Re: [PATCH v5] ALSA: usb-audio: scarlett2: Fix for loop increment in scarlett2_usb_get_config

Takashi Iwai
2021-07-01T17:12:05Z

On Sun, 27 Jun 2021 07:12:03 +0200,
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
> Acked-by: Geoffrey D. Bennett <g@b4.vu>
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> ---
>
> v1 -> v2:
>
> * Use temporary variables of proper type rather than casting, as
> requested by Takashi.
>
> * Mention that there is not a bug at the moment per Geoffrey's comment.
>
> v2 -> v3:
>
> * Restrict scope of buf_16 more, as requested by Geoffrey.
>
> * Add Geoffrey's ack.
>
> v3 -> v4:
>
> * Fix stray newline added below
>
> if (config_item->size >= 8) {
>
> leftover from buf_16's declaration.
>
> v4 -> v5 (or how many times does it take Nathan to get a patch right):
>
> * Re-add note about no bug that was dropped in v3 by accident, as
> noticed by Geoffrey. My apologies for the multiple revisions.

Thanks, applied now.

Takashi