nvmem: core: Implement force_ro sysfs attribute

Implement "force_ro" sysfs attribute to allow users to set read-write
devices as read-only and back to read-write from userspace. The choice
of the name is based on MMC core 'force_ro' attribute.

This solves a situation where an AT24 I2C EEPROM with GPIO based nWP
signal may have to be occasionally updated. Such I2C EEPROM device is
usually set as read-only during most of the regular system operation,
but in case it has to be updated in a controlled manner, it could be
unlocked using this new "force_ro" sysfs attribute and then re-locked
again.

The "read-only" DT property and config->read_only configuration is
respected and is used to set default state of the device, read-only
or read-write, for devices which do implement .reg_write function.
For devices which do not implement .reg_write function, the device
is unconditionally read-only and the "force_ro" attribute is not
visible.

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20240705074852.423202-16-srinivas.kandagatla@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/ABI/stable/sysfs-bus-nvmem

@@ -1,3 +1,20 @@
+What:		/sys/bus/nvmem/devices/.../force_ro
+Date:		June 2024
+KernelVersion:	6.11
+Contact:	Marek Vasut <marex@denx.de>
+Description:
+		This read/write attribute allows users to set read-write
+		devices as read-only and back to read-write from userspace.
+		This can be used to unlock and relock write-protection of
+		devices which are generally locked, except during sporadic
+		programming operation.
+		Read returns '0' or '1' for read-write or read-only modes
+		respectively.
+		Write parses one of 'YyTt1NnFf0', or [oO][NnFf] for "on"
+		and "off", i.e. what kstrbool() supports.
+		Note: This file is only present if CONFIG_NVMEM_SYSFS
+		is enabled.
+
 What:		/sys/bus/nvmem/devices/.../nvmem
 Date:		July 2015
 KernelVersion:	4.2

drivers/nvmem/core.c

@@ -184,7 +184,30 @@ static ssize_t type_show(struct device *dev,
 
 static DEVICE_ATTR_RO(type);
 
+static ssize_t force_ro_show(struct device *dev, struct device_attribute *attr,
+			     char *buf)
+{
+	struct nvmem_device *nvmem = to_nvmem_device(dev);
+
+	return sysfs_emit(buf, "%d\n", nvmem->read_only);
+}
+
+static ssize_t force_ro_store(struct device *dev, struct device_attribute *attr,
+			      const char *buf, size_t count)
+{
+	struct nvmem_device *nvmem = to_nvmem_device(dev);
+	int ret = kstrtobool(buf, &nvmem->read_only);
+
+	if (ret < 0)
+		return ret;
+
+	return count;
+}
+
+static DEVICE_ATTR_RW(force_ro);
+
 static struct attribute *nvmem_attrs[] = {
+	&dev_attr_force_ro.attr,
 	&dev_attr_type.attr,
 	NULL,
 };
@@ -285,6 +308,25 @@ static umode_t nvmem_bin_attr_is_visible(struct kobject *kobj,
 	return nvmem_bin_attr_get_umode(nvmem);
 }
 
+static umode_t nvmem_attr_is_visible(struct kobject *kobj,
+				     struct attribute *attr, int i)
+{
+	struct device *dev = kobj_to_dev(kobj);
+	struct nvmem_device *nvmem = to_nvmem_device(dev);
+
+	/*
+	 * If the device has no .reg_write operation, do not allow
+	 * configuration as read-write.
+	 * If the device is set as read-only by configuration, it
+	 * can be forced into read-write mode using the 'force_ro'
+	 * attribute.
+	 */
+	if (attr == &dev_attr_force_ro.attr && !nvmem->reg_write)
+		return 0;	/* Attribute not visible */
+
+	return attr->mode;
+}
+
 static struct nvmem_cell *nvmem_create_cell(struct nvmem_cell_entry *entry,
 					    const char *id, int index);
 
@@ -341,6 +383,7 @@ static const struct attribute_group nvmem_bin_group = {
 	.bin_attrs	= nvmem_bin_attributes,
 	.attrs		= nvmem_attrs,
 	.is_bin_visible = nvmem_bin_attr_is_visible,
+	.is_visible	= nvmem_attr_is_visible,
 };
 
 static const struct attribute_group *nvmem_dev_groups[] = {