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 8BCE0C678D5 for ; Fri, 24 Feb 2023 14:11:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229827AbjBXOLM (ORCPT ); Fri, 24 Feb 2023 09:11:12 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60318 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229622AbjBXOLK (ORCPT ); Fri, 24 Feb 2023 09:11:10 -0500 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3AA0412B; Fri, 24 Feb 2023 06:11:09 -0800 (PST) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id C836B61839; Fri, 24 Feb 2023 14:11:08 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id A512EC433EF; Fri, 24 Feb 2023 14:11:07 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1677247868; bh=ILec1+10culSs+4LFcu4lSKq2NAnqzI8CinlFn1UHX0=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=X5RRtLW6bsr/7UHfOmsN3l4DkaUpOu9tISvL05vIbTpOsR9fYGuvcHyaMvmyqaBMJ ZLsK7n4KM2xdKyXZoDZC2QjQcyQtNKZnLtxaG+bu3PkCd9Pj8Bx2Qm6NjymWSWuxJx 3kZpU2ead9fExo/nCjFMJLS+SK4UgO8ZYbOH/gv8= Date: Fri, 24 Feb 2023 15:11:05 +0100 From: Greg Kroah-Hartman To: Asahi Lina Cc: Miguel Ojeda , Alex Gaynor , Wedson Almeida Filho , Boqun Feng , Gary Guo , =?iso-8859-1?Q?Bj=F6rn?= 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 Subject: Re: [PATCH 2/5] rust: device: Add a minimal RawDevice trait Message-ID: References: <20230224-rust-iopt-rtkit-v1-0-49ced3391295@asahilina.net> <20230224-rust-iopt-rtkit-v1-2-49ced3391295@asahilina.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 :) 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? 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 :) 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. 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. thanks, greg k-h