From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-20.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id B6ED3C2B9F4 for ; Fri, 25 Jun 2021 20:26:10 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 857C161926 for ; Fri, 25 Jun 2021 20:26:10 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229878AbhFYU22 (ORCPT ); Fri, 25 Jun 2021 16:28:28 -0400 Received: from m.b4.vu ([203.16.231.148]:53606 "EHLO m.b4.vu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229573AbhFYU21 (ORCPT ); Fri, 25 Jun 2021 16:28:27 -0400 Received: by m.b4.vu (Postfix, from userid 1000) id E89B461E5F02; Sat, 26 Jun 2021 05:56:04 +0930 (ACST) Date: Sat, 26 Jun 2021 05:56:04 +0930 From: "Geoffrey D. Bennett" To: Nathan Chancellor Cc: Jaroslav Kysela , Takashi Iwai , Nick Desaulniers , alsa-devel@alsa-project.org, linux-kernel@vger.kernel.org, clang-built-linux@googlegroups.com Subject: Re: [PATCH v4] ALSA: usb-audio: scarlett2: Fix for loop increment in scarlett2_usb_get_config Message-ID: <20210625202604.GB23780@m.b4.vu> References: <20210625200549.1061113-1-nathan@kernel.org> <20210625201150.1523987-1-nathan@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210625201150.1523987-1-nathan@kernel.org> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jun 25, 2021 at 01:11:51PM -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. Your note about no bug which was added in v2 went missing in v3: > > 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 > Signed-off-by: Nathan Chancellor > --- > > 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. > > sound/usb/mixer_scarlett_gen2.c | 13 +++++++++---- > 1 file changed, 9 insertions(+), 4 deletions(-) > > diff --git a/sound/usb/mixer_scarlett_gen2.c b/sound/usb/mixer_scarlett_gen2.c > index fcba682cd422..161d832cafef 100644 > --- 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; > } > > base-commit: 0cbbeaf370221fc469c95945dd3c1198865c5fe4 > -- > 2.32.0.93.g670b81a890 >