drm/solomon: Add SSD130x OLED displays SPI support

The ssd130x driver only provides the core support for these devices but it
does not have any bus transport logic. Add a driver to interface over SPI.

There is a difference in the communication protocol when using 4-wire SPI
instead of I2C. For the latter, a control byte that contains a D/C# field
has to be sent. This field tells the controller whether the data has to be
written to the command register or to the graphics display data memory.

But for 4-wire SPI that control byte is not used, instead a real D/C# line
must be pulled HIGH for commands data and LOW for graphics display data.

For this reason the standard SPI regmap can't be used and a custom .write
bus handler is needed.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://patchwork.freedesktop.org/patch/msgid/20220419214824.335075-6-javierm@redhat.com

drivers/gpu/drm/solomon/Kconfig

@@ -20,3 +20,12 @@ config DRM_SSD130X_I2C
 	  I2C bus.
 
 	  If M is selected the module will be called ssd130x-i2c.
+
+config DRM_SSD130X_SPI
+	tristate "DRM support for Solomon SSD130X OLED displays (SPI bus)"
+	depends on DRM_SSD130X && SPI
+	select REGMAP
+	help
+	  Say Y here if the SSD130x OLED display is connected via SPI bus.
+
+	  If M is selected the module will be called ssd130x-spi.

drivers/gpu/drm/solomon/Makefile

@@ -1,2 +1,3 @@
 obj-$(CONFIG_DRM_SSD130X)	+= ssd130x.o
 obj-$(CONFIG_DRM_SSD130X_I2C)	+= ssd130x-i2c.o
+obj-$(CONFIG_DRM_SSD130X_SPI)	+= ssd130x-spi.o

