ath79: add Mikrotik rb4xx series drivers

This adds 3 Mikrotik rb4xx series drivers as follows:

rb4xx-cpld: This is in the mfd subsystem, and is the parent CPLD device
that interfaces between the SoC SPI bus and its two children below.
rb4xx-gpio: This is the GPIO expander.
rb4xx-nand: This is the NAND driver.

The history of this code comes in three phases.

1. The first is a May 2015 attempt to push the equivalient ar71xx rb4xx
drivers upstream. See https://lore.kernel.org/patchwork/patch/940880/.

Module-author: Gabor Juhos <juhosg@openwrt.org>
Module-author: Imre Kaloz <kaloz@openwrt.org>
Module-author: Bert Vermeulen <bert@biot.com>

2. Next several ar71xx patches were applied bringing the code current.

commit 7bbf4117c6fe4b764d9d7c62fb2bcf6dd93bff2c
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

commit af79fdbe4af32a287798b579141204bda056b8aa
commit 889272d92db689fd9c910243635e44c9d8323095
commit e21cb649a235180563363b8af5ba8296b9ac0baa
commit 7c09fa4a7492ca436f2c94bd9a465b7c5bbeed6f
Signed-off-by: Felix Fietkau <nbd@nbd.name>

3. Finally a heavy refactor to split the driver into the three new
subsystems, and updated to work with the device tree configuration, plus
updates and review feedback incorporated

Reviewed-by: Thibaut VARÈNE <hacks@slashdirt.org>
Signed-off-by: Christopher Hill <ch6574@gmail.com>

4 files changed, 676 insertions, 0 deletions

diff --git a/target/linux/ath79/files/drivers/gpio/gpio-rb4xx.c b/target/linux/ath79/files/drivers/gpio/gpio-rb4xx.c
new file mode 100644
index 0000000000..f8022436e0
--- /dev/null
+++ b/target/linux/ath79/files/drivers/gpio/gpio-rb4xx.c
@@ -0,0 +1,172 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * GPIO driver for the MikroTik RouterBoard 4xx series
+ *
+ * Copyright (C) 2008-2011 Gabor Juhos <juhosg@openwrt.org>
+ * Copyright (C) 2008 Imre Kaloz <kaloz@openwrt.org>
+ * Copyright (C) 2015 Bert Vermeulen <bert@biot.com>
+ * Copyright (C) 2020 Christopher Hill <ch6574@gmail.com>
+ *
+ * This file was based on the driver for Linux 2.6.22 published by
+ * MikroTik for their RouterBoard 4xx series devices.
+ *
+ * N.B. driver probe reports "DMA mask not set" warnings which are
+ * an artifact of using a platform_driver as an MFD device child.
+ * See conversation here https://lkml.org/lkml/2020/4/28/675
+ */
+#include <linux/platform_device.h>
+#include <linux/gpio/driver.h>
+#include <linux/module.h>
+#include <linux/of_device.h>
+
+#include <mfd/rb4xx-cpld.h>
+
+struct rb4xx_gpio {
+	struct rb4xx_cpld *cpld;
+	struct device *dev;
+
+	struct gpio_chip chip;
+	struct mutex lock;
+	u16 values;		/* bitfield of GPIO 0-8 current values */
+};
+
+static int rb4xx_gpio_cpld_set(struct rb4xx_gpio *gpio, unsigned int offset,
+			       int value)
+{
+	struct rb4xx_cpld *cpld = gpio->cpld;
+	u16 values;
+	int ret;
+
+	mutex_lock(&gpio->lock);
+	values = gpio->values;
+
+	if (value)
+		values |= BIT(offset);
+	else
+		values &= ~(BIT(offset));
+
+	if (values == gpio->values) {
+		ret = 0;
+		goto unlock;
+	}
+
+	if (offset < 8) {
+		ret = cpld->gpio_set_0_7(cpld, values & 0xff);
+	} else if (offset == 8) {
+		ret = cpld->gpio_set_8(cpld, values >> 8);
+	}
+
+	if(likely(!ret))
+		gpio->values = values;
+
+unlock:
+	mutex_unlock(&gpio->lock);
+	return ret;
+}
+
