[PATCH v1 1/3] nvmem: Add support for write-only instances

[Nicholas Johnson]

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@xxxxxxxxxxxxxx>
---
 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