[PATCH v2] ALSA: usb-audio: scarlett2: Fix for loop increment in scarlett2_usb_get_config

[Nathan Chancellor <nathan@kernel.org>]

Clang warns:

sound/usb/mixer_scarlett_gen2.c:1189:32: warning: expression result
unused [-Wunused-value]
                        for (i = 0; i < count; i++, (u16 *)buf++)
                                                    ^      ~~~~~
1 warning generated.

It appears the intention was to cast the void pointer to a u16 pointer
so that the data could be iterated through like an array of u16 values.
However, the cast happens after the increment because a cast is an
rvalue, whereas the post-increment operator only works on lvalues, so
the loop does not iterate as expected. This is not a bug in practice
because count is not greater than one at the moment but this could
change in the future so this should be fixed.

Replace the cast with a temporary variable of the proper type, which is
less error prone and fixes the iteration. Do the same thing for the
'u8 *' below this if block.

Fixes: ac34df733d2d ("ALSA: usb-audio: scarlett2: Update get_config to do endian conversion")
Link: https://github.com/ClangBuiltLinux/linux/issues/1408
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
---

v1 -> v2:

* Use temporary variables of proper type rather than casting, as
  requested by Takashi. I did not include Geoffrey's ack for this
  reason.

* Mention that there is not a bug at the moment per Geoffrey's comment.

 sound/usb/mixer_scarlett_gen2.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/sound/usb/mixer_scarlett_gen2.c b/sound/usb/mixer_scarlett_gen2.c
index fcba682cd422..b13903bed330 100644
--- a/sound/usb/mixer_scarlett_gen2.c
+++ b/sound/usb/mixer_scarlett_gen2.c
@@ -1177,17 +1177,22 @@ static int scarlett2_usb_get_config(
 	const struct scarlett2_config *config_item =
 		&scarlett2_config_items[info->has_mixer][config_item_num];
 	int size, err, i;
+	u8 *buf_8;
 	u8 value;
 
 	/* For byte-sized parameters, retrieve directly into buf */
 	if (config_item->size >= 8) {
+		u16 *buf_16;
+
 		size = config_item->size / 8 * count;
 		err = scarlett2_usb_get(mixer, config_item->offset, buf, size);
 		if (err < 0)
 			return err;
-		if (size == 2)
-			for (i = 0; i < count; i++, (u16 *)buf++)
-				*(u16 *)buf = le16_to_cpu(*(__le16 *)buf);
+		if (size == 2) {
+			buf_16 = buf;
+			for (i = 0; i < count; i++, buf_16++)
+				*buf_16 = le16_to_cpu(*(__le16 *)buf_16);
+		}
 		return 0;
 	}
 
@@ -1197,8 +1202,9 @@ static int scarlett2_usb_get_config(
 		return err;
 
 	/* then unpack from value into buf[] */
+	buf_8 = buf;
 	for (i = 0; i < 8 && i < count; i++, value >>= 1)
-		*(u8 *)buf++ = value & 1;
+		*buf_8++ = value & 1;
 
 	return 0;
 }

base-commit: 0cbbeaf370221fc469c95945dd3c1198865c5fe4
-- 
2.32.0.93.g670b81a890