+static int rb4xx_gpio_get_direction(struct gpio_chip *chip, unsigned int offset)
+{
+	return 0; /* All 9 GPIOs are out */
+}
+
+static int rb4xx_gpio_direction_input(struct gpio_chip *chip,
+				      unsigned int offset)
+{
+	return -EOPNOTSUPP;
+}
+
+static int rb4xx_gpio_direction_output(struct gpio_chip *chip,
+				       unsigned int offset, int value)
+{
+	return rb4xx_gpio_cpld_set(gpiochip_get_data(chip), offset, value);
+}
+
+static int rb4xx_gpio_get(struct gpio_chip *chip, unsigned int offset)
+{
+	struct rb4xx_gpio *gpio = gpiochip_get_data(chip);
+	int ret;
+
+	mutex_lock(&gpio->lock);
+	ret = (gpio->values >> offset) & 0x1;
+	mutex_unlock(&gpio->lock);
+
+	return ret;
+}
+
+static void rb4xx_gpio_set(struct gpio_chip *chip, unsigned int offset,
+			   int value)
+{
+	rb4xx_gpio_cpld_set(gpiochip_get_data(chip), offset, value);
+}
+
+static int rb4xx_gpio_probe(struct platform_device *pdev)
+{
+	struct device *dev = &pdev->dev;
+	struct device *parent = dev->parent;
+	struct rb4xx_gpio *gpio;
+	u32 val;
+
+	if (!parent)
+		return -ENODEV;
+
+	gpio = devm_kzalloc(dev, sizeof(*gpio), GFP_KERNEL);
+	if (!gpio)
+		return -ENOMEM;
+
+	platform_set_drvdata(pdev, gpio);
+	gpio->cpld	= dev_get_drvdata(parent);
+	gpio->dev	= dev;
+	gpio->values	= 0;
+	mutex_init(&gpio->lock);
+
+	gpio->chip.label		= "rb4xx-gpio";
+	gpio->chip.parent		= dev;
+	gpio->chip.owner		= THIS_MODULE;
+	gpio->chip.get_direction	= rb4xx_gpio_get_direction;
+	gpio->chip.direction_input	= rb4xx_gpio_direction_input;
+	gpio->chip.direction_output	= rb4xx_gpio_direction_output;
+	gpio->chip.get			= rb4xx_gpio_get;
+	gpio->chip.set			= rb4xx_gpio_set;
+	gpio->chip.ngpio		= 9;
+	gpio->chip.base			= -1;
+	gpio->chip.can_sleep		= 1;
+
+	if (!of_property_read_u32(dev->of_node, "base", &val))
+		gpio->chip.base = val;
+
+	return gpiochip_add_data(&gpio->chip, gpio);
+}
+
+static int rb4xx_gpio_remove(struct platform_device *pdev)
+{
+	struct rb4xx_gpio *gpio = platform_get_drvdata(pdev);
+
+	gpiochip_remove(&gpio->chip);
+	mutex_destroy(&gpio->lock);
+
+	return 0;
+}
+
+static const struct platform_device_id rb4xx_gpio_id_table[] = {
+	{ "mikrotik,rb4xx-gpio", },
+	{ },
+};
+MODULE_DEVICE_TABLE(platform, rb4xx_gpio_id_table);
+
+static struct platform_driver rb4xx_gpio_driver = {
+	.probe = rb4xx_gpio_probe,
+	.remove = rb4xx_gpio_remove,
+	.id_table = rb4xx_gpio_id_table,
+	.driver = {
+		.name = "rb4xx-gpio",
+	},
+};
+
+module_platform_driver(rb4xx_gpio_driver);
+
+MODULE_DESCRIPTION("Mikrotik RB4xx GPIO driver");
+MODULE_AUTHOR("Gabor Juhos <juhosg@openwrt.org>");
+MODULE_AUTHOR("Imre Kaloz <kaloz@openwrt.org>");
+MODULE_AUTHOR("Bert Vermeulen <bert@biot.com>");
+MODULE_AUTHOR("Christopher Hill <ch6574@gmail.com");
+MODULE_LICENSE("GPL v2");
diff --git a/target/linux/ath79/files/drivers/mfd/rb4xx-cpld.c b/target/linux/ath79/files/drivers/mfd/rb4xx-cpld.c
new file mode 100644
index 0000000000..da18424c63
--- /dev/null
+++ b/target/linux/ath79/files/drivers/mfd/rb4xx-cpld.c
@@ -0,0 +1,182 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * CPLD driver for the MikroTik RouterBoard 4xx series
+ *
+ * This driver provides access to a CPLD that interfaces between the SoC SPI bus
+ * and other devices. Behind the CPLD there is a NAND flash chip and five LEDs.
+ *
+ * The CPLD supports SPI two-wire mode, in which two bits are transferred per
+ * SPI clock cycle. The second bit is transmitted with the SoC's CS2 pin.
+ *
+ * The CPLD also acts as a GPIO expander.
+ *
+ * Copyright (C) 2008-2011 Gabor Juhos <juhosg@openwrt.org>
+ * Copyright (C) 2008 Imre Kaloz <kaloz@openwrt.org>
+ * Copyright (C) 2015 Bert Vermeulen <bert@biot.com>
+ * Copyright (C) 2020 Christopher Hill <ch6574@gmail.com>
+ *
+ * This file was based on the driver for Linux 2.6.22 published by
+ * MikroTik for their RouterBoard 4xx series devices.
+*/
+#include <linux/mfd/core.h>
+#include <linux/spi/spi.h>
+#include <linux/module.h>
+#include <linux/of_platform.h>
+
+#include <mfd/rb4xx-cpld.h>
+
+/* CPLD commands */
+#define CPLD_CMD_WRITE_NAND	0x08 /* send cmd, n x send data, send idle */
+#define CPLD_CMD_WRITE_CFG	0x09 /* send cmd, n x send cfg */
+#define CPLD_CMD_READ_NAND	0x0a /* send cmd, send idle, n x read data */
+#define CPLD_CMD_READ_FAST	0x0b /* send cmd, 4 x idle, n x read data */
+#define CPLD_CMD_GPIO8_HIGH	0x0c /* send cmd */
+#define CPLD_CMD_GPIO8_LOW	0x0d /* send cmd */
+
+static int rb4xx_cpld_write_nand(struct rb4xx_cpld *cpld, const void *tx_buf,
+				 unsigned int len)
+{
+	struct spi_message m;
+	static const u8 cmd = CPLD_CMD_WRITE_NAND;
+	struct spi_transfer t[3] = {
+		{
+			.tx_buf = &cmd,
+			.len = sizeof(cmd),
+		}, {
+			.tx_buf = tx_buf,
+			.len = len,
+			.tx_nbits = SPI_NBITS_DUAL,
+		}, {
+			.len = 1,
+			.tx_nbits = SPI_NBITS_DUAL,
+		},
+	};
+
+	spi_message_init(&m);
+	spi_message_add_tail(&t[0], &m);
+	spi_message_add_tail(&t[1], &m);
+	spi_message_add_tail(&t[2], &m);
+	return spi_sync(cpld->spi, &m);
+}
+
+static int rb4xx_cpld_read_nand(struct rb4xx_cpld *cpld, void *rx_buf,
+				unsigned int len)
+{
+	struct spi_message m;
+	static const u8 cmd[2] = {
+		CPLD_CMD_READ_NAND, 0
+	};
+	struct spi_transfer t[2] = {
+		{
+			.tx_buf = &cmd,
+			.len = sizeof(cmd),
+		}, {
+			.rx_buf = rx_buf,
+			.len = len,
+		},
+	};
+
+	spi_message_init(&m);
+	spi_message_add_tail(&t[0], &m);
+	spi_message_add_tail(&t[1], &m);
+	return spi_sync(cpld->spi, &m);
+}
+
+static int rb4xx_cpld_cmd(struct rb4xx_cpld *cpld, const void *tx_buf,
+			  unsigned int len)
+{
+	struct spi_message m;
+	struct spi_transfer t = {
+		.tx_buf = tx_buf,
+		.len = len,
+	};
+
+	spi_message_init(&m);
+	spi_message_add_tail(&t, &m);
+	return spi_sync(cpld->spi, &m);
+}
+
+static int rb4xx_cpld_gpio_set_0_7(struct rb4xx_cpld *cpld, u8 values)
+{
+	/* GPIO 0-7 change can be sent via command + bitfield */
+	u8 cmd[2] = {
+		CPLD_CMD_WRITE_CFG, values
+	};
+	return rb4xx_cpld_cmd(cpld, &cmd, 2);
+}
+
+static int rb4xx_cpld_gpio_set_8(struct rb4xx_cpld *cpld, u8 value)
+{
