soundwire: bus: Make sdw_nwrite() data pointer argument const

Idiomatically, write functions should take const pointers to the
data buffer, as they don't change the data. They are also likely
to be called from functions that receive a const data pointer.

Internally the pointer is passed to function/structs shared with
the read functions, requiring a cast, but this is an implementation
detail that should be hidden by the public API.

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Link: https://lore.kernel.org/r/20210616145901.29402-1-rf@opensource.cirrus.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>

Author: rfvirgil

drivers/soundwire/bus.c

@@ -394,13 +394,13 @@ sdw_nread_no_pm(struct sdw_slave *slave, u32 addr, size_t count, u8 *val)
 }
 
 static int
-sdw_nwrite_no_pm(struct sdw_slave *slave, u32 addr, size_t count, u8 *val)
+sdw_nwrite_no_pm(struct sdw_slave *slave, u32 addr, size_t count, const u8 *val)
 {
 	struct sdw_msg msg;
 	int ret;
 
 	ret = sdw_fill_msg(&msg, slave, addr, count,
-			   slave->dev_num, SDW_MSG_FLAG_WRITE, val);
+			   slave->dev_num, SDW_MSG_FLAG_WRITE, (u8 *)val);
 	if (ret < 0)
 		return ret;
 
@@ -535,9 +535,9 @@ EXPORT_SYMBOL(sdw_nread);
  * @slave: SDW Slave
  * @addr: Register address
  * @count: length
- * @val: Buffer for values to be read
+ * @val: Buffer for values to be written
  */
-int sdw_nwrite(struct sdw_slave *slave, u32 addr, size_t count, u8 *val)
+int sdw_nwrite(struct sdw_slave *slave, u32 addr, size_t count, const u8 *val)
 {
 	int ret;
 

include/linux/soundwire/sdw.h

@@ -1039,7 +1039,7 @@ int sdw_write(struct sdw_slave *slave, u32 addr, u8 value);
 int sdw_write_no_pm(struct sdw_slave *slave, u32 addr, u8 value);
 int sdw_read_no_pm(struct sdw_slave *slave, u32 addr);
 int sdw_nread(struct sdw_slave *slave, u32 addr, size_t count, u8 *val);
-int sdw_nwrite(struct sdw_slave *slave, u32 addr, size_t count, u8 *val);
+int sdw_nwrite(struct sdw_slave *slave, u32 addr, size_t count, const u8 *val);
 int sdw_compare_devid(struct sdw_slave *slave, struct sdw_slave_id id);
 void sdw_extract_slave_id(struct sdw_bus *bus, u64 addr, struct sdw_slave_id *id);