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=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 BFFD8C4338F for ; Fri, 13 Aug 2021 11:41:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A00A4610A5 for ; Fri, 13 Aug 2021 11:41:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240046AbhHMLmG (ORCPT ); Fri, 13 Aug 2021 07:42:06 -0400 Received: from aposti.net ([89.234.176.197]:46710 "EHLO aposti.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S239938AbhHMLmF (ORCPT ); Fri, 13 Aug 2021 07:42:05 -0400 Date: Fri, 13 Aug 2021 13:41:26 +0200 From: Paul Cercueil Subject: IIO, dmabuf, io_uring To: Jonathan Cameron , Sumit Semwal , Christian =?iso-8859-1?b?S/ZuaWc=?= , Christoph Hellwig Cc: linux-iio@vger.kernel.org, io-uring@vger.kernel.org, linux-media@vger.kernel.org, linux-kernel@vger.kernel.org, Michael Hennerich , Alexandru Ardelean Message-Id: <2H0SXQ.2KIK2PBVRFWH2@crapouillou.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org Hi, A few months ago we (ADI) tried to upstream the interface we use with our high-speed ADCs and DACs. It is a system with custom ioctls on the iio device node to dequeue and enqueue buffers (allocated with dma_alloc_coherent), that can then be mmap'd by userspace applications. Anyway, it was ultimately denied entry [1]; this API was okay in ~2014 when it was designed but it feels like re-inventing the wheel in 2021. Back to the drawing table, and we'd like to design something that we can actually upstream. This high-speed interface looks awfully similar to DMABUF, so we may try to implement a DMABUF interface for IIO, unless someone has a better idea. Our first usecase is, we want userspace applications to be able to dequeue buffers of samples (from ADCs), and/or enqueue buffers of samples (for DACs), and to be able to manipulate them (mmapped buffers). With a DMABUF interface, I guess the userspace application would dequeue a dma buffer from the driver, mmap it, read/write the data, unmap it, then enqueue it to the IIO driver again so that it can be disposed of. Does that sound sane? Our second usecase is - and that's where things get tricky - to be able to stream the samples to another computer for processing, over Ethernet or USB. Our typical setup is a high-speed ADC/DAC on a dev board with a FPGA and a weak soft-core or low-power CPU; processing the data in-situ is not an option. Copying the data from one buffer to another is not an option either (way too slow), so we absolutely want zero-copy. Usual userspace zero-copy techniques (vmsplice+splice, MSG_ZEROCOPY etc) don't really work with mmapped kernel buffers allocated for DMA [2] and/or have a huge overhead, so the way I see it, we would also need DMABUF support in both the Ethernet stack and USB (functionfs) stack. However, as far as I understood, DMABUF is mostly a DRM/V4L2 thing, so I am really not sure we have the right idea here. And finally, there is the new kid in town, io_uring. I am not very literate about the topic, but it does not seem to be able to handle DMA buffers (yet?). The idea that we could dequeue a buffer of samples from the IIO device and send it over the network in one single syscall is appealing, though. Any thoughts? Feedback would be greatly appreciated. Cheers, -Paul [1]: https://lore.kernel.org/linux-iio/20210217073638.21681-1-alexandru.ardelean@analog.com/T/#m6b853addb77959c55e078fbb06828db33d4bf3d7 [2]: https://newbedev.com/zero-copy-user-space-tcp-send-of-dma-mmap-coherent-mapped-memory