+	/* GPIO 8 uses dedicated high/low commands */
+	u8 cmd = CPLD_CMD_GPIO8_HIGH | !!(value);
+	return rb4xx_cpld_cmd(cpld, &cmd, 1);
+}
+
+static const struct mfd_cell rb4xx_cpld_cells[] = {
+	{
+		.name = "mikrotik,rb4xx-gpio",
+		.of_compatible = "mikrotik,rb4xx-gpio",
+	}, {
+		.name = "mikrotik,rb4xx-nand",
+		.of_compatible = "mikrotik,rb4xx-nand",
+	},
+};
+
+static int rb4xx_cpld_probe(struct spi_device *spi)
+{
+	struct device *dev = &spi->dev;
+	struct rb4xx_cpld *cpld;
+	int ret;
+
+	cpld = devm_kzalloc(dev, sizeof(*cpld), GFP_KERNEL);
+	if (!cpld)
+		return -ENOMEM;
+
+	dev_set_drvdata(dev, cpld);
+
+	cpld->spi		= spi;
+	cpld->write_nand	= rb4xx_cpld_write_nand;
+	cpld->read_nand		= rb4xx_cpld_read_nand;
+	cpld->gpio_set_0_7	= rb4xx_cpld_gpio_set_0_7;
+	cpld->gpio_set_8	= rb4xx_cpld_gpio_set_8;
+
+	spi->mode = SPI_MODE_0 | SPI_TX_DUAL;
+	ret = spi_setup(spi);
+	if (ret)
+		return ret;
+
+	return devm_mfd_add_devices(dev, PLATFORM_DEVID_NONE,
+				    rb4xx_cpld_cells,
+				    ARRAY_SIZE(rb4xx_cpld_cells),
+				    NULL, 0, NULL);
+}
+
+static int rb4xx_cpld_remove(struct spi_device *spi)
+{
+	return 0;
+}
+
+static const struct of_device_id rb4xx_cpld_dt_match[] = {
+	{ .compatible = "mikrotik,rb4xx-cpld", },
+	{ },
+};
+MODULE_DEVICE_TABLE(of, rb4xx_cpld_dt_match);
+
+static struct spi_driver rb4xx_cpld_driver = {
+	.probe = rb4xx_cpld_probe,
+	.remove = rb4xx_cpld_remove,
+	.driver = {
+		.name = "rb4xx-cpld",
+		.bus = &spi_bus_type,
+		.of_match_table = of_match_ptr(rb4xx_cpld_dt_match),
+	},
+};
+
+module_spi_driver(rb4xx_cpld_driver);
+
+MODULE_DESCRIPTION("Mikrotik RB4xx CPLD driver");
+MODULE_AUTHOR("Gabor Juhos <juhosg@openwrt.org>");
+MODULE_AUTHOR("Imre Kaloz <kaloz@openwrt.org>");
+MODULE_AUTHOR("Bert Vermeulen <bert@biot.com>");
+MODULE_AUTHOR("Christopher Hill <ch6574@gmail.com");
+MODULE_LICENSE("GPL v2");
diff --git a/target/linux/ath79/files/drivers/mtd/nand/raw/nand_rb4xx.c b/target/linux/ath79/files/drivers/mtd/nand/raw/nand_rb4xx.c
new file mode 100644
index 0000000000..922d7095cb
--- /dev/null
+++ b/target/linux/ath79/files/drivers/mtd/nand/raw/nand_rb4xx.c
@@ -0,0 +1,297 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * NAND driver for the MikroTik RouterBoard 4xx series
+ *
+ * Copyright (C) 2008-2011 Gabor Juhos <juhosg@openwrt.org>
+ * Copyright (C) 2008 Imre Kaloz <kaloz@openwrt.org>
+ * Copyright (C) 2015 Bert Vermeulen <bert@biot.com>
+ * Copyright (C) 2020 Christopher Hill <ch6574@gmail.com>
+ *
+ * This file was based on the driver for Linux 2.6.22 published by
+ * MikroTik for their RouterBoard 4xx series devices.
+ *
+ * N.B. driver probe reports "DMA mask not set" warnings which are
+ * an artifact of using a platform_driver as an MFD device child.
