Bootcount driver using NVMEM cell as backend.

Description
-----------
Implement a bootcount (1) related driver that uses NVMEM as a backend. The patchset will also update the snvs_lpgpr driver to support 2 bytes NVMEM cells.

1. https://www.denx.de/wiki/view/DULG/UBootBootCountLimit

Testing
-------
Hardware: i.MX6sx evaluation board
Kernel: linux-imx-5.4.24

1. Configure the bootcount hardware in DT to use a NVMEM cell provided by snvs_lpgpr driver as backend.
e.g. This will configure a 4 bytes long NVMEM cell

```
  bootcount_regs: bootcount-snvs-regs@0 {
      reg = <0x0 0x04>;
  };
```

2. Record the current NVMEM cell content:

```
~ # hexdump -C
/sys/devices/soc0/soc/2000000.aips-bus/20cc000.snvs/20cc000.snvs:snvs-lpgpr/20cc000.snvs:snvs-lpgpr0/nvmem
00000000  00 00 01 b0                                       |....|
00000004
```

3. Write the bootcount and check that is successful: PASS

```
~ # echo 1 > /sys/bus/platform/drivers/bootcount-nvmem/bootcount/value
bootcount: Write regval: 0xb0010001
~ # hexdump -C
/sys/devices/soc0/soc/2000000.aips-bus/20cc000.snvs/20cc000.snvs:snvs-lpgpr/20cc000.snvs:snvs-lpgpr0/nvmem
00000000  01 00 01 b0                                       |....|
00000004
``` 

Note: similar tests were done also for 2 bytes NVMEM cell size.

Kernel: linux-next, tag: next-20210322
1. Enable bootcount and snvs_lpgpr drivers
2. Verify that they compile succesfully: PASS

```
kernel-master> make -j2 drivers/nvmem/
  DESCEND  objtool
  CALL    scripts/atomic/check-atomics.sh
  CC      arch/x86/kernel/asm-offsets.s
  CALL    scripts/checksyscalls.sh
  CC      drivers/nvmem/core.o
  CC      drivers/nvmem/bootcount-nvmem.o
  CC      drivers/nvmem/snvs_lpgpr.o
  AR      drivers/nvmem/built-in.
```

Testing the bootcount YAML document:
1. Check the document by running the command:
make DT_CHECKER_FLAGS=-m dt_binding_check
2. Verify that is successful: PASS
3. Verify that no warnings are generated by bootcount related yaml file: PASS

```
...
CHECK Documentation/devicetree/bindings/nvmem/brcm,nvram.example.dt.yaml                                                                      DTC Documentation/devicetree/bindings/nvmem/bootcount-nvmem.example.dt.yaml
CHECK Documentation/devicetree/bindings/nvmem/bootcount-nvmem.example.dt.yaml
DTC Documentation/devicetree/bindings/opp/allwinner,sun50i-h6-operating-points.example.dt.yaml
...
```

Changes since v1
----------------
- Fix the dt-bindings YAML document.

Changes since v2
----------------
- Fix the dt-bindings YAML document warnings.

Nandor Han (4):
dt-bindings: nvmem: Add bootcount-nvmem
nvmem: bootcount: add bootcount driver
nvmem: snvs_lpgpr: use cell stride for regmap size calculation
nvmem: snvs_lpgpr: support two bytes NVMEM cell size

 .../bindings/nvmem/bootcount-nvmem.yaml       |  74 +++++++
 drivers/nvmem/Kconfig                         |  10 +
 drivers/nvmem/Makefile                        |   1 +
 drivers/nvmem/bootcount-nvmem.c               | 195 ++++++++++++++++++
 drivers/nvmem/snvs_lpgpr.c                    |  67 +++++-
 5 files changed, 341 insertions(+), 6 deletions(-)
 create mode 100644 Documentation/devicetree/bindings/nvmem/bootcount-nvmem.yaml
 create mode 100644 drivers/nvmem/bootcount-nvmem.c

2.26.3

Documents the device tree bindings for bootcount-nvmem driver.

---
 .../bindings/nvmem/bootcount-nvmem.yaml       | 74 +++++++++++++++++++
 1 file changed, 74 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/nvmem/bootcount-nvmem.yaml

