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 gabe.freedesktop.org (gabe.freedesktop.org [131.252.210.177]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 13404C88E50 for ; Mon, 14 Sep 2026 12:17:47 +0000 (UTC) Received: from gabe.freedesktop.org (localhost [127.0.0.1]) by gabe.freedesktop.org (Postfix) with ESMTP id 7708710E18D; Mon, 14 Sep 2026 12:17:46 +0000 (UTC) Authentication-Results: gabe.freedesktop.org; dkim=fail reason="signature verification failed" (1024-bit key; unprotected) header.d=arm.com header.i=@arm.com header.b="ntSiCFzF"; dkim-atps=neutral Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by gabe.freedesktop.org (Postfix) with ESMTP id 96DE410E18D for ; Mon, 14 Sep 2026 12:17:44 +0000 (UTC) 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 7DE821576; Mon, 14 Sep 2026 05:17:40 -0700 (PDT) Received: from [10.2.212.23] (e121345-lin.cambridge.arm.com [10.2.212.23]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 452523F86F; Mon, 14 Sep 2026 05:17:42 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1789388264; bh=c0CFsUaosLdBaAY1WVZXpEpvXD8k8XfgcUQrkA6InPM=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=ntSiCFzFkuEdj3K33SmO+PzjbPECbuW1/EaQ1yQvrkgJeIHb8P1qXEcBjuw4L3n/a 5feEXdS3bbHFTDLKA/lTSqqgfUdDt3PfSSPiCEpcXVf6A3/3eJtvrHG14tCb2NKu62 oqcozB1jlR3R9fus2DDr7VYmt1u52rQT3C1hWedc= Message-ID: Date: Mon, 14 Sep 2026 13:17:40 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 2/2] rust: scatterlist: honor the device's maximum segment size To: Gary Guo , Danilo Krummrich , Alexandre Courbot Cc: Matteo Kloiber , aliceryhl@google.com, ojeda@kernel.org, airlied@gmail.com, simona@ffwll.ch, abdiel.janulgue@gmail.com, daniel.almeida@collabora.com, a.hindborg@kernel.org, nova-gpu@lists.linux.dev, dri-devel@lists.freedesktop.org, driver-core@lists.linux.dev, rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org References: <20260831233215.287881-1-kernel@matt3o12.de> <20260831233215.287881-3-kernel@matt3o12.de> <20260913210806.125589-1-kernel@matt3o12.de> From: Robin Murphy Content-Language: en-GB In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-BeenThere: dri-devel@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Direct Rendering Infrastructure - Development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" On 14/09/2026 11:22 am, Gary Guo wrote: > On Mon Sep 14, 2026 at 10:58 AM BST, Danilo Krummrich wrote: >> On Mon Sep 14, 2026 at 2:45 AM CEST, Alexandre Courbot wrote: >>> On Mon Sep 14, 2026 at 6:08 AM JST, Matteo Kloiber wrote: >>>> On Mon Sep 7, 2026 at 11:43 AM JST, Alexandre Courbot wrote: >>>>> It also means that without patch 1, nova-core would split the firmware >>>>> into hundreds of 64KB SG entries, which is not breaking but still >>>>> something we want to avoid. The correct fix is to make sure that >>>>> `dma_set_max_seg_size` is called by the driver, and while we are at it >>>>> we also want every driver to call `dma_set_mask_and_coherent`. Ideally >>>>> we would use the type system to make sure that both functions are called >>>>> before any DMA operation can take place (using a safe interface), but >>>>> I'm not quite sure yet how we can do this. >>>> >>>> This sounds sensible indeed. Should I open a thread regarding that on Zulip? >>> >>> Probably not necessary, the mailing-list has a larger audience and is >>> the right place for this. I expect people will jump in here with their >>> thoughts. >> >> The problem with those is not that they must strictly be called before >> allocating DMA memory, but they must not be called concurrently with other DMA >> operations, such as allocating DMA memory, as it would technically be a data >> race. > > Do they really have to be called *before* allocating DMA memory, not do they > just need not be called *concurrent* to DMA memory allocation? Similar to DMA masks, the segment parameters should be set appropriately before any dma_map_sg() operation. Plus since they likely influence scatterlist geometry, that means typically they're also going to need to be set before building the scatterlist to be mapped in the first place. I don't think there's any expectation that they would ever change *between* different mappings - especially given the underlying hardware properties they represent - so while that might technically be possible in the C API if the caller can enforce sufficient ordering, it should be fine to rule it out in the Rust abstraction if that makes things easier to reason about. Thanks, Robin. > If it's the former, we can require these to require mutable reference instead, > so the probe takes `Pin<&'bound mut Device>>` which still derefs to > `&'bound Device`, but Rust will require the shared reference to not > co-exist with the mutable reference. > > Best, > Gary >