drivers/gpu/drm/solomon/ssd130x-spi.c

@@ -0,0 +1,178 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * DRM driver for Solomon SSD130X OLED displays (SPI bus)
+ *
+ * Copyright 2022 Red Hat Inc.
+ * Authors: Javier Martinez Canillas <javierm@redhat.com>
+ */
+#include <linux/spi/spi.h>
+#include <linux/module.h>
+
+#include "ssd130x.h"
+
+#define DRIVER_NAME	"ssd130x-spi"
+#define DRIVER_DESC	"DRM driver for Solomon SSD130X OLED displays (SPI)"
+
+struct ssd130x_spi_transport {
+	struct spi_device *spi;
+	struct gpio_desc *dc;
+};
+
+static const struct regmap_config ssd130x_spi_regmap_config = {
+	.reg_bits = 8,
+	.val_bits = 8,
+};
+
+/*
+ * The regmap bus .write handler, it is just a wrapper around spi_write()
+ * but toggling the Data/Command control pin (D/C#). Since for 4-wire SPI
+ * a D/C# pin is used, in contrast with I2C where a control byte is sent,
+ * prior to every data byte, that contains a bit with the D/C# value.
+ *
+ * These control bytes are considered registers by the ssd130x core driver
+ * and can be used by the ssd130x SPI driver to determine if the data sent
+ * is for a command register or for the Graphic Display Data RAM (GDDRAM).
+ */
+static int ssd130x_spi_write(void *context, const void *data, size_t count)
+{
+	struct ssd130x_spi_transport *t = context;
+	struct spi_device *spi = t->spi;
+	const u8 *reg = data;
+
+	if (*reg == SSD130X_COMMAND)
+		gpiod_set_value_cansleep(t->dc, 0);
+
+	if (*reg == SSD130X_DATA)
+		gpiod_set_value_cansleep(t->dc, 1);
+
+	/* Remove control byte since is not used in a 4-wire SPI interface */
+	return spi_write(spi, reg + 1, count - 1);
+}
+
+/* The ssd130x driver does not read registers but regmap expects a .read */
+static int ssd130x_spi_read(void *context, const void *reg, size_t reg_size,
+			    void *val, size_t val_size)
+{
+	return -EOPNOTSUPP;
+}
+
+/*
+ * A custom bus is needed due the special write that toggles a D/C# pin,
+ * another option could be to just have a .reg_write() callback but that
+ * will prevent to do data writes in bulk.
+ *
+ * Once the regmap API is extended to support defining a bulk write handler
+ * in the struct regmap_config, this can be simplified and the bus dropped.
+ */
+static struct regmap_bus regmap_ssd130x_spi_bus = {
+	.write = ssd130x_spi_write,
+	.read = ssd130x_spi_read,
+};
+
+static int ssd130x_spi_probe(struct spi_device *spi)
+{
+	struct ssd130x_spi_transport *t;
+	struct ssd130x_device *ssd130x;
+	struct regmap *regmap;
+	struct gpio_desc *dc;
+	struct device *dev = &spi->dev;
+
+	dc = devm_gpiod_get(dev, "dc", GPIOD_OUT_LOW);
+	if (IS_ERR(dc))
+		return dev_err_probe(dev, PTR_ERR(dc),
+				     "Failed to get dc gpio\n");
+
+	t = devm_kzalloc(dev, sizeof(*t), GFP_KERNEL);
+	if (!t)
+		return dev_err_probe(dev, -ENOMEM,
+				     "Failed to allocate SPI transport data\n");
+
+	t->spi = spi;
+	t->dc = dc;
+
+	regmap = devm_regmap_init(dev, &regmap_ssd130x_spi_bus, t,
+				  &ssd130x_spi_regmap_config);
+	if (IS_ERR(regmap))
+		return PTR_ERR(regmap);
+
+	ssd130x = ssd130x_probe(dev, regmap);
+	if (IS_ERR(ssd130x))
+		return PTR_ERR(ssd130x);
+
+	spi_set_drvdata(spi, ssd130x);
+
+	return 0;
+}
+
+static void ssd130x_spi_remove(struct spi_device *spi)
+{
+	struct ssd130x_device *ssd130x = spi_get_drvdata(spi);
+
+	ssd130x_remove(ssd130x);
+}
+
+static void ssd130x_spi_shutdown(struct spi_device *spi)
+{
+	struct ssd130x_device *ssd130x = spi_get_drvdata(spi);
+
+	ssd130x_shutdown(ssd130x);
+}
+
+static const struct of_device_id ssd130x_of_match[] = {
+	{
+		.compatible = "sinowealth,sh1106",
+		.data = &ssd130x_variants[SH1106_ID],
+	},
+	{
+		.compatible = "solomon,ssd1305",
+		.data = &ssd130x_variants[SSD1305_ID],
+	},
+	{
+		.compatible = "solomon,ssd1306",
+		.data = &ssd130x_variants[SSD1306_ID],
+	},
+	{
+		.compatible = "solomon,ssd1307",
+		.data = &ssd130x_variants[SSD1307_ID],
+	},
+	{
+		.compatible = "solomon,ssd1309",
+		.data = &ssd130x_variants[SSD1309_ID],
+	},
+	{ /* sentinel */ }
+};
+MODULE_DEVICE_TABLE(of, ssd130x_of_match);
+
+/*
+ * The SPI core always reports a MODALIAS uevent of the form "spi:<dev>", even
+ * if the device was registered via OF. This means that the module will not be
+ * auto loaded, unless it contains an alias that matches the MODALIAS reported.
+ *
+ * To workaround this issue, add a SPI device ID table. Even when this should
+ * not be needed for this driver to match the registered SPI devices.
+ */
+static const struct spi_device_id ssd130x_spi_table[] = {
+	{ "sh1106",  SH1106_ID },
+	{ "ssd1305", SSD1305_ID },
+	{ "ssd1306", SSD1306_ID },
+	{ "ssd1307", SSD1307_ID },
+	{ "ssd1309", SSD1309_ID },
+	{ /* sentinel */ }
+};
+MODULE_DEVICE_TABLE(spi, ssd130x_spi_table);
+
+static struct spi_driver ssd130x_spi_driver = {
+	.driver = {
+		.name = DRIVER_NAME,
+		.of_match_table = ssd130x_of_match,
+	},
+	.probe = ssd130x_spi_probe,
+	.remove = ssd130x_spi_remove,
+	.shutdown = ssd130x_spi_shutdown,
+};
+module_spi_driver(ssd130x_spi_driver);
+
+MODULE_DESCRIPTION(DRIVER_DESC);
+MODULE_AUTHOR("Javier Martinez Canillas <javierm@redhat.com>");
+MODULE_LICENSE("GPL");
+MODULE_IMPORT_NS(DRM_SSD130X);