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=-5.5 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED,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 92F01C433DB for ; Mon, 8 Feb 2021 17:58:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4F75464E54 for ; Mon, 8 Feb 2021 17:58:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235203AbhBHR55 (ORCPT ); Mon, 8 Feb 2021 12:57:57 -0500 Received: from marcansoft.com ([212.63.210.85]:33448 "EHLO mail.marcansoft.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229623AbhBHPcN (ORCPT ); Mon, 8 Feb 2021 10:32:13 -0500 Received: from [127.0.0.1] (localhost [127.0.0.1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits)) (No client certificate requested) (Authenticated sender: marcan@marcan.st) by mail.marcansoft.com (Postfix) with ESMTPSA id D408541F47; Mon, 8 Feb 2021 15:31:20 +0000 (UTC) To: Marc Zyngier , Arnd Bergmann List-Id: Cc: SoC Team , Linux ARM , Rob Herring , "linux-kernel@vger.kernel.org" , DTML , Olof Johansson References: <20210204203951.52105-1-marcan@marcan.st> <20210204203951.52105-16-marcan@marcan.st> <87eehqlxlr.wl-maz@kernel.org> <87a6selrkt.wl-maz@kernel.org> From: Hector Martin Subject: Re: [PATCH 15/18] irqchip/apple-aic: Add support for the Apple Interrupt Controller Message-ID: Date: Tue, 9 Feb 2021 00:31:18 +0900 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 MIME-Version: 1.0 In-Reply-To: <87a6selrkt.wl-maz@kernel.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: es-ES Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 08/02/2021 20.36, Marc Zyngier wrote: > On Mon, 08 Feb 2021 10:29:23 +0000, > Arnd Bergmann wrote: >> >> On Mon, Feb 8, 2021 at 10:25 AM Marc Zyngier wrote: >>> On Thu, 04 Feb 2021 20:39:48 +0000, Hector Martin wrote: >> >>>> +{ >>>> + return readl(ic->base + reg); >>> >>> Please consider using the _relaxed accessors, as I don't think any of >>> these interacts with memory (apart from IPIs, of course). >> >> MSI interrupts require serializing with DMA, so at the minimum I think there >> needs to be something that ensures that DMA from device into memory >> has completed before delivering the completion interrupt to a driver. This >> may already be implied when the AIC is entered, but this is hard to know >> without actual hardware specs. > > If there is a sync with memory required, it should happen at the point > where it is Acked, not when masked/unmasked or anything else. And > given that you want to sync with an external agent (the DMA producer), > the DMB generated by readl won't save you, as it only orders CPU > accesses AFAICT. Found an doc that talks about this, but then... how does the current Linux code work anyway for normal use cases? https://elinux.org/images/7/73/Deacon-weak-to-weedy.pdf That says dmb is not enough for DMA-control to DMA-data dependencies due to speculation, which is what we have here and the situation I described (with an IRQ along the way, but that's irrelevant). But that's what readl does: a read followed by a dmb(oshld) followed by a control dependency (but that needs an isb to take effect). How does this not break drivers that read DMA-accessed memory after a readl of a status register? I thought that was the point of the non-relaxed functions. -- Hector Martin (marcan@marcan.st) Public Key: https://mrcn.st/pub