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Subject[PATCH v1 1/3] nvmem: Add support for write-only instances
Date
Mika Westerberg requires write-only nvmem for the Thunderbolt driver.
Refer to 03cd45d2e219 ("thunderbolt: Prevent crash if non-active NVMem
file is read"). Hence, there is at least one real-world use for
write-only nvmem instances.

Add support for write-only nvmem instances by changing the nvmem attrs
to 0222 if the .reg_read callback is not populated.

Add a WARN_ON in case a driver populates neither .reg_read nor
.reg_write because this behaviour should clearly never occur.

Signed-off-by: Nicholas Johnson <nicholas.johnson-opensource@outlook.com.au>
---
drivers/nvmem/nvmem-sysfs.c | 77 +++++++++++++++++++++++++++++++++----
1 file changed, 70 insertions(+), 7 deletions(-)

diff --git a/drivers/nvmem/nvmem-sysfs.c b/drivers/nvmem/nvmem-sysfs.c
index 9e0c429cd..be3b94f0b 100644
--- a/drivers/nvmem/nvmem-sysfs.c
+++ b/drivers/nvmem/nvmem-sysfs.c
@@ -147,6 +147,30 @@ static const struct attribute_group *nvmem_ro_dev_groups[] = {
NULL,
};

+/* write only permission */
+static struct bin_attribute bin_attr_wo_nvmem = {
+ .attr = {
+ .name = "nvmem",
+ .mode = 0222,
+ },
+ .write = bin_attr_nvmem_write,
+};
+
+static struct bin_attribute *nvmem_bin_wo_attributes[] = {
+ &bin_attr_wo_nvmem,
+ NULL,
+};
+
+static const struct attribute_group nvmem_bin_wo_group = {
+ .bin_attrs = nvmem_bin_wo_attributes,
+ .attrs = nvmem_attrs,
+};
+
+static const struct attribute_group *nvmem_wo_dev_groups[] = {
+ &nvmem_bin_wo_group,
+ NULL,
+};
+
/* default read/write permissions, root only */
static struct bin_attribute bin_attr_rw_root_nvmem = {
.attr = {
@@ -196,16 +220,50 @@ static const struct attribute_group *nvmem_ro_root_dev_groups[] = {
NULL,
};

+/* write only permission, root only */
+static struct bin_attribute bin_attr_wo_root_nvmem = {
+ .attr = {
+ .name = "nvmem",
+ .mode = 0200,
+ },
+ .write = bin_attr_nvmem_write,
+};
+
+static struct bin_attribute *nvmem_bin_wo_root_attributes[] = {
+ &bin_attr_wo_root_nvmem,
+ NULL,
+};
+
+static const struct attribute_group nvmem_bin_wo_root_group = {
+ .bin_attrs = nvmem_bin_wo_root_attributes,
+ .attrs = nvmem_attrs,
+};
+
+static const struct attribute_group *nvmem_wo_root_dev_groups[] = {
+ &nvmem_bin_wo_root_group,
+ NULL,
+};
+
const struct attribute_group **nvmem_sysfs_get_groups(
struct nvmem_device *nvmem,
const struct nvmem_config *config)
{
- if (config->root_only)
- return nvmem->read_only ?
- nvmem_ro_root_dev_groups :
- nvmem_rw_root_dev_groups;
-
- return nvmem->read_only ? nvmem_ro_dev_groups : nvmem_rw_dev_groups;
+ /*
+ * If neither reg_read nor reg_write are provided, we cannot use this
+ * nvmem entry, as any operation will cause kernel mode NULL reference.
+ */
+ WARN_ON(!nvmem->reg_read && !nvmem->reg_write);
+
+ if (nvmem->reg_read && nvmem->reg_write)
+ return config->root_only ?
+ nvmem_rw_root_dev_groups : nvmem_rw_dev_groups;
+
+ if (nvmem->reg_read && !nvmem->reg_write)
+ return config->root_only ?
+ nvmem_ro_root_dev_groups : nvmem_ro_dev_groups;
+
+ return config->root_only ?
+ nvmem_wo_root_dev_groups : nvmem_wo_dev_groups;
}

/*
@@ -224,11 +282,16 @@ int nvmem_sysfs_setup_compat(struct nvmem_device *nvmem,
if (!config->base_dev)
return -EINVAL;

- if (nvmem->read_only) {
+ if (nvmem->reg_read && !nvmem->reg_write) {
if (config->root_only)
nvmem->eeprom = bin_attr_ro_root_nvmem;
else
nvmem->eeprom = bin_attr_ro_nvmem;
+ } else if (!nvmem->reg_read && nvmem->reg_write) {
+ if (config->root_only)
+ nvmem->eeprom = bin_attr_wo_root_nvmem;
+ else
+ nvmem->eeprom = bin_attr_wo_nvmem;
} else {
if (config->root_only)
nvmem->eeprom = bin_attr_rw_root_nvmem;
--
2.25.1
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