[PATCH v2 3/4] nvmem: provide consumer access to cell size metrics

Add nvmem_cell_get_size() to let a consumer query a cell's size before
reading or writing it. Without this, a consumer that assumes a fixed
cell size can pass nvmem_cell_write() a buffer larger than the cell,
which fails with an incorrect-size error instead of succeeding with
the cell's actual capacity.

Co-developed-by: Oleksij Rempel <[email protected]>
Signed-off-by: Oleksij Rempel <[email protected]>
Reviewed-by: Sebastian Reichel <[email protected]>
Link: https://lore.kernel.org/lkml/dixmyhyj6b66vyizfotpqw6yzaavi2o2mauloa5x6zn3qgk5ub@huoal5ucdd44/
Signed-off-by: Faruque Ansari <[email protected]>
---
 drivers/nvmem/core.c           | 29 +++++++++++++++++++++++++++++
 include/linux/nvmem-consumer.h |  7 +++++++
 2 files changed, 36 insertions(+)

diff --git a/drivers/nvmem/core.c b/drivers/nvmem/core.c
index 0556d140170a..49afcff4c72c 100644
--- a/drivers/nvmem/core.c
+++ b/drivers/nvmem/core.c
@@ -1790,6 +1790,35 @@ int nvmem_cell_write(struct nvmem_cell *cell, void *buf, size_t len)
 
 EXPORT_SYMBOL_GPL(nvmem_cell_write);
 
+/**
+ * nvmem_cell_get_size() - Retrieve the storage size of an NVMEM cell.
+ * @cell: Pointer to the NVMEM cell structure.
+ * @bytes: Optional pointer to store the cell size in bytes (can be NULL).
+ * @bits: Optional pointer to store the cell size in bits (can be NULL).
+ *
+ * This function allows consumers to retrieve the size of a specific NVMEM
+ * cell before performing read/write operations. It is useful for validating
+ * buffer sizes to prevent mismatched writes.
+ *
+ * Return: 0 on success or negative on failure.
+ */
+int nvmem_cell_get_size(struct nvmem_cell *cell, size_t *bytes, size_t *bits)
+{
+	struct nvmem_cell_entry *entry = cell->entry;
+
+	if (!entry->nvmem)
+		return -EINVAL;
+
+	if (bytes)
+		*bytes = entry->bytes;
+
+	if (bits)
+		*bits = entry->nbits;
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(nvmem_cell_get_size);
+
 static int nvmem_cell_read_common(struct device *dev, const char *cell_id,
 				  void *val, size_t count)
 {
diff --git a/include/linux/nvmem-consumer.h b/include/linux/nvmem-consumer.h
index 34c0e58dfa26..bcb0e17e415d 100644
--- a/include/linux/nvmem-consumer.h
+++ b/include/linux/nvmem-consumer.h
@@ -56,6 +56,7 @@ void nvmem_cell_put(struct nvmem_cell *cell);
 void devm_nvmem_cell_put(struct device *dev, struct nvmem_cell *cell);
 void *nvmem_cell_read(struct nvmem_cell *cell, size_t *len);
 int nvmem_cell_write(struct nvmem_cell *cell, void *buf, size_t len);
+int nvmem_cell_get_size(struct nvmem_cell *cell, size_t *bytes, size_t *bits);
 int nvmem_cell_read_u8(struct device *dev, const char *cell_id, u8 *val);
 int nvmem_cell_read_u16(struct device *dev, const char *cell_id, u16 *val);
 int nvmem_cell_read_u32(struct device *dev, const char *cell_id, u32 *val);
@@ -128,6 +129,12 @@ static inline int nvmem_cell_write(struct nvmem_cell *cell,
 	return -EOPNOTSUPP;
 }
 
+static inline int nvmem_cell_get_size(struct nvmem_cell *cell, size_t *bytes,
+				      size_t *bits)
+{
+	return -EOPNOTSUPP;
+}
+
 static inline int nvmem_cell_read_u8(struct device *dev,
 				     const char *cell_id, u8 *val)
 {

-- 
2.34.1