Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753668AbbHFNvq (ORCPT ); Thu, 6 Aug 2015 09:51:46 -0400 Received: from pandora.arm.linux.org.uk ([78.32.30.218]:39438 "EHLO pandora.arm.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753212AbbHFNvn (ORCPT ); Thu, 6 Aug 2015 09:51:43 -0400 Date: Thu, 6 Aug 2015 14:51:29 +0100 From: Russell King - ARM Linux To: Vignesh R Cc: Michal Suchanek , Mark Brown , devicetree , Brian Norris , Tony Lindgren , Linux Kernel Mailing List , linux-spi , Huang Shijie , MTD Maling List , linux-omap@vger.kernel.org, David Woodhouse , "linux-arm-kernel@lists.infradead.org" Subject: Re: [RFC PATCH 1/5] spi: introduce flag for memory mapped read Message-ID: <20150806135129.GJ7576@n2100.arm.linux.org.uk> References: <20150804155148.GR20873@sirena.org.uk> <55C0FD98.1090107@ti.com> <20150805115013.GJ20873@sirena.org.uk> <20150805124412.GN20873@sirena.org.uk> <20150806090202.GO20873@sirena.org.uk> <20150806102225.GI7576@n2100.arm.linux.org.uk> <55C35233.2000105@ti.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <55C35233.2000105@ti.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5389 Lines: 123 On Thu, Aug 06, 2015 at 05:55:23PM +0530, Vignesh R wrote: > On the whole following are my requirements: > 1. to be able to communicate with non -flash SPI devices via config port > ( this functionality is supported by current driver, I dont want to > break it). Or pump any spi_message on to SPI bus directly. > 2. take advantage of memory mapped port in order to increase read > throughput( and use dma in future) when the slave is a m25p80 type flash. > 3. handle m25p80 as well as other slave on multiple chipselects. > > I just need to know whether the user that requested the transfer is > m25p80 driver. If yes, ti-qspi driver can take advantage of memory > mapped interface, else just use config port to access SPI bus directly. The problem with this approach is that it's an abomination. It's adding a SPI-user specific hack which is detected by a specific driver. That's really not sane - what happens when we have lots of these kinds of "I'm an X SPI-user" with drivers detecting that? It's not maintainable in the long term. Yes, your requirements _today_ seem simple and easy, but you're only thinking about today, not tomorrow when you've moved on and someone else has to maintain the mess left behind (or delete it from mainline because they're sick of dealing with a hack.) > The spi_message that is received in transfer_one_message() is too > generic to imply the slave device that is on the other side of the wire. > IMO, the read command does not imply that the slave is m25p80 flash > (besides the read opcodes vary across vendors of m25p80 and across modes). I can see both sides of the argument. Mark is saying: if the SPI driver detects that the message to be transmitted is a read command followed by the appropriate number of dummy bytes, and then the data being read _and_ it's using quad-mode access, and the hardware generates _exactly_ that in hardware using the memory mapped mode, there is no reason _not_ to use the hardware to achieve that SPI transaction. The bus activity will be identical to what happens when the SPI controller is used manually to achieve that bus sequence. You're saying: but the documentation says you can't use it for anything except m25p80. If you look at 24.5.4.1.2, it tells you what the SFI generates on the bus, which is: 1. CS active 2. Read command byte sent 3. 1-4 address bytes sent 4. 0-3 dummy bytes sent 5. data bytes read from bus 6. CS inactive So, Mark's point is "if we can detect a transaction which fits _that_ bus activity, there's no reason not to use this acceleration for the transaction." What you're failing to counter with is: we don't have enough information in the SPI driver to know how many dummy bytes there are between the address bytes and the data read from the bus. The M25P80 driver just appends additional bytes to the message to achieve this: struct m25p *flash = nor->priv; unsigned int dummy = nor->read_dummy; /* convert the dummy cycles to the number of bytes */ dummy /= 8; flash->command[0] = nor->read_opcode; m25p_addr2cmd(nor, from, flash->command); t[0].tx_buf = flash->command; t[0].len = m25p_cmdsz(nor) + dummy; spi_message_add_tail(&t[0], &m); The reason that the number of dummy bytes can't be detected is because it's all hidden in the first transaction as the total number of bytes to be transmitted - and the dummy bytes are uninitialised, so you can't make any assumptions what value they are. There is no way for the SPI driver to know whether these dummy bytes are dummy bytes or whether they have an effect on the targetted device. I think that comprehensively rules out Mark's idea with the SPI core as it stands today: there is no way for the SPI driver to reliably detect a message like a SFI read command by parsing the SPI transmitted message prior to sending it as we can never be sure whether there are dummy bytes to be transmitted. What may make more sense from the SPI point of view is to communicate to all SPI drivers how many dummy bytes are to be transferred. I'm not fully up on SPI, but maybe something like this: t[0].tx_buf = flash->command; t[0].len = m25p_cmdsz(nor); spi_message_add_tail(&t[0], &m); t[1].tx_buf = dummy_buffer; t[1].len = dummy; t[1].dummy = 1; spi_message_add_tail(&t[1], &m); This way, we're describing the transfer to the SPI core, and explicitly indicating that there are some dummy bytes. The SPI driver can then tell that these are dummy bytes, and if the SPI message consists of: - transmit 2 to 5 bytes where the first byte is a recognised read command - transmit 0 to 3 dummy bytes - read some bytes then it can make use of the SFI mode to accelerate the operation. This would not be a hack to the SPI code: we're describing to the SPI code what we want to achieve in terms of the activity on the bus, and providing that level of description then allows the SPI driver to make informed decisions on whether it can handle the transfer using some non-standard feature. -- FTTC broadband for 0.8mile line: currently at 10.5Mbps down 400kbps up according to speedtest.net. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/