[PATCH v2 4/4] nvmem: core: Implement force_ro sysfs attribute

Marek Vasut
2024-06-23T03:18:37Z

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>
---
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Bartosz Golaszewski <brgl@bgdev.pl>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Cc: linux-i2c@vger.kernel.org
---
V2: - Document the force_ro attribute
- Use sysfs_emit()
---
Documentation/ABI/stable/sysfs-bus-nvmem | 13 +++++++
drivers/nvmem/core.c | 44 ++++++++++++++++++++++++
drivers/nvmem/internals.h | 1 +
3 files changed, 58 insertions(+)

diff --git a/Documentation/ABI/stable/sysfs-bus-nvmem b/Documentation/ABI/stable/sysfs-bus-nvmem
index 0e3a8492a1082..c713c4e6cecb1 100644
--- a/Documentation/ABI/stable/sysfs-bus-nvmem
+++ b/Documentation/ABI/stable/sysfs-bus-nvmem
@@ -1,3 +1,16 @@
+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.
+ Note: This file is only present if CONFIG_NVMEM_SYSFS
+ is enabled
+
What: /sys/bus/nvmem/devices/.../nvmem
Date: July 2015
KernelVersion: 4.2
diff --git a/drivers/nvmem/core.c b/drivers/nvmem/core.c
index b31abffcdfc00..2321fb7da1e92 100644
--- a/drivers/nvmem/core.c
+++ b/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,
};
@@ -299,6 +322,25 @@ 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);

@@ -355,6 +397,7 @@ 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,
};

/* Cell attributes will be dynamically allocated */
@@ -971,6 +1014,7 @@ struct nvmem_device *nvmem_register(const struct nvmem_config *config)

nvmem->read_only = device_property_present(config->dev, "read-only") ||
config->read_only || !nvmem->reg_write;
+ nvmem->default_read_only = nvmem->read_only;

#ifdef CONFIG_NVMEM_SYSFS
nvmem->dev.groups = nvmem_dev_groups;
diff --git a/drivers/nvmem/internals.h b/drivers/nvmem/internals.h
index 18fed57270e5e..0667937ebb86b 100644
--- a/drivers/nvmem/internals.h
+++ b/drivers/nvmem/internals.h
@@ -16,6 +16,7 @@ struct nvmem_device {
int id;
struct kref refcnt;
size_t size;
+ bool default_read_only;
bool read_only;
bool root_only;
int flags;
--
2.43.0