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, 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 800FCC4320A for ; Mon, 16 Aug 2021 09:27:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5918361B64 for ; Mon, 16 Aug 2021 09:27:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235431AbhHPJ2V convert rfc822-to-8bit (ORCPT ); Mon, 16 Aug 2021 05:28:21 -0400 Received: from aposti.net ([89.234.176.197]:51460 "EHLO aposti.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235190AbhHPJ2V (ORCPT ); Mon, 16 Aug 2021 05:28:21 -0400 Date: Mon, 16 Aug 2021 11:27:40 +0200 From: Paul Cercueil Subject: Re: IIO, dmabuf, io_uring To: Christoph Hellwig Cc: Jonathan Cameron , Sumit Semwal , Christian =?iso-8859-1?b?S/ZuaWc=?= , linux-iio@vger.kernel.org, io-uring@vger.kernel.org, linux-media@vger.kernel.org, linux-kernel@vger.kernel.org, Michael Hennerich , Alexandru Ardelean , dri-devel@lists.freedesktop.org, linaro-mm-sig@lists.linaro.org Message-Id: <4AEXXQ.7Z97EUWQOO0Q3@crapouillou.net> In-Reply-To: <20210814073019.GC21175@lst.de> References: <2H0SXQ.2KIK2PBVRFWH2@crapouillou.net> <20210814073019.GC21175@lst.de> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1; format=flowed Content-Transfer-Encoding: 8BIT Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org Hi Christoph, Le sam., août 14 2021 at 09:30:19 +0200, Christoph Hellwig a écrit : > On Fri, Aug 13, 2021 at 01:41:26PM +0200, Paul Cercueil wrote: >> 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. > > To me this does sound a lot like a dma buf use case. The interesting > question to me is how to signal arrival of new data, or readyness to > consume more data. I suspect that people that are actually using > dmabuf heavily at the moment (dri/media folks) might be able to chime > in a little more on that. Thanks for the feedback. I haven't looked too much into how dmabuf works; but IIO device nodes right now have a regular stdio interface, so I believe poll() flags can be used to signal arrival of new data. >> 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. > > Think of io_uring really just as an async syscall layer. It doesn't > replace DMA buffers, but can be used as a different and for some > workloads more efficient way to dispatch syscalls. That was my thought, yes. Thanks. Cheers, -Paul