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

On Sat, Jun 26, 2021 at 04:13:42AM +0930, Geoffrey D. Bennett wrote:

On Fri, Jun 25, 2021 at 10:54:19AM -0700, Nathan Chancellor wrote:

...

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;
  

I would prefer that the u16 *buf_16 declaration above be removed from there...

...

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;
  

...and combined with the assignment here, like: u16 *buf_16 = buf;

Thanks for pointing it out, I was not paying enough attention to realize that the scope could be reduced. v3 sent with your Ack added, thank you for taking a look in a quick manner!

Cheers, Nathan

Regardless:

Acked-by: Geoffrey D. Bennett g@b4.vu

And, thanks again!

...

• 	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