Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751749AbdCAO3N convert rfc822-to-8bit (ORCPT ); Wed, 1 Mar 2017 09:29:13 -0500 Received: from mail.free-electrons.com ([62.4.15.54]:41902 "EHLO mail.free-electrons.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750787AbdCAO3L (ORCPT ); Wed, 1 Mar 2017 09:29:11 -0500 Date: Wed, 1 Mar 2017 15:28:55 +0100 From: Boris Brezillon To: Cyrille Pitchen Cc: Vignesh R , Richard Weinberger , David Woodhouse , Brian Norris , Marek Vasut , , , , Frode Isaksen , , Mark Brown Subject: Re: [RFC PATCH 2/2] mtd: devices: m25p80: Enable spi-nor bounce buffer support Message-ID: <20170301152855.0cfcdf0f@bbrezillon> In-Reply-To: <8a2c9b3b-dd5f-fca7-fa5c-690e5bed949f@atmel.com> References: <20170227120839.16545-1-vigneshr@ti.com> <20170227120839.16545-3-vigneshr@ti.com> <8f999a27-c3ce-2650-452c-b21c3e44989d@ti.com> <8a2c9b3b-dd5f-fca7-fa5c-690e5bed949f@atmel.com> X-Mailer: Claws Mail 3.13.2 (GTK+ 2.24.30; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4645 Lines: 105 On Wed, 1 Mar 2017 15:21:24 +0100 Cyrille Pitchen wrote: > + Mark > > Le 01/03/2017 à 12:46, Vignesh R a écrit : > > > > > > On Wednesday 01 March 2017 04:13 PM, Cyrille Pitchen wrote: > >> Le 01/03/2017 à 05:54, Vignesh R a écrit : > >>> > >>> > >>> On Wednesday 01 March 2017 03:11 AM, Richard Weinberger wrote: > >>>> Vignesh, > >>>> > >>>> Am 27.02.2017 um 13:08 schrieb Vignesh R: > >>>>> Many SPI controller drivers use DMA to read/write from m25p80 compatible > >>>>> flashes. Therefore enable bounce buffers support provided by spi-nor > >>>>> framework to take care of handling vmalloc'd buffers which may not be > >>>>> DMA'able. > >>>>> > >>>>> Signed-off-by: Vignesh R > >>>>> --- > >>>>> drivers/mtd/devices/m25p80.c | 1 + > >>>>> 1 file changed, 1 insertion(+) > >>>>> > >>>>> diff --git a/drivers/mtd/devices/m25p80.c b/drivers/mtd/devices/m25p80.c > >>>>> index c4df3b1bded0..d05acf22eadf 100644 > >>>>> --- a/drivers/mtd/devices/m25p80.c > >>>>> +++ b/drivers/mtd/devices/m25p80.c > >>>>> @@ -241,6 +241,7 @@ static int m25p_probe(struct spi_device *spi) > >>>>> else > >>>>> flash_name = spi->modalias; > >>>>> > >>>>> + nor->flags |= SNOR_F_USE_BOUNCE_BUFFER; > >>>> > >>>> Isn't there a better way to detect whether a bounce buffer is needed or not? > >>> > >> > >> I agree with Richard: the bounce buffer should be enabled only if needed > >> by the SPI controller. > >> > >>> Yes, I can poke the spi->master struct to see of dma channels are > >>> populated and request SNOR_F_USE_BOUNCE_BUFFER accordingly: > >>> > >>> - nor->flags |= SNOR_F_USE_BOUNCE_BUFFER; > >>> + if (spi->master->dma_tx || spi->master->dma_rx) > >>> + nor->flags |= SNOR_F_USE_BOUNCE_BUFFER; > >>> + > >>> > >> > >> However I don't agree with this solution: master->dma_{tx|rx} can be set > >> for SPI controllers which already rely on spi_map_msg() to handle > >> vmalloc'ed memory during DMA transfers. > >> Such SPI controllers don't need the spi-nor bounce buffer. > >> > >> spi_map_msg() can build a scatter-gather list from vmalloc'ed buffer > >> then map this sg list with dma_map_sg(). AFAIK, It is safe to do so for > >> architectures using PIPT caches since the possible cache aliases issue > >> present for VIPT or VIVT caches is always avoided for PIPT caches. > >> > >> For instance, the drivers/spi/spi-atmel.c driver relies on spi_map_sg() > >> to be called from the SPI sub-system to handle vmalloc'ed buffers and > >> both master->dma_tx and master->dma_rx are set by the this driver. > >> > >> > >> By the way, Is there any case where the same physical page is actually > >> mapped into two different virtual addresses for the buffers allocated by > >> the MTD sub-system? Because for a long time now I wonder whether the > >> cache aliases issue is a real or only theoretical issue but I have no > >> answer to that question. > >> > > > > I have atleast one evidence of VIVT aliasing causing problem. Please see > > this thread on DMA issues with davinci-spi driver > > https://www.spinics.net/lists/arm-kernel/msg563420.html > > https://www.spinics.net/lists/arm-kernel/msg563445.html > > > >> Then my next question: is spi_map_msg() enough in every case, even with > >> VIPT or VIVT caches? > >> > > > > 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. > > > > Both Frode and you confirmed that the alias issue does occur at least > with VIVT caches, hence we can't rely on spi_map_msg() in that case. > So I agree with you: adding a bounce buffer in spi-nor seems to be a > good solution at least till some rework is done in the ubifs layer, as > proposed by Boris, to replace vmalloc'ed buffers by kmalloc'ed memory. We should keep it even after reworking UBI/UBIFS, because UBI is just one user of MTD, and other users might pass vmalloc-ed or kmap-ed bufs.