ALSA: usb-audio: scarlett2: Fix for loop increment in scarlett2_usb_get_config

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
Acked-by: Geoffrey D. Bennett <g@b4.vu>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/20210627051202.1888250-1-nathan@kernel.org
Signed-off-by: Takashi Iwai <tiwai@suse.de>

diff --git a/sound/usb/mixer_scarlett_gen2.c b/sound/usb/mixer_scarlett_gen2.c
index 346551599dd68c6b4cc4d43f02c1d44514a14fef..f9d698a371539d716754bad57cbe35a86a9f9ce9 100644 (file)
--- a/sound/usb/mixer_scarlett_gen2.c
+++ b/sound/usb/mixer_scarlett_gen2.c
@@ -1177,6 +1177,7 @@ 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 */
@@ -1185,9 +1186,12 @@ static int scarlett2_usb_get_config(
                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) {
+                       u16 *buf_16 = buf;
+
+                       for (i = 0; i < count; i++, buf_16++)
+                               *buf_16 = le16_to_cpu(*(__le16 *)buf_16);
+               }
                return 0;
        }
 
@@ -1197,8 +1201,9 @@ 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;
 }