+ * See conversation here https://lkml.org/lkml/2020/4/28/675
+ */
+#include <linux/platform_device.h>
+#include <linux/mtd/rawnand.h>
+#include <linux/gpio/consumer.h>
+#include <linux/module.h>
+#include <linux/of_device.h>
+#include <linux/version.h>
+
+#include <mfd/rb4xx-cpld.h>
+
+struct rb4xx_nand {
+	struct rb4xx_cpld *cpld;
+	struct device *dev;
+
+	struct nand_chip chip;
+	struct gpio_desc *ale;
+	struct gpio_desc *cle;
+	struct gpio_desc *nce;
+	struct gpio_desc *rdy;
+};
+
+static int rb4xx_ooblayout_ecc(struct mtd_info *mtd, int section,
+			       struct mtd_oob_region *oobregion)
+{
+	switch (section) {
+	case 0:
+		oobregion->offset = 8;
+		oobregion->length = 3;
+		return 0;
+	case 1:
+		oobregion->offset = 13;
+		oobregion->length = 3;
+		return 0;
+	default:
+		return -ERANGE;
+	}
+}
+
+static int rb4xx_ooblayout_free(struct mtd_info *mtd, int section,
+				struct mtd_oob_region *oobregion)
+{
+	switch (section) {
+	case 0:
+		oobregion->offset = 0;
+		oobregion->length = 4;
+		return 0;
+	case 1:
+		oobregion->offset = 4;
+		oobregion->length = 1;
+		return 0;
+	case 2:
+		oobregion->offset = 6;
+		oobregion->length = 2;
+		return 0;
+	case 3:
+		oobregion->offset = 11;
+		oobregion->length = 2;
+		return 0;
+	default:
+		return -ERANGE;
+	}
+}
+
+static const struct mtd_ooblayout_ops rb4xx_nand_ecclayout_ops = {
+	.ecc = rb4xx_ooblayout_ecc,
+	.free = rb4xx_ooblayout_free,
+};
+
+#if LINUX_VERSION_CODE < KERNEL_VERSION(4,20,0)
+static uint8_t rb4xx_nand_read_byte(struct mtd_info *mtd)
+{
+	struct rb4xx_nand *nand = mtd->priv;
+#else
+static u8 rb4xx_nand_read_byte(struct nand_chip *chip)
+{
+	struct rb4xx_nand *nand = chip->priv;
+#endif
+	struct rb4xx_cpld *cpld = nand->cpld;
+	u8 data;
+	int ret;
+
+	ret = cpld->read_nand(cpld, &data, 1);
+	if (unlikely(ret))
+		return 0xff;
+
+	return data;
+}
+
+#if LINUX_VERSION_CODE < KERNEL_VERSION(4,20,0)
+static void rb4xx_nand_write_buf(struct mtd_info *mtd, const uint8_t *buf,
+				 int len)
+{
+	struct rb4xx_nand *nand = mtd->priv;
+#else
+static void rb4xx_nand_write_buf(struct nand_chip *chip, const u8 *buf, int len)
+{
+	struct rb4xx_nand *nand = chip->priv;
+#endif
+	struct rb4xx_cpld *cpld = nand->cpld;
+
+	cpld->write_nand(cpld, buf, len);
+}
+
+#if LINUX_VERSION_CODE < KERNEL_VERSION(4,20,0)
+static void rb4xx_nand_read_buf(struct mtd_info *mtd, uint8_t *buf, int len)
+{
+	struct rb4xx_nand *nand = mtd->priv;
+#else
+static void rb4xx_nand_read_buf(struct nand_chip *chip, u8 *buf, int len)
+{
+	struct rb4xx_nand *nand = chip->priv;
+#endif
+	struct rb4xx_cpld *cpld = nand->cpld;
+
+	cpld->read_nand(cpld, buf, len);
+}
+
+#if LINUX_VERSION_CODE < KERNEL_VERSION(4,20,0)
+static void rb4xx_nand_cmd_ctrl(struct mtd_info *mtd, int dat, unsigned int ctrl)
+{
+	struct rb4xx_nand *nand = mtd->priv;
