From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail.free-electrons.com ([62.4.15.54]) by bombadil.infradead.org with esmtp (Exim 4.87 #1 (Red Hat Linux)) id 1cjSce-0001ZD-NY for linux-mtd@lists.infradead.org; Thu, 02 Mar 2017 15:26:30 +0000 Date: Thu, 2 Mar 2017 16:25:56 +0100 From: Boris Brezillon To: Frode Isaksen Cc: Vignesh R , 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 Subject: Re: [RFC PATCH 2/2] mtd: devices: m25p80: Enable spi-nor bounce buffer support Message-ID: <20170302162556.76b0ae8c@bbrezillon> In-Reply-To: <341ef45d-bad5-fd7c-aa05-807041c35f42@baylibre.com> 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> <341ef45d-bad5-fd7c-aa05-807041c35f42@baylibre.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit List-Id: Linux MTD discussion mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Thu, 2 Mar 2017 16:03:17 +0100 Frode Isaksen wrote: > 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. It's a bit more complicated than that. You may have operations running in background that are using those big vmalloc-ed buffers at runtime. To optimize things, we really need to split LEB/PEB buffers into multiple ->max_write_size (or ->min_io_size) kmalloc-ed buffers. > 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. Hm, the buffer itself is bigger than 11 bytes, it's just that the same buffer is used in different use cases, and sometime we're only partially filling it.