCE(cs > pinfo->ngpios || !pinfo->immr_spi_cs))
+			return;
+		iowrite32be(on ? SPI_BOOT_SEL_BIT : 0, pinfo->immr_spi_cs);
+	}
 }
 
 static int of_fsl_spi_get_chipselects(struct device *dev)
@@ -712,12 +727,15 @@ static int of_fsl_spi_get_chipselects(struct device *dev)
 	struct device_node *np = dev->of_node;
 	struct fsl_spi_platform_data *pdata = dev_get_platdata(dev);
 	struct mpc8xxx_spi_probe_info *pinfo = to_of_pinfo(pdata);
+	bool spisel_boot = IS_ENABLED(CONFIG_FSL_SOC) &&
+		of_property_read_bool(np, "fsl,spisel_boot");
 	int ngpios;
 	int i = 0;
 	int ret;
 
 	ngpios = of_gpio_count(np);
-	if (ngpios <= 0) {
+	ngpios = max(ngpios, 0);
+	if (ngpios == 0 && !spisel_boot) {
 		/*
 		 * SPI w/o chip-select line. One SPI device is still permitted
 		 * though.
@@ -726,6 +744,7 @@ static int of_fsl_spi_get_chipselects(struct device *dev)
 		return 0;
 	}
 
+	pinfo->ngpios = ngpios;
 	pinfo->gpios = kmalloc_array(ngpios, sizeof(*pinfo->gpios),
 				     GFP_KERNEL);
 	if (!pinfo->gpios)
@@ -769,7 +788,18 @@ static int of_fsl_spi_get_chipselects(struct device *dev)
 		}
 	}
 
-	pdata->max_chipselect = ngpios;
+#if IS_ENABLED(CONFIG_FSL_SOC)
+	if (spisel_boot) {
+		pinfo->immr_spi_cs = ioremap(get_immrbase() + IMMR_SPI_CS_OFFSET, 4);
+		if (!pinfo->immr_spi_cs) {
+			ret = -ENOMEM;
+			i = ngpios - 1;
+			goto err_loop;
+		}
+	}
+#endif
+
+	pdata->max_chipselect = ngpios + spisel_boot;
 	pdata->cs_control = fsl_spi_cs_control;
 
 	return 0;