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>

diff --git a/Documentation/ABI/stable/sysfs-bus-nvmem b/Documentation/ABI/stable/sysfs-bus-nvmem
index 3f0a952..aa89adf 100644 (file)
--- 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@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

diff --git a/drivers/nvmem/core.c b/drivers/nvmem/core.c
index 89f632f..12df7d0 100644 (file)
--- 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[] = {