nvmem: sunxi_sid: Optimize register read-out method

SID cells are 32-bit aligned, and a multiple of 32 bits in length. The
only outlier is the thermal sensor calibration data, which is 16 bits
per sensor. However a whole 64 bits is allocated for this purpose, so
we could consider it conforming to the rule above.

Also, the register read-out method assumes native endian, unlike the
direct MMIO method, which assumes big endian. Thus no endian conversion
is involved.

Under these assumptions, the register read-out method can be slightly
optimized. Instead of reading one word then discarding 3 bytes, read
the whole word directly into the buffer. However, for reads under 4
bytes or trailing bytes, we still use a scratch buffer to extract the
requested bytes.

We could go one step further if .word_size was 4, but changing that
would affect the sysfs interface's behavior.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/nvmem/sunxi_sid.c

@@ -115,36 +115,34 @@ static int sun8i_sid_register_readout(const struct sunxi_sid *sid,
  * to be not reliable at all.
  * Read by the registers instead.
  */
-static int sun8i_sid_read_byte_by_reg(const struct sunxi_sid *sid,
-				      const unsigned int offset,
-				      u8 *out)
-{
-	u32 word;
-	int ret;
-
-	ret = sun8i_sid_register_readout(sid, offset & ~0x03, &word);
-
-	if (ret)
-		return ret;
-
-	*out = (word >> ((offset & 0x3) * 8)) & 0xff;
-
-	return 0;
-}
-
 static int sun8i_sid_read_by_reg(void *context, unsigned int offset,
 				 void *val, size_t bytes)
 {
 	struct sunxi_sid *sid = context;
-	u8 *buf = val;
+	u32 word;
 	int ret;
 
-	while (bytes--) {
-		ret = sun8i_sid_read_byte_by_reg(sid, offset++, buf++);
+	/* .stride = 4 so offset is guaranteed to be aligned */
+	while (bytes >= 4) {
+		ret = sun8i_sid_register_readout(sid, offset, val);
 		if (ret)
 			return ret;
+
+		val += 4;
+		offset += 4;
+		bytes -= 4;
 	}
 
+	if (!bytes)
+		return 0;
+
+	/* Handle any trailing bytes */
+	ret = sun8i_sid_register_readout(sid, offset, &word);
+	if (ret)
+		return ret;
+
+	memcpy(val, &word, bytes);
+
 	return 0;
 }