soc: at91: Add Atmel SFR SN (Serial Number) support

Add support to read SFR's read-only registers providing the SoC
Serial Numbers (SN0+SN1) to userspace.

~ #  hexdump -n 8  -e'"%d\n"' /sys/bus/nvmem/devices/atmel-sfr0/nvmem
959527243
371539274

Signed-off-by: Kamel Bouhara <kamel.bouhara@bootlin.com>
Tested-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20191004151802.21793-1-kamel.bouhara@bootlin.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/soc/atmel/Kconfig

@@ -5,3 +5,14 @@ config AT91_SOC_ID
 	default ARCH_AT91
 	help
 	  Include support for the SoC bus on the Atmel ARM SoCs.
+
+config AT91_SOC_SFR
+	tristate "Special Function Registers support"
+	depends on ARCH_AT91 || COMPILE_TEST
+	help
+	  This is a driver for the Special Function Registers available on
+	  Atmel SAMA5Dx SoCs, providing access to specific aspects of the
+	  integrated memory, bridge implementations, processor etc.
+
+	  This driver can also be built as a module. If so, the module
+	  will be called sfr.

drivers/soc/atmel/Makefile

@@ -1,2 +1,3 @@
 # SPDX-License-Identifier: GPL-2.0-only
 obj-$(CONFIG_AT91_SOC_ID) += soc.o
+obj-$(CONFIG_AT91_SOC_SFR) += sfr.o

drivers/soc/atmel/sfr.c

@@ -0,0 +1,99 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * sfr.c - driver for special function registers
+ *
+ * Copyright (C) 2019 Bootlin.
+ *
+ */
+#include <linux/mfd/syscon.h>
+#include <linux/module.h>
+#include <linux/nvmem-provider.h>
+#include <linux/random.h>
+#include <linux/of.h>
+#include <linux/of_device.h>
+#include <linux/platform_device.h>
+#include <linux/regmap.h>
+
+#define SFR_SN0		0x4c
+#define SFR_SN_SIZE	8
+
+struct atmel_sfr_priv {
+	struct regmap			*regmap;
+};
+
+static int atmel_sfr_read(void *context, unsigned int offset,
+			  void *buf, size_t bytes)
+{
+	struct atmel_sfr_priv *priv = context;
+
+	return regmap_bulk_read(priv->regmap, SFR_SN0 + offset,
+				buf, bytes / 4);
+}
+
+static struct nvmem_config atmel_sfr_nvmem_config = {
+	.name = "atmel-sfr",
+	.read_only = true,
+	.word_size = 4,
+	.stride = 4,
+	.size = SFR_SN_SIZE,
+	.reg_read = atmel_sfr_read,
+};
+
+static int atmel_sfr_probe(struct platform_device *pdev)
+{
+	struct device *dev = &pdev->dev;
+	struct device_node *np = dev->of_node;
+	struct nvmem_device *nvmem;
+	struct atmel_sfr_priv *priv;
+	u8 sn[SFR_SN_SIZE];
+	int ret;
+
+	priv = devm_kmalloc(dev, sizeof(*priv), GFP_KERNEL);
+	if (!priv)
+		return -ENOMEM;
+
+	priv->regmap = syscon_node_to_regmap(np);
+	if (IS_ERR(priv->regmap)) {
+		dev_err(dev, "cannot get parent's regmap\n");
+		return PTR_ERR(priv->regmap);
+	}
+
+	atmel_sfr_nvmem_config.dev = dev;
+	atmel_sfr_nvmem_config.priv = priv;
+
+	nvmem = devm_nvmem_register(dev, &atmel_sfr_nvmem_config);
+	if (IS_ERR(nvmem)) {
+		dev_err(dev, "error registering nvmem config\n");
+		return PTR_ERR(nvmem);
+	}
+
+	ret = atmel_sfr_read(priv, 0, sn, SFR_SN_SIZE);
+	if (ret == 0)
+		add_device_randomness(sn, SFR_SN_SIZE);
+
+	return ret;
+}
+
+static const struct of_device_id atmel_sfr_dt_ids[] = {
+	{
+		.compatible = "atmel,sama5d2-sfr",
+	}, {
+		.compatible = "atmel,sama5d4-sfr",
+	}, {
+		/* sentinel */
+	},
+};
+MODULE_DEVICE_TABLE(of, atmel_sfr_dt_ids);
+
+static struct platform_driver atmel_sfr_driver = {
+	.probe = atmel_sfr_probe,
+	.driver = {
+		.name = "atmel-sfr",
+		.of_match_table = atmel_sfr_dt_ids,
+	},
+};
+module_platform_driver(atmel_sfr_driver);
+
+MODULE_AUTHOR("Kamel Bouhara <kamel.bouhara@bootlin.com>");
+MODULE_DESCRIPTION("Atmel SFR SN driver for SAMA5D2/4 SoC family");
+MODULE_LICENSE("GPL v2");