From mboxrd@z Thu Jan 1 00:00:00 1970 From: Guenter Roeck Subject: Re: [PATCH] eeprom: at24: Add support for large EEPROMs connected to SMBus adapters Date: Wed, 18 Mar 2015 20:24:18 -0700 Message-ID: <550A4162.8000009@roeck-us.net> References: <1423067017-27607-1-git-send-email-linux@roeck-us.net> <20150204174723.GB30930@katana> <20150204190819.GA15467@roeck-us.net> <20150204233516.GA1677@katana> <20150205002630.GA396@roeck-us.net> <20150205144028.GA4865@katana> <20150205175326.GA26691@roeck-us.net> <20150212040126.GA1691@roeck-us.net> <20150216120951.GA2840@katana> <20150317042049.GA6765@roeck-us.net> <20150318132707.GD3580@katana> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20150318132707.GD3580@katana> Sender: linux-kernel-owner@vger.kernel.org To: Wolfram Sang Cc: linux-i2c@vger.kernel.org, linux-kernel@vger.kernel.org List-Id: linux-i2c@vger.kernel.org On 03/18/2015 06:27 AM, Wolfram Sang wrote: > On Mon, Mar 16, 2015 at 09:20:49PM -0700, Guenter Roeck wrote: >> On Mon, Feb 16, 2015 at 01:09:51PM +0100, Wolfram Sang wrote: >>> Hi Guenter, >>> >>>> I wonder where we are with thisp patch; I don't recall a reply to my previous >>>> e-mail. >>> >>> Sorry for the late reply. I needed to recover from a HDD headcrash :( >>> >>>> Do you need some more time to think about it ? Otherwise I'll publish an >>>> out-of-tree version of the at24 driver with the patch applied on github, >>>> for those who might need the functionality provided by this patch. >>> >>> Your last mail made me aware of why we were missing each other before. I >>> see your point now, but yes, still need to think about it. My plan is to >>> have a decision until the 3.21 merge window. >>> >> Hi Wolfram, >> >> any news ? > > Yes :) > > The main misunderstanding we had before was: You were talking about > multi-master safety between transfers, while I was thinking about > multi-master safety between messages. While we need to guarantee this > for the latter, you are right about the former, sadly. True multi-master > safety between transfers is probably like a can of worms currently. > > Still, I think we have a race with your patch when having two read > processes. If b) kicks in after a) has just set the eeprom pointer, a) > will not read the data it wants. For that to prevent, we should take the > adapter_lock during those two transfers needed for the read you > implemented. My preferred solution would be to have __smbus_transfer > like we have __i2c_transfer and then using that. Some mux code could > also make use out of that. But if you are going to use > adapter->algo->smbus_xfer() directly, well, then be it. > You are right, that is a problem. Not for eeprom access itself - that already has a mutex - but for parallel access to the chip through the eeprom file and, say, by i2cdump. I don't call that multi-master, though, so I guess we may have a bit of a terminology problem. I'll see what I can come up with, but I am not sure if I'll find the time before the 4.1 commit window opens. Company has a working solution (kind of), so now I'll have to do this on my own time ;-). Thanks, Guenter