[PATCH] nvmem: core: add nvmem_cell_nbits()

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

A cell with a bits property is narrower than the bytes nvmem_cell_read()
returns and nvmem_cell_write() doesn't mask the value written to it, so
anything that doesn't fit spills into the neighbouring bits.

A consumer that computes the value it writes back, like a counter, thus
needs to know the width of the cell. Let's add a helper that returns it.

Assisted-by: Claude:opus-5.5
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
---
 drivers/nvmem/core.c           | 16 ++++++++++++++++
 include/linux/nvmem-consumer.h |  6 ++++++
 2 files changed, 22 insertions(+)

diff --git a/drivers/nvmem/core.c b/drivers/nvmem/core.c
index 79a36bb9b996..9fb7049c0116 100644
--- a/drivers/nvmem/core.c
+++ b/drivers/nvmem/core.c
@@ -928,6 +928,22 @@ void *nvmem_cell_read(struct nvmem_cell *cell, size_t *len)
 }
 EXPORT_SYMBOL_GPL(nvmem_cell_read);
 
+/**
+ * nvmem_cell_nbits() - Get the width of a nvmem cell in bits
+ *
+ * @cell: nvmem cell to be queried.
+ *
+ * Return: the number of bits in the cell, which can be less than its size in
+ * bytes times 8 if the cell has a bits property.
+ */
+unsigned int nvmem_cell_nbits(struct nvmem_cell *cell)
+{
+	struct nvmem_cell_entry *entry = cell->entry;
+
+	return entry->nbits ?: entry->bytes * BITS_PER_BYTE;
+}
+EXPORT_SYMBOL_GPL(nvmem_cell_nbits);
+
 static inline const void *nvmem_cell_prepare_write_buffer(struct nvmem_cell_entry *cell,
 							  const u8 *_buf, int len)
 {
diff --git a/include/linux/nvmem-consumer.h b/include/linux/nvmem-consumer.h
index ff9b87442852..efce9bb0e593 100644
--- a/include/linux/nvmem-consumer.h
+++ b/include/linux/nvmem-consumer.h
@@ -28,6 +28,7 @@ extern struct class nvmem_class;
 struct nvmem_cell *nvmem_cell_get(struct device *dev, const char *name);
 void nvmem_cell_put(struct nvmem_cell *cell);
 void *nvmem_cell_read(struct nvmem_cell *cell, size_t *len);
+unsigned int nvmem_cell_nbits(struct nvmem_cell *cell);
 void *nvmem_cell_get_and_read(struct device_node *np, const char *cell_name,
 			      size_t bytes);
 int nvmem_cell_read_variable_le_u32(struct device *dev, const char *cell_id,
@@ -68,6 +69,11 @@ static inline char *nvmem_cell_read(struct nvmem_cell *cell, size_t *len)
 	return ERR_PTR(-EOPNOTSUPP);
 }
 
+static inline unsigned int nvmem_cell_nbits(struct nvmem_cell *cell)
+{
+	return 0;
+}
+
 static inline void *nvmem_cell_get_and_read(struct device_node *np,
 					    const char *cell_name,
 					    size_t bytes)
-- 
2.47.3