From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-wr0-x233.google.com ([2a00:1450:400c:c0c::233]) by bombadil.infradead.org with esmtps (Exim 4.87 #1 (Red Hat Linux)) id 1cjSGd-0005NY-1S for linux-mtd@lists.infradead.org; Thu, 02 Mar 2017 15:03:45 +0000 Received: by mail-wr0-x233.google.com with SMTP id g10so54210567wrg.2 for ; Thu, 02 Mar 2017 07:03:20 -0800 (PST) Subject: Re: [RFC PATCH 2/2] mtd: devices: m25p80: Enable spi-nor bounce buffer support To: Boris Brezillon , Vignesh R 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> <4cd22ddd-b108-f697-0bde-ad844a386e62@ti.com> <20170302152921.1c031b57@bbrezillon> Cc: Mark Brown , Cyrille Pitchen , Richard Weinberger , David Woodhouse , Brian Norris , Marek Vasut , linux-mtd@lists.infradead.org, linux-kernel@vger.kernel.org, linux-omap@vger.kernel.org, linux-spi@vger.kernel.org From: Frode Isaksen Message-ID: <341ef45d-bad5-fd7c-aa05-807041c35f42@baylibre.com> Date: Thu, 2 Mar 2017 16:03:17 +0100 MIME-Version: 1.0 In-Reply-To: <20170302152921.1c031b57@bbrezillon> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit List-Id: Linux MTD discussion mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On 02/03/2017 15:29, Boris Brezillon wrote: > On Thu, 2 Mar 2017 19:24:43 +0530 > Vignesh R wrote: > >>>>>> >>>>> 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. > Hm, that's not really true. For all cases where you have a DMA-able > buffer it would still use DMA. For other cases (like the UBI+SPI-NOR > case we're talking about here), yes, it will be slower, but slower is > still better than buggy. > So, in any case, I think the fixes pointed by Frode are needed. Also, I think the UBIFS layer only uses vmalloc'ed buffers during mount/unmount and not for read/write, so the performance hit is not that big. In most cases the buffer is the size of the erase block, but I've seen vmalloc'ed buffer of size only 11 bytes ! So, to optimize this, the best solution is probably to change how the UBIFS layer is using vmalloc'ed vs kmalloc'ed buffers, since vmalloc'ed should only be used for large (> 128K) buffers. Frode > >>>> 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. > Well, that's my opinion. The only one that can decide when to do > PIO, when to use DMA or when to use a bounce buffer+DMA is the SPI > controller. > If you move this logic to the SPI NOR layer, you'll have to guess what > is the best approach, and I fear the decision will be wrong on some > platforms (leading to perf degradation). > > You're mentioning code duplication in each SPI controller, I agree, > this is far from ideal, but what you're suggesting is not necessarily > better. What if another SPI user starts passing vmalloc-ed buffers to > the SPI controller? You'll have to duplicate the bounce-buffer logic in > this user as well. > >> 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. > That's a better approach IMHO. Note that the decision should not only > be based on the buffer type, but also on the transfer length and/or > whether the controller supports transferring non physically contiguous > buffers. > > Maybe we should just extend ->can_dma() to let the core know if it > should use a bounce buffer. > > Regarding the bounce buffer allocation logic, I'm not sure how it > should be done. The SPI user should be able to determine a max transfer > len (at least this is the case for SPI NORs) and inform the SPI layer > about this boundary so that the SPI core can allocate a bounce buffer > of this size. But we also have limitations at the SPI master level > (->max_transfer_size(), ->max_message_size()). > >