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@xxxxxxx>
---
Cc: Alexandre Belloni <alexandre.belloni@xxxxxxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Bartosz Golaszewski <brgl@xxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Srinivas Kandagatla <srinivas.kandagatla@xxxxxxxxxx>
Cc: linux-i2c@xxxxxxxxxxxxxxx
---
V2: - Document the force_ro attribute
- Use sysfs_emit()
V3: - Expand ABI documentation with what the force_ro attribute
returns and what it can parse.
V4: - Drop default_read_only , that is superseded by nvmem_attr_is_visible
which outright hides the force_ro attribute for read-only devices,
so they cannot be switched to read-write.
---
Documentation/ABI/stable/sysfs-bus-nvmem | 17 ++++++++++
drivers/nvmem/core.c | 43 ++++++++++++++++++++++++
2 files changed, 60 insertions(+)
diff --git a/Documentation/ABI/stable/sysfs-bus-nvmem b/Documentation/ABI/stable/sysfs-bus-nvmem
index 3f0a95250aa84..aa89adf18bc55 100644
--- a/Documentation/ABI/stable/sysfs-bus-nvmem
+++ b/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@xxxxxxx>
+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
diff --git a/drivers/nvmem/core.c b/drivers/nvmem/core.c
index 5b3606114628b..628d4f940a2b2 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,
};
@@ -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[] = {
--
2.43.0
