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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 429F4C433EF for ; Fri, 20 May 2022 08:52:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1347400AbiETIwE (ORCPT ); Fri, 20 May 2022 04:52:04 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:53436 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1344968AbiETIwB (ORCPT ); Fri, 20 May 2022 04:52:01 -0400 Received: from theia.8bytes.org (8bytes.org [IPv6:2a01:238:4383:600:38bc:a715:4b6d:a889]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 39C335DD00 for ; Fri, 20 May 2022 01:52:00 -0700 (PDT) Received: by theia.8bytes.org (Postfix, from userid 1000) id BC47C1E9; Fri, 20 May 2022 10:51:58 +0200 (CEST) Date: Fri, 20 May 2022 10:51:57 +0200 From: Joerg Roedel To: Christoph Hellwig Cc: Robin Murphy , John Garry , will@kernel.org, m.szyprowski@samsung.com, chenxiang66@hisilicon.com, thunder.leizhen@huawei.com, iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org, liyihang6@hisilicon.com Subject: Re: [RFC PATCH] dma-iommu: Add iommu_dma_max_mapping_size() Message-ID: References: <1652706361-92557-1-git-send-email-john.garry@huawei.com> <9160031b-50be-6993-5a8e-f238391962c5@huawei.com> <8f193bdd-3a0a-f9ed-0726-e6081f374320@arm.com> <20220518131353.GB26019@lst.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220518131353.GB26019@lst.de> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, May 18, 2022 at 03:13:53PM +0200, Christoph Hellwig wrote: > On Tue, May 17, 2022 at 01:02:00PM +0100, Robin Murphy wrote: > >> So how to inform the SCSI driver of this caching limit then so that it may > >> limit the SGL length? > > > > Driver-specific mechanism; block-layer-specific mechanism; redefine this > > whole API to something like dma_opt_mapping_size(), as a limit above which > > mappings might become less efficient or start to fail (callback to my > > thoughts on [1] as well, I suppose); many options. Just not imposing a > > ridiculously low *maximum* on everyone wherein mapping calls "should not be > > larger than the returned value" when that's clearly bollocks. > > Well, for swiotlb it is a hard limit. So if we want to go down that > route we need two APIs, one for the optimal size and one for the > hard limit. I agree with Robin, and if it really helps some drivers I am all for doing a dma_opt_mapping_size() instead. Limiting DMA mapping sizes to make drivers perform better gets a clear NAK from my side. Regards, Joerg