linux-kernel - [PATCH 2/7] nvmem: return -EOPNOTSUPP to in-kernel users on missing callbacks

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Message-Id: <20260116-nvmem-unbind-v1-2-7bb401ab19a8@oss.qualcomm.com>
Date: Fri, 16 Jan 2026 12:01:09 +0100
From: Bartosz Golaszewski <bartosz.golaszewski@....qualcomm.com>
To: Srinivas Kandagatla <srini@...nel.org>,
        Bartosz Golaszewski <brgl@...nel.org>
Cc: linux-kernel@...r.kernel.org,
        Bartosz Golaszewski <bartosz.golaszewski@....qualcomm.com>
Subject: [PATCH 2/7] nvmem: return -EOPNOTSUPP to in-kernel users on
 missing callbacks

__nvmem_reg_read/write() currently return -EINVAL if the relevant
callback is not present. User-space helpers again check the presence
of the callbacks to see if they should return -EPERM.

Ahead of adding SRCU synchronization: change the error code returned to
in-kernel users to -EOPNOTSUPP which is more indicative of the actual
reason for the failure as well as allows us to translate it easily to
-EPERM in sysfs callbacks without having to dereference the callback
pointers. This will allow us to limit the number of SRCU critical
sections in the follow-up commits.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@....qualcomm.com>
---
 drivers/nvmem/core.c | 37 +++++++++++++++++++------------------
 1 file changed, 19 insertions(+), 18 deletions(-)

diff --git a/drivers/nvmem/core.c b/drivers/nvmem/core.c
index 387c88c55259541446901f0e539bbb0dd8c4c3de..c4a5f8ef65164ef6994343e6d26d3d302c9b00c3 100644
--- a/drivers/nvmem/core.c
+++ b/drivers/nvmem/core.c
@@ -55,10 +55,10 @@ static BLOCKING_NOTIFIER_HEAD(nvmem_notifier);
 static int __nvmem_reg_read(struct nvmem_device *nvmem, unsigned int offset,
 			    void *val, size_t bytes)
 {
-	if (nvmem->reg_read)
-		return nvmem->reg_read(nvmem->priv, offset, val, bytes);
+	if (!nvmem->reg_read)
+		return -EOPNOTSUPP;
 
-	return -EINVAL;
+	return nvmem->reg_read(nvmem->priv, offset, val, bytes);
 }
 
 static int __nvmem_reg_write(struct nvmem_device *nvmem, unsigned int offset,
@@ -66,14 +66,14 @@ static int __nvmem_reg_write(struct nvmem_device *nvmem, unsigned int offset,
 {
 	int ret;
 
-	if (nvmem->reg_write) {
-		gpiod_set_value_cansleep(nvmem->wp_gpio, 0);
-		ret = nvmem->reg_write(nvmem->priv, offset, val, bytes);
-		gpiod_set_value_cansleep(nvmem->wp_gpio, 1);
-		return ret;
-	}
+	if (!nvmem->reg_write)
+		return -EOPNOTSUPP;
+
+	gpiod_set_value_cansleep(nvmem->wp_gpio, 0);
+	ret = nvmem->reg_write(nvmem->priv, offset, val, bytes);
+	gpiod_set_value_cansleep(nvmem->wp_gpio, 1);
 
-	return -EINVAL;
+	return ret;
 }
 
 static int nvmem_access_with_keepouts(struct nvmem_device *nvmem,
@@ -231,13 +231,12 @@ static ssize_t bin_attr_nvmem_read(struct file *filp, struct kobject *kobj,
 
 	count = round_down(count, nvmem->word_size);
 
-	if (!nvmem->reg_read)
-		return -EPERM;
-
 	rc = nvmem_reg_read(nvmem, pos, buf, count);
-
-	if (rc)
+	if (rc) {
+		if (rc == -EOPNOTSUPP)
+			rc = -EPERM;
 		return rc;
+	}
 
 	return count;
 }
@@ -264,13 +263,15 @@ static ssize_t bin_attr_nvmem_write(struct file *filp, struct kobject *kobj,
 
 	count = round_down(count, nvmem->word_size);
 
-	if (!nvmem->reg_write || nvmem->read_only)
+	if (nvmem->read_only)
 		return -EPERM;
 
 	rc = nvmem_reg_write(nvmem, pos, buf, count);
-
-	if (rc)
+	if (rc) {
+		if (rc == -EOPNOTSUPP)
+			rc = -EPERM;
 		return rc;
+	}
 
 	return count;
 }

-- 
2.47.3