new file mode 100644
index 000000000000..d17406449c6d
@@ -0,0 +1,74 @@
+# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
+# Copyright (c) Vaisala Oyj. All rights reserved.
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/nvmem/bootcount-nvmem.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Bootcount NVMEM bindings
+
+maintainers:
+  - Nandor Han <[email protected]>
+
+description: |
+  This binding is intendent to describe the hardware location for
+  storing the bootcount value and magic combo.
+
+  The NVMEM cell size should be 2 or 4 bytes.
+
+allOf:
+  - $ref: "nvmem-consumer.yaml#"
+
+properties:
+  compatible:
+    enum:
+      - linux,bootcount-nvmem
+
+  nvmem-cells:
+    description: Phandle to reboot mode nvmem data cell.
+    $ref: /schemas/types.yaml#/definitions/phandle-array
+
+  nvmem-cell-names:
+    description: Name of the NVMEM cell.
+    $ref: /schemas/types.yaml#/definitions/string-array
+    enum:
+      - bootcount-regs
+
+  linux,bootcount-magic:
+    description: Override default mask value.
+    $ref: /schemas/types.yaml#/definitions/uint32
+
+dependencies:
+  nvmem-cell-names: [ nvmem-cells ]
+
+required:
+  - compatible
+  - nvmem-cells
+  - nvmem-cell-names
+
+additionalProperties: false
+
+examples:
+  # example with 16 bit nvram cell:
+  - |
+    bootcount {
+        compatible = "linux,bootcount-nvmem";
+        nvmem-cells = <&bootcount_regs>;
+        nvmem-cell-names = "bootcount-regs";
+    };
+
+    rtc: rtc@68 {
+        #address-cells = <1>;
+        #size-cells = <1>;
+
+        reg = <0x68 0x10>;
+
+        bootcount_regs: bootcount_nvmem_regs@e {
+            #address-cells = <1>;
+            #size-cells = <1>;
+
+            reg = <0x0e 0x2>;
+        };
+    };
+
+...

2.26.3

In order to have a robust system we want to be able to identify and take actions if a boot loop occurs. This is possible by using the bootcount feature, which can be used to identify the number of times device has booted since bootcount was last time reset. Bootcount feature (1) requires a collaboration between bootloader and user-space, where the bootloader will increase a counter and user-space reset it. If the counter is not reset and a pre-established threshold is reached, bootloader can react and take action.

This is the kernel side implementation, which can be used to identify the number of times device has booted since bootcount was last time reset.

The driver supports both 16 and 32 bits NVMEM cell size.

1) https://www.denx.de/wiki/DULG/UBootBootCountLimit

---
 drivers/nvmem/Kconfig           |  10 ++
 drivers/nvmem/Makefile          |   1 +
 drivers/nvmem/bootcount-nvmem.c | 195 ++++++++++++++++++++++++++++++++
 3 files changed, 206 insertions(+)
 create mode 100644 drivers/nvmem/bootcount-nvmem.c

index dd2019006838..d5413c937350 100644
@@ -288,4 +288,14 @@ config NVMEM_BRCM_NVRAM
 	  This driver provides support for Broadcom's NVRAM that can be accessed
 	  using I/O mapping.
 
+config BOOTCOUNT_NVMEM
+	bool "Bootcount driver using nvmem registers"
+	depends on OF
+	depends on NVMEM
+	help
+	  Driver that implements the bootcount feature support using a
+	  NVMEM cell as a backend. The driver supports 2 and 4 bytes
+	  size cells.
+
+	  Say y here to enable bootcount support.
 endif

index bbea1410240a..4c77679bbf0d 100644
@@ -59,3 +59,4 @@ obj-$(CONFIG_NVMEM_RMEM) 	+= nvmem-rmem.o
 nvmem-rmem-y			:= rmem.o
 obj-$(CONFIG_NVMEM_BRCM_NVRAM)	+= nvmem_brcm_nvram.o
 nvmem_brcm_nvram-y		:= brcm_nvram.o
+obj-$(CONFIG_BOOTCOUNT_NVMEM)	+= bootcount-nvmem.o

2.26.3

Using a hard-coded value for calculating the number of registers to read makes future changes more challenging.

Change the calculation to use the NVMEM cell stride instead of a hard coded value. This will allow specifying different NVMEM cell sizes.

---
 drivers/nvmem/snvs_lpgpr.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

index 4692aa985bd6..35457421314a 100644
@@ -72,7 +72,7 @@ static int snvs_lpgpr_write(void *context, unsigned int offset, void *val,
 		return -EPERM;
 
 	return regmap_bulk_write(priv->regmap, dcfg->offset + offset, val,
-				bytes / 4);
+				 bytes / priv->cfg.stride);
 }
 
 static int snvs_lpgpr_read(void *context, unsigned int offset, void *val,

@@ -81,8 +81,8 @@ static int snvs_lpgpr_read(void *context, unsigned int offset, void *val,
 	struct snvs_lpgpr_priv *priv = context;
 	const struct snvs_lpgpr_cfg *dcfg = priv->dcfg;
 
-	return regmap_bulk_read(priv->regmap, dcfg->offset + offset,
-			       val, bytes / 4);
+	return regmap_bulk_read(priv->regmap, dcfg->offset + offset, val,
+				bytes / priv->cfg.stride);
 }
 
 static int snvs_lpgpr_probe(struct platform_device *pdev)

2.26.3

In some situation is desired to use less than 4 bytes for storing data. This will allow using the same register for multiple purposes.

Add support for allowing 2 bytes granularity for NVMEM cells.

