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 1C867C433DF for ; Tue, 9 Jun 2020 10:01:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id F01B8207C3 for ; Tue, 9 Jun 2020 10:01:02 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728267AbgFIKA5 (ORCPT ); Tue, 9 Jun 2020 06:00:57 -0400 Received: from foss.arm.com ([217.140.110.172]:39456 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726765AbgFIKA5 (ORCPT ); Tue, 9 Jun 2020 06:00:57 -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 2F73A1FB; Tue, 9 Jun 2020 03:00:56 -0700 (PDT) Received: from [10.57.11.57] (unknown [10.57.11.57]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 850DF3F73D; Tue, 9 Jun 2020 03:00:47 -0700 (PDT) Subject: Re: [PATCH v9 RESEND 01/13] spi: imx: add dma_sync_sg_for_device after fallback from dma To: Robin Gong , Mark Brown Cc: "mark.rutland@arm.com" , "devicetree@vger.kernel.org" , "matthias.schiffer@ew.tq-group.com" , "martin.fuzzey@flowbird.group" , "catalin.marinas@arm.com" , "s.hauer@pengutronix.de" , "will.deacon@arm.com" , "linux-kernel@vger.kernel.org" , "linux-spi@vger.kernel.org" , "vkoul@kernel.org" , "robh+dt@kernel.org" , dl-linux-imx , "festevam@gmail.com" , "u.kleine-koenig@pengutronix.de" , "dmaengine@vger.kernel.org" , "dan.j.williams@intel.com" , "shawnguo@kernel.org" , "kernel@pengutronix.de" , "linux-arm-kernel@lists.infradead.org" References: <1591485677-20533-1-git-send-email-yibin.gong@nxp.com> <1591485677-20533-2-git-send-email-yibin.gong@nxp.com> <20200608143458.GH4593@sirena.org.uk> <20200608153139.GI4593@sirena.org.uk> <59ce3620-00b9-bac1-30e1-011a29583642@arm.com> From: Robin Murphy Message-ID: Date: Tue, 9 Jun 2020 11:00:33 +0100 User-Agent: Mozilla/5.0 (Windows NT 10.0; rv:68.0) Gecko/20100101 Thunderbird/68.8.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 7bit Sender: dmaengine-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: dmaengine@vger.kernel.org On 2020-06-09 06:21, Robin Gong wrote: > On 2020/06/09 0:44 Robin Murphy wrote: >> On 2020-06-08 16:31, Mark Brown wrote: >>> On Mon, Jun 08, 2020 at 03:08:45PM +0000, Robin Gong wrote: >>> >>>>>> + if (transfer->rx_sg.sgl) { >>>>>> + struct device *rx_dev = spi->controller->dma_rx->device->dev; >>>>>> + >>>>>> + dma_sync_sg_for_device(rx_dev, transfer->rx_sg.sgl, >>>>>> + transfer->rx_sg.nents, DMA_TO_DEVICE); >>>>>> + } >>>>>> + >>> >>>>> This is confusing - why are we DMA mapping to the device after doing >>>>> a PIO transfer? >>> >>>> 'transfer->rx_sg.sgl' condition check that's the case fallback PIO >>>> after DMA transfer failed. But the spi core still think the buffer >>>> should be in 'device' while spi driver touch it by PIO(CPU), so sync it back to >> device to ensure all received data flush to DDR. >>> >>> So we sync it back to the device so that we can then do another sync >>> to CPU? TBH I'm a bit surprised that there's a requirement that we >>> explicitly undo a sync and that a redundant double sync in the same >>> direction might be an issue but I've not had a need to care so I'm >>> perfectly prepared to believe there is. >>> >>> At the very least this needs a comment. >> >> Yeah, something's off here - at the very least, syncing with DMA_TO_DEVICE on >> the Rx buffer that was mapped with DMA_FROM_DEVICE is clearly wrong. >> CONFIG_DMA_API_DEBUG should scream about that. >> >> If the device has written to the buffer at all since dma_map_sg() was called >> then you do need a dma_sync_sg_for_cpu() call before touching it from a CPU >> fallback path, but if nobody's going to touch it from that point until it's >> unmapped then there's no point syncing it again. The >> my_card_interrupt_handler() example in DMA-API_HOWTO.txt demonstrates >> this. > Thanks for you post, but sorry, that's not spi-imx case now, because the rx data in device memory is not truly updated from 'device'/DMA, but from PIO, so that dma_sync_sg_for_cpu with DMA_FROM_DEVICE can't be used, otherwise the fresh data in cache will be invalidated. > But you're right, kernel warning comes out if CONFIG_DMA_API_DEBUG enabled... Ah, I think I understand what's going on now. That's... really ugly :( Looking at the SPI core code, I think a better way to handle this would be to have your fallback path call spi_unmap_buf() directly (or perform the same actions, if exporting that to drivers is unacceptable), then make sure ->can_dma() returns false after that such that spi_unmap_msg() won't try to unmap it again. That's a lot more reasonable than trying to fake up a DMA_TO_DEVICE transfer in the middle of a DMA_FROM_DEVICE operation on the same buffer. Alternatively, is it feasible to initiate a dummy DMA request during probe, such that you can detect the failure condition and give up on the DMA channel early, and not have to deal with it during a real SPI transfer? Robin.