ARM: imx: Add serial number support for i.MX6/7 SoCs

i.MX6/7 SoCs have a 64-bit SoC unique ID stored in OCOTP,
it can be used as SoC serial number, add this support for
i.MX6Q/6DL/6SL/6SX/6SLL/6UL/6ULL/6ULZ/7D, see below example
on i.MX6Q:

root@imx6qpdlsolox:~# cat /sys/devices/soc0/serial_number
240F31D4E1FDFCA7
Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/mach-imx/cpu.c

 // SPDX-License-Identifier: GPL-2.0
 #include <linux/err.h>
+#include <linux/mfd/syscon.h>
 #include <linux/module.h>
 #include <linux/io.h>
 #include <linux/of.h>
 #include <linux/of_address.h>
+#include <linux/regmap.h>
 #include <linux/slab.h>
 #include <linux/sys_soc.h>
 
 #include "hardware.h"
 #include "common.h"
 
+#define OCOTP_UID_H	0x420
+#define OCOTP_UID_L	0x410
+
 unsigned int __mxc_cpu_type;
 static unsigned int imx_soc_revision;
 
@@ -76,9 +81,13 @@ void __init imx_aips_allow_unprivileged_access(
 struct device * __init imx_soc_device_init(void)
 {
 	struct soc_device_attribute *soc_dev_attr;
+	const char *ocotp_compat = NULL;
 	struct soc_device *soc_dev;
 	struct device_node *root;
+	struct regmap *ocotp;
 	const char *soc_id;
+	u64 soc_uid = 0;
+	u32 val;
 	int ret;
 
 	soc_dev_attr = kzalloc(sizeof(*soc_dev_attr), GFP_KERNEL);
@@ -119,30 +128,39 @@ struct device * __init imx_soc_device_init(void)
 		soc_id = "i.MX53";
 		break;
 	case MXC_CPU_IMX6SL:
+		ocotp_compat = "fsl,imx6sl-ocotp";
 		soc_id = "i.MX6SL";
 		break;
 	case MXC_CPU_IMX6DL:
+		ocotp_compat = "fsl,imx6q-ocotp";
 		soc_id = "i.MX6DL";
 		break;
 	case MXC_CPU_IMX6SX:
+		ocotp_compat = "fsl,imx6sx-ocotp";
 		soc_id = "i.MX6SX";
 		break;
 	case MXC_CPU_IMX6Q:
+		ocotp_compat = "fsl,imx6q-ocotp";
 		soc_id = "i.MX6Q";
 		break;
 	case MXC_CPU_IMX6UL:
+		ocotp_compat = "fsl,imx6ul-ocotp";
 		soc_id = "i.MX6UL";
 		break;
 	case MXC_CPU_IMX6ULL:
+		ocotp_compat = "fsl,imx6ul-ocotp";
 		soc_id = "i.MX6ULL";
 		break;
 	case MXC_CPU_IMX6ULZ:
+		ocotp_compat = "fsl,imx6ul-ocotp";
 		soc_id = "i.MX6ULZ";
 		break;
 	case MXC_CPU_IMX6SLL:
+		ocotp_compat = "fsl,imx6sll-ocotp";
 		soc_id = "i.MX6SLL";
 		break;
 	case MXC_CPU_IMX7D:
+		ocotp_compat = "fsl,imx7d-ocotp";
 		soc_id = "i.MX7D";
 		break;
 	case MXC_CPU_IMX7ULP:
@@ -153,18 +171,36 @@ struct device * __init imx_soc_device_init(void)
 	}
 	soc_dev_attr->soc_id = soc_id;
 
+	if (ocotp_compat) {
+		ocotp = syscon_regmap_lookup_by_compatible(ocotp_compat);
+		if (IS_ERR(ocotp))
+			pr_err("%s: failed to find %s regmap!\n", __func__, ocotp_compat);
+
+		regmap_read(ocotp, OCOTP_UID_H, &val);
+		soc_uid = val;
+		regmap_read(ocotp, OCOTP_UID_L, &val);
+		soc_uid <<= 32;
+		soc_uid |= val;
+	}
+
 	soc_dev_attr->revision = kasprintf(GFP_KERNEL, "%d.%d",
 					   (imx_soc_revision >> 4) & 0xf,
 					   imx_soc_revision & 0xf);
 	if (!soc_dev_attr->revision)
 		goto free_soc;
 
+	soc_dev_attr->serial_number = kasprintf(GFP_KERNEL, "%016llX", soc_uid);
+	if (!soc_dev_attr->serial_number)
+		goto free_rev;
+
 	soc_dev = soc_device_register(soc_dev_attr);
 	if (IS_ERR(soc_dev))
-		goto free_rev;
+		goto free_serial_number;
 
 	return soc_device_to_device(soc_dev);
 
+free_serial_number:
+	kfree(soc_dev_attr->serial_number);
 free_rev:
 	kfree(soc_dev_attr->revision);
 free_soc: