From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1762032Ab0HMSn0 (ORCPT ); Fri, 13 Aug 2010 14:43:26 -0400 Received: from ns2.cypress.com ([157.95.67.5]:45040 "EHLO ns2.cypress.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753184Ab0HMSnY (ORCPT ); Fri, 13 Aug 2010 14:43:24 -0400 Subject: Re: EXPORT_SYMBOL(fat_get_block) From: David Cross Reply-To: david.cross@cypress.com To: Greg KH Cc: hirofumi@mail.parknet.co.jp, linux-kernel@vger.kernel.org In-Reply-To: <20100813175402.GA15024@suse.de> References: <1281721539.5839.4.camel@odc-laptop> <20100813175402.GA15024@suse.de> Content-Type: text/plain Organization: Cypress Date: Fri, 13 Aug 2010 11:43:12 -0700 Message-Id: <1281724992.12189.16.camel@odc-laptop> Mime-Version: 1.0 X-Mailer: Evolution 2.26.1 Content-Transfer-Encoding: 7bit X-Brightmail-Tracker: AAAAAA== X-Brightmail-Tracker: AAAAAA== X-MailScanner: Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 2010-08-13 at 10:54 -0700, Greg KH wrote: > On Fri, Aug 13, 2010 at 10:45:39AM -0700, David Cross wrote: > > Hello Hirofumi, > > I would like to export this symbol from the vfat code and add this patch > > to the Linux kernel. You are listed as the MAINTAINER for FAT and VFAT. > > As such, I need your approval to do it. > > The reason that I need to export this symbol is to allow for file based > > DMA in which the file needs to be pre-allocated. The pre-allocated block > > addresses are passed to a DMA engine which then directly transfers the > > data to non-volatile storage. > > Please let me know if you have any objections to exporting this symbol > > or if you need additional information from me concerning this change > > request. I am happy to answer any questions that you may have about this > > in the meantime. > > Wait, _I_ have an objection to exporting this symbol. > > I really don't think it is needed at this point in time, as we really > need to figure out exactly what your driver is doing to want to need to > call this function. OK, I am trying to answer all questions on this topic that I am getting, but honestly I have not gotten a lot so far. > So please, let's not export it for now, when we get to the point that we > all agree that this driver is doing the "correct" thing, then we can > export the symbol, ok? Sure, I think that the driver is doing the correct thing, but that is mostly because I have not found another way to solve this issue. If there is a different, correct way I am happy to implement it. > thanks, >Also, why do this at the FAT level, and not more correctly at the block > layer level? The short answer is that the block level does not have the file system awareness needed to determine where a file should be stored on the NVM device. The longer answer is to consider the case of an MTP host, which operates at the file level. It sends the mobile (Linux) device "SendObject" which tells it to store the file on its file system. When the device receives this, it has two options: 1) receive all data, buffer it, then write the file. This is typically a slow process USB->Processor->SDRAM->Processor/DMA engine->Media 2) pre-allocate the file as soon as it knows that it is coming and how big it is, and then send the block addresses to an external DMA engine and let it transfer the data from the MTP host directly The West Bridge driver goes for option two for performance reasons. In doing this, it needs to get information from the file system on where to store the file. >What happens if this isn't a FAT partition on the >device? Good question. So far, it has been stored on a FAT partition in most use cases because the user typically wants the option to enumerate the device as mass storage as well or be able to see content on a PC if the SD card is removed. However, there is no reason that this could not be done with ext2 or other filesystems on non-removable media. This would require some work to figure out how to implement a pre-allocation interface with it. Alternatively a more general pre-allocation interface could be added (falloc()) to implemented file systems to make this process easier. Please let me know if there are any additional questions or implementation feedback on this. Thanks, David --------------------------------------------------------------- This message and any attachments may contain Cypress (or its subsidiaries) confidential information. If it has been received in error, please advise the sender and immediately delete this message. ---------------------------------------------------------------