---
 drivers/nvmem/snvs_lpgpr.c | 67 ++++++++++++++++++++++++++++++++++----
 1 file changed, 61 insertions(+), 6 deletions(-)

index 35457421314a..44614f3d68f0 100644
@@ -21,6 +21,9 @@
 #define IMX_GPR_SL		BIT(5)
 #define IMX_GPR_HL		BIT(5)
 
+#define REGMAP_FIELD_SIZE 16
+#define REGMAP_FIELDS_PER_REG 2
+
 struct snvs_lpgpr_cfg {
 	int offset;
 	int offset_hplr;

@@ -33,6 +36,7 @@ struct snvs_lpgpr_priv {
 	struct regmap			*regmap;
 	struct nvmem_config		cfg;
 	const struct snvs_lpgpr_cfg	*dcfg;
+	struct regmap_field **reg_fields;
 };
 
 static const struct snvs_lpgpr_cfg snvs_lpgpr_cfg_imx6q = {

@@ -56,6 +60,11 @@ static int snvs_lpgpr_write(void *context, unsigned int offset, void *val,
 	const struct snvs_lpgpr_cfg *dcfg = priv->dcfg;
 	unsigned int lock_reg;
 	int ret;
+	u32 regval;
+	unsigned int field_id;
+
+	if (offset + bytes > dcfg->size)
+		return -EINVAL;
 
 	ret = regmap_read(priv->regmap, dcfg->offset_hplr, &lock_reg);
 	if (ret < 0)

@@ -71,8 +80,16 @@ static int snvs_lpgpr_write(void *context, unsigned int offset, void *val,
 	if (lock_reg & IMX_GPR_HL)
 		return -EPERM;
 
-	return regmap_bulk_write(priv->regmap, dcfg->offset + offset, val,
-				 bytes / priv->cfg.stride);
+	if (bytes == (REGMAP_FIELD_SIZE >> 3)) {
+		regval = *(u16 *)(val);
+		field_id = offset / REGMAP_FIELDS_PER_REG;
+		ret = regmap_field_write(priv->reg_fields[field_id], regval);
+	} else {
+		ret = regmap_bulk_write(priv->regmap, dcfg->offset + offset,
+					val, bytes / priv->cfg.stride);
+	}
+
+	return ret;
 }
 
 static int snvs_lpgpr_read(void *context, unsigned int offset, void *val,

@@ -80,9 +97,27 @@ static int snvs_lpgpr_read(void *context, unsigned int offset, void *val,
 {
 	struct snvs_lpgpr_priv *priv = context;
 	const struct snvs_lpgpr_cfg *dcfg = priv->dcfg;
+	int ret;
+	u32 regval;
+	unsigned int field_id;
 
-	return regmap_bulk_read(priv->regmap, dcfg->offset + offset, val,
-				bytes / priv->cfg.stride);
+	if (offset + bytes > dcfg->size)
+		return -EINVAL;
+
+	if (bytes == (REGMAP_FIELD_SIZE >> 3)) {
+		field_id = offset / REGMAP_FIELDS_PER_REG;
+		ret = regmap_field_read(priv->reg_fields[field_id], &regval);
+		if (ret)
+			return ret;
+
+		*(u16 *)(val) = regval;
+	} else {
+		ret = regmap_bulk_read(priv->regmap, dcfg->offset + offset, val,
+				       bytes / priv->cfg.stride);
+		if (ret)
+			return ret;
+	}
+	return 0;
 }
 
 static int snvs_lpgpr_probe(struct platform_device *pdev)

@@ -94,6 +129,8 @@ static int snvs_lpgpr_probe(struct platform_device *pdev)
 	struct nvmem_config *cfg;
 	struct nvmem_device *nvmem;
 	const struct snvs_lpgpr_cfg *dcfg;
+	int i;
+	int fields_count;
 
 	if (!node)
 		return -ENOENT;

@@ -121,13 +158,31 @@ static int snvs_lpgpr_probe(struct platform_device *pdev)
 	cfg->priv = priv;
 	cfg->name = dev_name(dev);
 	cfg->dev = dev;
-	cfg->stride = 4;
-	cfg->word_size = 4;
+	cfg->stride = 2;
+	cfg->word_size = 2;
 	cfg->size = dcfg->size;
 	cfg->owner = THIS_MODULE;
 	cfg->reg_read  = snvs_lpgpr_read;
 	cfg->reg_write = snvs_lpgpr_write;
 
+	fields_count = priv->dcfg->size / priv->cfg.stride;
+	priv->reg_fields = devm_kzalloc(
+		dev, sizeof(struct regmap_field *) * fields_count, GFP_KERNEL);
+	if (!priv->reg_fields)
+		return -ENOMEM;
+
+	for (i = 0; i < fields_count; i++) {
+		size_t field_start = i * REGMAP_FIELD_SIZE;
+		size_t field_end = field_start + REGMAP_FIELD_SIZE - 1;
+		const struct reg_field field =
+			REG_FIELD(dcfg->offset, field_start, field_end);
+
+		priv->reg_fields[i] =
+			devm_regmap_field_alloc(dev, priv->regmap, field);
+		if (IS_ERR(priv->reg_fields[i]))
+			return PTR_ERR(priv->reg_fields[i]);
+	}
+
 	nvmem = devm_nvmem_register(dev, cfg);
 
 	return PTR_ERR_OR_ZERO(nvmem);

2.26.3