+#else
+static void rb4xx_nand_cmd_ctrl(struct nand_chip *chip, int dat,
+				unsigned int ctrl)
+{
+	struct rb4xx_nand *nand = chip->priv;
+#endif
+	struct rb4xx_cpld *cpld = nand->cpld;
+	u8 data = dat;
+
+	if (ctrl & NAND_CTRL_CHANGE) {
+		gpiod_set_value_cansleep(nand->cle, !!(ctrl & NAND_CLE));
+		gpiod_set_value_cansleep(nand->ale, !!(ctrl & NAND_ALE));
+		gpiod_set_value_cansleep(nand->nce,  !(ctrl & NAND_NCE));
+	}
+
+	if (dat != NAND_CMD_NONE)
+		cpld->write_nand(cpld, &data, 1);
+}
+
+#if LINUX_VERSION_CODE < KERNEL_VERSION(4,20,0)
+static int rb4xx_nand_dev_ready(struct mtd_info *mtd)
+{
+	struct rb4xx_nand *nand = mtd->priv;
+#else
+static int rb4xx_nand_dev_ready(struct nand_chip *chip)
+{
+	struct rb4xx_nand *nand = chip->priv;
+#endif
+
+	return gpiod_get_value_cansleep(nand->rdy);
+}
+
+static int rb4xx_nand_probe(struct platform_device *pdev)
+{
+	struct device *dev = &pdev->dev;
+	struct device *parent = dev->parent;
+	struct rb4xx_nand *nand;
+	struct mtd_info *mtd;
+	int ret;
+
+	if (!parent)
+		return -ENODEV;
+
+	nand = devm_kzalloc(dev, sizeof(*nand), GFP_KERNEL);
+	if (!nand)
+		return -ENOMEM;
+
+	platform_set_drvdata(pdev, nand);
+	nand->cpld	= dev_get_drvdata(parent);
+	nand->dev	= dev;
+
+	nand->ale = devm_gpiod_get_index(dev, NULL, 0, GPIOD_OUT_LOW);
+	if (IS_ERR(nand->ale))
+		dev_err(dev, "missing gpio ALE: %ld\n", PTR_ERR(nand->ale));
+
+	nand->cle = devm_gpiod_get_index(dev, NULL, 1, GPIOD_OUT_LOW);
+	if (IS_ERR(nand->cle))
+		dev_err(dev, "missing gpio CLE: %ld\n", PTR_ERR(nand->cle));
+
+	nand->nce = devm_gpiod_get_index(dev, NULL, 2, GPIOD_OUT_LOW);
+	if (IS_ERR(nand->nce))
+		dev_err(dev, "missing gpio nCE: %ld\n", PTR_ERR(nand->nce));
+
+	nand->rdy = devm_gpiod_get_index(dev, NULL, 3, GPIOD_IN);
+	if (IS_ERR(nand->rdy))
+		dev_err(dev, "missing gpio RDY: %ld\n", PTR_ERR(nand->rdy));
+
+	if (IS_ERR(nand->ale) || IS_ERR(nand->cle) ||
+	    IS_ERR(nand->nce) || IS_ERR(nand->rdy))
+		return -ENOENT;
+
+	gpiod_set_consumer_name(nand->ale, "mikrotik:nand:ALE");
+	gpiod_set_consumer_name(nand->cle, "mikrotik:nand:CLE");
+	gpiod_set_consumer_name(nand->nce, "mikrotik:nand:nCE");
+	gpiod_set_consumer_name(nand->rdy, "mikrotik:nand:RDY");
+
+	mtd = nand_to_mtd(&nand->chip);
+	mtd->priv	= nand;
+	mtd->owner	= THIS_MODULE;
+	mtd->dev.parent	= dev;
+	mtd_set_of_node(mtd, dev->of_node);
+
+	if (mtd->writesize == 512)
+		mtd_set_ooblayout(mtd, &rb4xx_nand_ecclayout_ops);
+
+	nand->chip.ecc.mode	= NAND_ECC_SOFT;
+	nand->chip.ecc.algo	= NAND_ECC_HAMMING;
+	nand->chip.options	= NAND_NO_SUBPAGE_WRITE;
+	nand->chip.priv		= nand;
+
+#if LINUX_VERSION_CODE < KERNEL_VERSION(4,20,0)
+	nand->chip.read_byte	= rb4xx_nand_read_byte;
+	nand->chip.write_buf	= rb4xx_nand_write_buf;
+	nand->chip.read_buf	= rb4xx_nand_read_buf;
+	nand->chip.cmd_ctrl	= rb4xx_nand_cmd_ctrl;
+	nand->chip.dev_ready	= rb4xx_nand_dev_ready;
+	nand->chip.chip_delay	= 25;
+
+	ret = nand_scan(mtd, 1);
+#else
+	nand->chip.legacy.read_byte	= rb4xx_nand_read_byte;
+	nand->chip.legacy.write_buf	= rb4xx_nand_write_buf;
+	nand->chip.legacy.read_buf	= rb4xx_nand_read_buf;
+	nand->chip.legacy.cmd_ctrl	= rb4xx_nand_cmd_ctrl;
+	nand->chip.legacy.dev_ready	= rb4xx_nand_dev_ready;
+	nand->chip.legacy.chip_delay	= 25;
+
+	ret = nand_scan(&nand->chip, 1);
+#endif
+	if (ret)
+		return -ENXIO;
+
+	ret = mtd_device_register(mtd, NULL, 0);
+	if (ret) {
+#if LINUX_VERSION_CODE < KERNEL_VERSION(4,20,0)
+		nand_release(mtd);
+#else
+		nand_release(&nand->chip);
+#endif
+		return ret;
+	}
+
+	return 0;
+}
+
+static int rb4xx_nand_remove(struct platform_device *pdev)
+{
+	struct rb4xx_nand *nand = platform_get_drvdata(pdev);
+
+#if LINUX_VERSION_CODE < KERNEL_VERSION(4,20,0)
+	nand_release(nand_to_mtd(&nand->chip));
+#else
+	nand_release(&nand->chip);
+#endif
+
+	return 0;
+}
+
+static const struct platform_device_id rb4xx_nand_id_table[] = {
+	{ "mikrotik,rb4xx-nand", },
+	{ },
+};
+MODULE_DEVICE_TABLE(platform, rb4xx_nand_id_table);
+
+static struct platform_driver rb4xx_nand_driver = {
+	.probe = rb4xx_nand_probe,
+	.remove = rb4xx_nand_remove,
+	.id_table = rb4xx_nand_id_table,
+	.driver = {
+		.name = "rb4xx-nand",
+	},
+};
+
+module_platform_driver(rb4xx_nand_driver);
+
+MODULE_DESCRIPTION("Mikrotik RB4xx NAND driver");
+MODULE_AUTHOR("Gabor Juhos <juhosg@openwrt.org>");
+MODULE_AUTHOR("Imre Kaloz <kaloz@openwrt.org>");
+MODULE_AUTHOR("Bert Vermeulen <bert@biot.com>");
+MODULE_AUTHOR("Christopher Hill <ch6574@gmail.com");
+MODULE_LICENSE("GPL v2");
diff --git a/target/linux/ath79/files/include/mfd/rb4xx-cpld.h b/target/linux/ath79/files/include/mfd/rb4xx-cpld.h
new file mode 100644
index 0000000000..d73617bfb1
--- /dev/null
+++ b/target/linux/ath79/files/include/mfd/rb4xx-cpld.h
@@ -0,0 +1,25 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * CPLD driver for the MikroTik RouterBoard 4xx series
+ *
+ * Copyright (C) 2008-2011 Gabor Juhos <juhosg@openwrt.org>
+ * Copyright (C) 2008 Imre Kaloz <kaloz@openwrt.org>
+ * Copyright (C) 2015 Bert Vermeulen <bert@biot.com>
+ * Copyright (C) 2020 Christopher Hill <ch6574@gmail.com>
+ *
+ * This file was based on the driver for Linux 2.6.22 published by
+ * MikroTik for their RouterBoard 4xx series devices.
+ */
+#include <linux/spi/spi.h>
+
+struct rb4xx_cpld {
+	struct spi_device *spi;
+
+	int (*write_nand)(struct rb4xx_cpld *self, const void *tx_buf,
+			  unsigned int len);
+	int (*read_nand)(struct rb4xx_cpld *self, void *rx_buf,
+			 unsigned int len);
+
+	int (*gpio_set_0_7)(struct rb4xx_cpld *self, u8 values);
+	int (*gpio_set_8)(struct rb4xx_cpld *self, u8 value);
+};