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=-15.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_SANE_1 autolearn=unavailable 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 7432EC4361B for ; Mon, 14 Dec 2020 13:02:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2FB2D20728 for ; Mon, 14 Dec 2020 13:02:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2407919AbgLNNCR (ORCPT ); Mon, 14 Dec 2020 08:02:17 -0500 Received: from foss.arm.com ([217.140.110.172]:47166 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2407907AbgLNNCO (ORCPT ); Mon, 14 Dec 2020 08:02:14 -0500 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 092FF1FB; Mon, 14 Dec 2020 05:01:28 -0800 (PST) Received: from [10.57.33.60] (unknown [10.57.33.60]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id DE7313F66B; Mon, 14 Dec 2020 05:01:26 -0800 (PST) Subject: Re: [PATCH v2] dma-mapping: add unlikely hint for error path in dma_mapping_error To: Heiner Kallweit , Christoph Hellwig , Marek Szyprowski , Barry Song Cc: "open list:AMD IOMMU (AMD-VI)" , Linux Kernel Mailing List References: <39581f9a-0066-ad98-094d-d41082145e23@gmail.com> From: Robin Murphy Message-ID: Date: Mon, 14 Dec 2020 13:01:25 +0000 User-Agent: Mozilla/5.0 (Windows NT 10.0; rv:78.0) Gecko/20100101 Thunderbird/78.5.1 MIME-Version: 1.0 In-Reply-To: <39581f9a-0066-ad98-094d-d41082145e23@gmail.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2020-12-13 16:32, Heiner Kallweit wrote: > Zillions of drivers use the unlikely() hint when checking the result of > dma_mapping_error(). This is an inline function anyway, so we can move > the hint into this function and remove it from drivers. Reviewed-by: Robin Murphy FWIW I consider this case similar to the same hint in WARN_ON() and friends - it's a pretty severe condition that should never be expected to be hit in normal operation, so it's entirely logical for it to be implicitly unlikely. I struggle to imagine any case that would specifically *not* want that (or worse, want to hint it as likely). Some DMA API backends may spend considerable time trying as hard as possible to make a mapping work before eventually admitting defeat, so the idea of ever trying to optimise at the driver level for failure in hot paths just makes no sense. Thanks, Robin. > Signed-off-by: Heiner Kallweit > --- > v2: > Split the big patch into the change for dma-mapping.h and follow-up > patches per subsystem that will go through the trees of the respective > maintainers. > --- > include/linux/dma-mapping.h | 2 +- > kernel/dma/map_benchmark.c | 2 +- > 2 files changed, 2 insertions(+), 2 deletions(-) > > diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h > index 2e49996a8..6177e20b5 100644 > --- a/include/linux/dma-mapping.h > +++ b/include/linux/dma-mapping.h > @@ -95,7 +95,7 @@ static inline int dma_mapping_error(struct device *dev, dma_addr_t dma_addr) > { > debug_dma_mapping_error(dev, dma_addr); > > - if (dma_addr == DMA_MAPPING_ERROR) > + if (unlikely(dma_addr == DMA_MAPPING_ERROR)) > return -ENOMEM; > return 0; > } > diff --git a/kernel/dma/map_benchmark.c b/kernel/dma/map_benchmark.c > index b1496e744..901420a5d 100644 > --- a/kernel/dma/map_benchmark.c > +++ b/kernel/dma/map_benchmark.c > @@ -78,7 +78,7 @@ static int map_benchmark_thread(void *data) > > map_stime = ktime_get(); > dma_addr = dma_map_single(map->dev, buf, PAGE_SIZE, map->dir); > - if (unlikely(dma_mapping_error(map->dev, dma_addr))) { > + if (dma_mapping_error(map->dev, dma_addr)) { > pr_err("dma_map_single failed on %s\n", > dev_name(map->dev)); > ret = -ENOMEM; >