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=-7.1 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=no 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 E117AC4727E for ; Wed, 30 Sep 2020 09:55:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9B6F3206D4 for ; Wed, 30 Sep 2020 09:55:47 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=ffwll.ch header.i=@ffwll.ch header.b="eT9cqWNg" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728695AbgI3Jzr (ORCPT ); Wed, 30 Sep 2020 05:55:47 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:47344 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725776AbgI3Jzq (ORCPT ); Wed, 30 Sep 2020 05:55:46 -0400 Received: from mail-wr1-x443.google.com (mail-wr1-x443.google.com [IPv6:2a00:1450:4864:20::443]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 94AE9C061755 for ; Wed, 30 Sep 2020 02:55:46 -0700 (PDT) Received: by mail-wr1-x443.google.com with SMTP id z4so1069257wrr.4 for ; Wed, 30 Sep 2020 02:55:46 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=ffwll.ch; s=google; h=date:from:to:cc:subject:message-id:mail-followup-to:references :mime-version:content-disposition:content-transfer-encoding :in-reply-to; bh=QnJosaKk+6UlgjPbZmsy+2/c8gwAv95gBGvvrsjNxEg=; b=eT9cqWNgDxAhVmLEzVeN5IAAnwRLaybn8Mw9iEkk16/nBoIH2EO+om9rA1o9iTl2A+ b3pC94nsrkanU9ZVS+jp50g4xt7tyJV8pijzVboQKo0nr+Ph6t1pgySRTqV1BDcss6hH ys1OUca6Z1XZq45Hy8JxEIia8LP5mrsHYGJtY= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id :mail-followup-to:references:mime-version:content-disposition :content-transfer-encoding:in-reply-to; bh=QnJosaKk+6UlgjPbZmsy+2/c8gwAv95gBGvvrsjNxEg=; b=n8XOfhTy+WusGvY5Xjk9zg29tHSV5gcux8g+wE+yy7xUDSsMIdlfKezlK5F4VQj1RJ Gf/OKl3Lf6BfAggb4UuACq98/EsrxXXt+V7k6W0nL7VJ2IWvZHcT9/QUwYBmthFNttAs dYzrJWh4lBa4q5BO3niD8fFQJLWsTkG0vloCLs9ujCHV4yZ9xP6IedmpAcex54syzwRP TNJiFw569YHSJ4Kjx3d8alyooEllWIWz0/esVXdvW5K8wpn3n8LwLn0eHQJelg6S3mb0 2q2yx20xSNGnJvBvDXbt/lCaZPmJyS7HjujO+sX2KWsQcCwxHlNaQwIebsb8jGVqVaZb BZ4A== X-Gm-Message-State: AOAM531TOKAcP63bCPiIDGO6TtFxjEftVCwC6MmzklJ8RuO4vI5lpt2T Tsuqb8ruAoCNC5uD5Bd9gNNs0Q== X-Google-Smtp-Source: ABdhPJzwgGtmE4gHk7un+VHQGLY1/OmHMFNtHzY+QeRZgyppjz9cdlSzmPs669vfGphmd66u+lMFrw== X-Received: by 2002:adf:edd2:: with SMTP id v18mr2173577wro.242.1601459745199; Wed, 30 Sep 2020 02:55:45 -0700 (PDT) Received: from phenom.ffwll.local ([2a02:168:57f4:0:efd0:b9e5:5ae6:c2fa]) by smtp.gmail.com with ESMTPSA id x10sm1832792wmi.37.2020.09.30.02.55.43 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 30 Sep 2020 02:55:44 -0700 (PDT) Date: Wed, 30 Sep 2020 11:55:42 +0200 From: Daniel Vetter To: Michel =?iso-8859-1?Q?D=E4nzer?= Cc: Jason Ekstrand , Chenbo Feng , daniels@collabora.com, daniel.vetter@ffwll.ch, jajones@nvidia.com, linux-kernel@vger.kernel.org, Greg Hackmann , linaro-mm-sig@lists.linaro.org, hoegsberg@google.com, dri-devel@lists.freedesktop.org, jessehall@google.com, airlied@redhat.com, christian.koenig@amd.com, linux-media@vger.kernel.org Subject: Re: [PATCH 3/3] RFC: dma-buf: Add an API for importing and exporting sync files (v5) Message-ID: <20200930095542.GY438822@phenom.ffwll.local> Mail-Followup-To: Michel =?iso-8859-1?Q?D=E4nzer?= , Jason Ekstrand , Chenbo Feng , daniels@collabora.com, jajones@nvidia.com, linux-kernel@vger.kernel.org, Greg Hackmann , linaro-mm-sig@lists.linaro.org, hoegsberg@google.com, dri-devel@lists.freedesktop.org, jessehall@google.com, airlied@redhat.com, christian.koenig@amd.com, linux-media@vger.kernel.org References: <20200311034351.1275197-3-jason@jlekstrand.net> <20200317212115.419358-1-jason@jlekstrand.net> <64eed158-22a8-10a7-7686-c972f8542649@daenzer.net> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <64eed158-22a8-10a7-7686-c972f8542649@daenzer.net> X-Operating-System: Linux phenom 5.7.0-1-amd64 Precedence: bulk List-ID: X-Mailing-List: linux-media@vger.kernel.org On Wed, Sep 30, 2020 at 11:39:06AM +0200, Michel Dänzer wrote: > On 2020-03-17 10:21 p.m., Jason Ekstrand wrote: > > Explicit synchronization is the future. At least, that seems to be what > > most userspace APIs are agreeing on at this point. However, most of our > > Linux APIs (both userspace and kernel UAPI) are currently built around > > implicit synchronization with dma-buf. While work is ongoing to change > > many of the userspace APIs and protocols to an explicit synchronization > > model, switching over piecemeal is difficult due to the number of > > potential components involved. On the kernel side, many drivers use > > dma-buf including GPU (3D/compute), display, v4l, and others. In > > userspace, we have X11, several Wayland compositors, 3D drivers, compute > > drivers (OpenCL etc.), media encode/decode, and the list goes on. > > > > This patch provides a path forward by allowing userspace to manually > > manage the fences attached to a dma-buf. Alternatively, one can think > > of this as making dma-buf's implicit synchronization simply a carrier > > for an explicit fence. This is accomplished by adding two IOCTLs to > > dma-buf for importing and exporting a sync file to/from the dma-buf. > > This way a userspace component which is uses explicit synchronization, > > such as a Vulkan driver, can manually set the write fence on a buffer > > before handing it off to an implicitly synchronized component such as a > > Wayland compositor or video encoder. In this way, each of the different > > components can be upgraded to an explicit synchronization model one at a > > time as long as the userspace pieces connecting them are aware of it and > > import/export fences at the right times. > > > > There is a potential race condition with this API if userspace is not > > careful. A typical use case for implicit synchronization is to wait for > > the dma-buf to be ready, use it, and then signal it for some other > > component. Because a sync_file cannot be created until it is guaranteed > > to complete in finite time, userspace can only signal the dma-buf after > > it has already submitted the work which uses it to the kernel and has > > received a sync_file back. There is no way to atomically submit a > > wait-use-signal operation. This is not, however, really a problem with > > this API so much as it is a problem with explicit synchronization > > itself. The way this is typically handled is to have very explicit > > ownership transfer points in the API or protocol which ensure that only > > one component is using it at any given time. Both X11 (via the PRESENT > > extension) and Wayland provide such ownership transfer points via > > explicit present and idle messages. > > > > The decision was intentionally made in this patch to make the import and > > export operations IOCTLs on the dma-buf itself rather than as a DRM > > IOCTL. This makes it the import/export operation universal across all > > components which use dma-buf including GPU, display, v4l, and others. > > It also means that a userspace component can do the import/export > > without access to the DRM fd which may be tricky to get in cases where > > the client communicates with DRM via a userspace API such as OpenGL or > > Vulkan. At a future date we may choose to add direct import/export APIs > > to components such as drm_syncobj to avoid allocating a file descriptor > > and going through two ioctls. However, that seems to be something of a > > micro-optimization as import/export operations are likely to happen at a > > rate of a few per frame of rendered or decoded video. > > > > v2 (Jason Ekstrand): > > - Use a wrapper dma_fence_array of all fences including the new one > > when importing an exclusive fence. > > > > v3 (Jason Ekstrand): > > - Lock around setting shared fences as well as exclusive > > - Mark SIGNAL_SYNC_FILE as a read-write ioctl. > > - Initialize ret to 0 in dma_buf_wait_sync_file > > > > v4 (Jason Ekstrand): > > - Use the new dma_resv_get_singleton helper > > > > v5 (Jason Ekstrand): > > - Rename the IOCTLs to import/export rather than wait/signal > > - Drop the WRITE flag and always get/set the exclusive fence > > > > Signed-off-by: Jason Ekstrand > > What's the status of this? DMA_BUF_IOCTL_EXPORT_SYNC_FILE would be useful > for Wayland compositors to wait for client buffers to become ready without > being prone to getting delayed by later HW access to them, so it would be > nice to merge that at least (if DMA_BUF_IOCTL_IMPORT_SYNC_FILE is still > controversial). I think the missing bits are just the usual stuff - igt testcases - userspace using the new ioctls - review of the entire pile I don't think there's any fundamental objections aside from "no one ever pushed this over the finish line". Cheers, Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch