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From:   Nicholas Johnson <nicholas.johnson-opensource@outlook.com.au>
To:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
CC:     Mika Westerberg <mika.westerberg@linux.intel.com>,
        Srinivas Kandagatla <srinivas.kandagatla@linaro.org>,
        Nicholas Johnson <nicholas.johnson-opensource@outlook.com.au>
Subject: [PATCH v2 1/3] nvmem: Add support for write-only instances
Thread-Topic: [PATCH v2 1/3] nvmem: Add support for write-only instances
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Date:   Mon, 2 Mar 2020 15:42:34 +0000
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There is at least one real-world use-case for write-only nvmem
instances. Refer to 03cd45d2e219 ("thunderbolt: Prevent crash if
non-active NVMem file is read").

Add support for write-only nvmem instances by adding attrs for 0200.

Change nvmem_register() to abort if NULL group is returned from
nvmem_sysfs_get_groups().

Return NULL from nvmem_sysfs_get_groups() in invalid cases.

Signed-off-by: Nicholas Johnson <nicholas.johnson-opensource@outlook.com.au>
---
 drivers/nvmem/core.c        |  2 ++
 drivers/nvmem/nvmem-sysfs.c | 53 ++++++++++++++++++++++++++++++++-----
 2 files changed, 48 insertions(+), 7 deletions(-)

diff --git a/drivers/nvmem/core.c b/drivers/nvmem/core.c
index ef326f243..27bd4c4e3 100644
--- a/drivers/nvmem/core.c
+++ b/drivers/nvmem/core.c
@@ -388,6 +388,8 @@ struct nvmem_device *nvmem_register(const struct nvmem_config *config)
 			   config->read_only || !nvmem->reg_write;
 
 	nvmem->dev.groups = nvmem_sysfs_get_groups(nvmem, config);
+	if (!nvmem->dev.groups)
+		return NULL;
 
 	device_initialize(&nvmem->dev);
 
diff --git a/drivers/nvmem/nvmem-sysfs.c b/drivers/nvmem/nvmem-sysfs.c
index 9e0c429cd..00d3259ea 100644
--- a/drivers/nvmem/nvmem-sysfs.c
+++ b/drivers/nvmem/nvmem-sysfs.c
@@ -196,16 +196,50 @@ 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;
+	/* Read-only */
+	if (nvmem->reg_read && (!nvmem->reg_write || nvmem->read_only))
+		return config->root_only ?
+			nvmem_ro_root_dev_groups : nvmem_ro_dev_groups;
+
+	/* Read-write */
+	if (nvmem->reg_read && nvmem->reg_write)
+		return config->root_only ?
+			nvmem_rw_root_dev_groups : nvmem_rw_dev_groups;
+
+	/* Write-only, do not honour request for global writable entry */
+	if (!nvmem->reg_read && nvmem->reg_write)
+		return config->root_only ? nvmem_wo_root_dev_groups : NULL;
+
+	/* Neither reg_read nor reg_write are provided, abort */
+	return NULL;
 }
 
 /*
@@ -224,11 +258,16 @@ 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 || nvmem->read_only)) {
 		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
+			return -EPERM;
 	} else {
 		if (config->root_only)
 			nvmem->eeprom = bin_attr_rw_root_nvmem;
-- 
2.25.1