Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752180AbdCCWHx (ORCPT ); Fri, 3 Mar 2017 17:07:53 -0500 Received: from lelnx194.ext.ti.com ([198.47.27.80]:62375 "EHLO lelnx194.ext.ti.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751869AbdCCWHv (ORCPT ); Fri, 3 Mar 2017 17:07:51 -0500 Subject: Re: [RFC PATCH 2/2] mtd: devices: m25p80: Enable spi-nor bounce buffer support To: Frode Isaksen , Boris Brezillon , Mark Brown References: <20170227120839.16545-1-vigneshr@ti.com> <20170227120839.16545-3-vigneshr@ti.com> <8f999a27-c3ce-2650-452c-b21c3e44989d@ti.com> <20170301175506.202cb478@bbrezillon> <09ffe06d-565d-afe8-8b7d-d1a0b575595b@baylibre.com> CC: Cyrille Pitchen , Richard Weinberger , David Woodhouse , Brian Norris , Marek Vasut , , , , From: Vignesh R Message-ID: <4cd22ddd-b108-f697-0bde-ad844a386e62@ti.com> Date: Thu, 2 Mar 2017 19:24:43 +0530 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.7.0 MIME-Version: 1.0 In-Reply-To: <09ffe06d-565d-afe8-8b7d-d1a0b575595b@baylibre.com> Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1912 Lines: 45 >>>> >>> Not really, I am debugging another issue with UBIFS on DRA74 EVM (ARM >>> cortex-a15) wherein pages allocated by vmalloc are in highmem region >>> that are not addressable using 32 bit addresses and is backed by LPAE. >>> So, a 32 bit DMA cannot access these buffers at all. >>> When dma_map_sg() is called to map these pages by spi_map_buf() the >>> physical address is just truncated to 32 bit in pfn_to_dma() (as part of >>> dma_map_sg() call). This results in random crashes as DMA starts >>> accessing random memory during SPI read. >>> >>> IMO, there may be more undiscovered caveat with using dma_map_sg() for >>> non kmalloc'd buffers and its better that spi-nor starts handling these >>> buffers instead of relying on spi_map_msg() and working around every >>> time something pops up. >>> >> Ok, I had a closer look at the SPI framework, and it seems there's a >> way to tell to the core that a specific transfer cannot use DMA >> (->can_dam()). The first thing you should do is fix the spi-davinci >> driver: >> >> 1/ implement ->can_dma() >> 2/ patch davinci_spi_bufs() to take the decision to do DMA or not on a >> per-xfer basis and not on a per-device basis >> This would lead to poor perf defeating entire purpose of using DMA. >> Then we can start thinking about how to improve perfs by using a bounce >> buffer for large transfers, but I'm still not sure this should be done >> at the MTD level... If its at SPI level, then I guess each individual drivers which cannot handle vmalloc'd buffers will have to implement bounce buffer logic. Or SPI core can be extended in a way similar to this RFC. That is, SPI master driver will set a flag to request SPI core to use of bounce buffer for vmalloc'd buffers. And spi_map_buf() just uses bounce buffer in case buf does not belong to kmalloc region based on the flag. Mark, Cyrille, Is that what you prefer? -- Regards Vignesh