Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Thu, 01 Jul 2021 19:12:01 +0200
Message-ID: <s5hfswyhsbi.wl-tiwai@suse.de>
From:   Takashi Iwai <tiwai@suse.de>
To:     Nathan Chancellor <nathan@kernel.org>
Cc:     "Geoffrey D. Bennett" <g@b4.vu>, Jaroslav Kysela <perex@perex.cz>,
        Takashi Iwai <tiwai@suse.com>,
        clang-built-linux@googlegroups.com, alsa-devel@alsa-project.org,
        Nick Desaulniers <ndesaulniers@google.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v5] ALSA: usb-audio: scarlett2: Fix for loop increment in scarlett2_usb_get_config
In-Reply-To: <20210627051202.1888250-1-nathan@kernel.org>
References: <20210625202604.GB23780@m.b4.vu>
        <20210627051202.1888250-1-nathan@kernel.org>
User-Agent: Wanderlust/2.15.9 (Almost Unreal) SEMI/1.14.6 (Maruoka)
 FLIM/1.14.9 (=?UTF-8?B?R29qxY0=?=) APEL/10.8 Emacs/25.3
 (x86_64-suse-linux-gnu) MULE/6.0 (HANACHIRUSATO)
MIME-Version: 1.0 (generated by SEMI 1.14.6 - "Maruoka")
Content-Type: text/plain; charset=US-ASCII
Precedence: bulk

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