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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 10453C433DF for ; Thu, 11 Jun 2020 14:11:16 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E5BE62083E for ; Thu, 11 Jun 2020 14:11:15 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728033AbgFKOLP (ORCPT ); Thu, 11 Jun 2020 10:11:15 -0400 Received: from foss.arm.com ([217.140.110.172]:53094 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726444AbgFKOLM (ORCPT ); Thu, 11 Jun 2020 10:11:12 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id E45301F1; Thu, 11 Jun 2020 07:11:11 -0700 (PDT) Received: from gaia (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id A4C0F3F6CF; Thu, 11 Jun 2020 07:11:08 -0700 (PDT) Date: Thu, 11 Jun 2020 15:11:02 +0100 From: Catalin Marinas To: Wooyeon Kim Cc: 'Dave Martin' , 'Mark Rutland' , 'Bhupesh Sharma' , 'Julien Grall' , 'Vincenzo Frascino' , 'Will Deacon' , yhwan.joo@samsung.com, 'Anisse Astier' , 'Marc Zyngier' , 'Allison Randal' , 'Sanghoon Lee' , jihun.kim@samsung.com, 'Kees Cook' , 'Suzuki K Poulose' , 'Wooki Min' , 'Kristina Martsenko' , 'Jeongtae Park' , 'Thomas Gleixner' , linux-arm-kernel@lists.infradead.org, 'Steve Capper' , 'Greg Kroah-Hartman' , linux-kernel@vger.kernel.org, 'James Morse' , 'Sudeep Holla' , dh.han@samsung.com Subject: Re: [PATCH] arm64: fpsimd: Added API to manage fpsimd state inside kernel Message-ID: <20200611141101.GA31408@gaia> References: <20200605073052.23044-1-wooy88.kim@samsung.com> <20200605103705.GD85498@C02TD0UTHF1T.local> <20200608103340.GA31466@arm.com> <001401d63fd4$95646690$c02d33b0$@samsung.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <001401d63fd4$95646690$c02d33b0$@samsung.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jun 11, 2020 at 06:42:12PM +0900, Wooyeon Kim wrote: > I am in charge of camera driver development in Samsung S.LSI division. > > In order to guarantee real time processing such as Camera 3A algorithm in > current or ongoing projects, prebuilt binary is loaded and used in kernel > space, rather than user space. Thanks for the additional details. If you do such intensive processing in an IRQ context you'd probably introduce additional IRQ latency. Wouldn't offloading such work to a real-time (user) thread help? In a non-preempt-rt kernel, I don't think you can get much in terms of (soft) guarantees for IRQ latency anyway. > Because the binary is built with other standard library which could use > FPSIMD register, kernel API should keep the original FPSIMD state for other > user tasks. Can you not recompile those libraries not to use FP? As Mark said, for a kernel API we require at least an in-kernel, upstreamed, user of that functionality. > In the case of the kernel_neon_begin / kernel_neon_end that you mentioned, > there is a limitation that cannot be used in hardirq context. > Also, if another kernel task switching occurs while kernel API is being > used, fpsimd register corruption may occur. kernel_neon_begin/end disable preemption, so you can't have a task switch (you can have interrupts though but we don't allow FPSIMD in IRQ context). -- Catalin