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 2160BC61DA3 for ; Fri, 24 Feb 2023 14:44:19 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230030AbjBXOoS (ORCPT ); Fri, 24 Feb 2023 09:44:18 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:32956 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229978AbjBXOoG (ORCPT ); Fri, 24 Feb 2023 09:44:06 -0500 Received: from mail.marcansoft.com (marcansoft.com [212.63.210.85]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id EE7151689D; Fri, 24 Feb 2023 06:44:03 -0800 (PST) Received: from [127.0.0.1] (localhost [127.0.0.1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) (Authenticated sender: lina@asahilina.net) by mail.marcansoft.com (Postfix) with ESMTPSA id 2AB393FA55; Fri, 24 Feb 2023 14:43:56 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=asahilina.net; s=default; t=1677249842; bh=D5wNXpCLthvKzU0v4nv/lr+ty+Pv+guL5ET4c/EH8nE=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Irlcd4vdNdqExEiwyDlVv808VxXgX9m/G5YGPBtDAzaBbtvHVqk+M6pp6lLJRb/Vl 6PrAkVvLBUdlAoLtVdfffN7wpTeIp9MFE/g0j0JNbXJ+l6auXoQVRW5MpDE1y2N5mh nPJqUnPdkN8C7BIV5xy8t8S2oK9uEfw0IEBybH4Nqp+XtB/abdrfPIS5jMAOH3drxP DcMRWEwlj3kRo+dDtOQ2XYy/fpEYNkcZcCGzvD3ZTYRlhGSqRoMtRKpCIVHDFV/3xJ 3pdo74roDva34gPGO/b/MIfnHB1QxdQJKmEclCybOaUuS9owqxmRxG1HfeiMgx1XuK 7jU8V9xYpKXIw== Message-ID: <0e3c15bd-a1d6-a363-d7b6-087b6e4cddac@asahilina.net> Date: Fri, 24 Feb 2023 23:43:54 +0900 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux aarch64; rv:102.0) Gecko/20100101 Thunderbird/102.7.2 Subject: Re: [PATCH 2/5] rust: device: Add a minimal RawDevice trait Content-Language: en-US To: Greg Kroah-Hartman Cc: Miguel Ojeda , Alex Gaynor , Wedson Almeida Filho , Boqun Feng , Gary Guo , =?UTF-8?Q?Bj=c3=b6rn_Roy_Baron?= , Will Deacon , Robin Murphy , Joerg Roedel , Hector Martin , Sven Peter , Arnd Bergmann , "Rafael J. Wysocki" , Alyssa Rosenzweig , Neal Gompa , rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org, asahi@lists.linux.dev References: <20230224-rust-iopt-rtkit-v1-0-49ced3391295@asahilina.net> <20230224-rust-iopt-rtkit-v1-2-49ced3391295@asahilina.net> From: Asahi Lina In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2023/02/24 23:11, Greg Kroah-Hartman wrote: > Thanks for the detailed rust explainations, I'd like to just highlight > one thing: > > On Fri, Feb 24, 2023 at 10:15:12PM +0900, Asahi Lina wrote: >> On 24/02/2023 20.23, Greg Kroah-Hartman wrote: >>> And again, why are bindings needed for a "raw" struct device at all? >>> Shouldn't the bus-specific wrappings work better? >> >> Because lots of kernel subsystems need to be able to accept "any" device >> and don't care about the bus! That's what this is for. > > That's great, but: > >> All the bus >> wrappers would implement this so they can be used as an argument for all >> those subsystems (plus a generic one when you just need to pass around >> an actual owned generic reference and no longer need bus-specific >> operations - you can materialize that out of a RawDevice impl, which is >> when get_device() would be called). That's why I'm introducing this now, >> because both io_pgtable and rtkit need to take `struct device` pointers >> on the C side so we need some "generic struct device" view on the Rust side. > > In looking at both ftkit and io_pgtable, those seem to be good examples > of how "not to use a struct device", so trying to make safe bindings > from Rust to these frameworks is very ironic :) And this is why I want to use Rust, and why writing the abstractions for C code is so difficult... Rust encodes all these rules in the type system, but C doesn't, and so many kernel APIs don't document any of this or what the requirements are... > rtkit takes a struct device pointer and then never increments it, > despite saving it off, which is unsafe. It then only uses it to print > out messages if things go wrong (or right in some cases), which is odd. > So it can get away from using a device pointer entirely, except for the > devm_apple_rtkit_init() call, which I doubt you want to call from rust > code, right? That sounds like we need to fix the C side to grab a reference ^^ We do need to pass the device to the init function though (apple_rtkit_init(), this is in the SoC tree which I mentioned as a prequisite and already on the way to 6.3-rc1), since at the very least it has to pick up the mailbox and all that to initialize. Alternatively we could say that the C API contract is that the user of rtkit has to own a reference, and then the Rust abstraction would have to take that reference to make a safe abstraction, but that doesn't sound like the better option. What do you recommend for things that want to print device-associated messages, if not holding a reference to the device? Or did I misunderstand what you meant? Just pr_foo() isn't great because we have a lot of instances of rtkit and then you wouldn't know which device the messages are about... > for io_pgtable, that's a bit messier, you want to pass in a device that > io_pgtable treats as a "device" but again, it is NEVER properly > reference counted, AND, it is only needed to try to figure out the bus > operations that dma memory should be allocated from for this device. So > what would be better to save off there would be a pointer to the bus, > which is constant and soon will be read-only so there are no lifetime > rules needed at all (see the major struct bus_type changes going into > 6.3-rc1 that will enable that to happen). > > So the two subsystems you want to call from rust code don't properly > handle the reference count of the object you are going to pass into it, > and only need it for debugging and iommu stuff, which is really only the > bus that the device is on, not good examples to start out with :) Well, they're two examples that are dependencies for the driver I wrote, and I don't think you want me picking easy examples with zero known upcoming users... ^^;; > Yeah, this is yack-shaving, sorry, but it's how we clean up core > subsystems for apis and implementations that are not really correct and > were not noticed at the time. I'm fine with helping fix all this, and I don't expect all the underlying C code to be perfect already either! I already fixed one locking bug in DRM and spent a lot of time trying to figure out lifetime rules there, but I didn't dig into rtkit/io_pgtable and didn't realize they don't take references properly... > Can we see some users of this code posted so I can see how struct device > is going to work in a rust driver? That's the thing I worry most about > the rust/C interaction here as we have two different ways of thinking > about reference counts from the two worlds and putting them together is > going to be "interesting", as can be seen here already. I linked a tree with everything in the cover letter ([4]), look in drivers/gpu/drm/asahi for the actual driver. But there are a lot of other dependencies that have to go in before that will compile (everything else in that branch...) I know it's hard to review without examples, but I also can't just post the driver and everything else as one series now, there's still a lot to be improved and fixed and I'm working with the Rust folks on figuring out a roadmap for that... and waiting until "everything" is ready and perfect would mean we don't get anything done in the meantime and fall into a pit of endless rebasing and coordinating downstream trees, which also isn't good... ~~ Lina