From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 2E2A0C41513 for ; Thu, 12 Oct 2023 22:18:20 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1442986AbjJLWST (ORCPT ); Thu, 12 Oct 2023 18:18:19 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41574 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1442960AbjJLWST (ORCPT ); Thu, 12 Oct 2023 18:18:19 -0400 X-Greylist: delayed 999 seconds by postgrey-1.37 at lindbergh.monkeyblade.net; Thu, 12 Oct 2023 15:18:17 PDT Received: from stcim.de (stcim.de [IPv6:2a01:4f8:151:40c4::2]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 2B72BB8; Thu, 12 Oct 2023 15:18:17 -0700 (PDT) Received: from stc by stcim with local (Exim 4.92) (envelope-from ) id 1qr3kF-0007rM-Hb; Fri, 13 Oct 2023 00:01:27 +0200 Date: Fri, 13 Oct 2023 00:01:27 +0200 From: Stefan Lengfeld To: Krzysztof =?utf-8?Q?Ha=C5=82asa?= Cc: linux-media , lkml , Dave Stevenson , Oleksij Rempel , Pengutronix Kernel Team , Shawn Guo , Sascha Hauer , Fabio Estevam , NXP Linux Team , linux-i2c@vger.kernel.org Subject: Re: Sony IMX290/462 image sensors I2C xfer peculiarity Message-ID: <20231012220127.GB27838@stcim.de> References: <20231011101553.we3r73xejvqdql5j@porty> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-i2c@vger.kernel.org Hi Chris, > > My understand is that an ordinary I2C device would just use normal (and > > sleepable) I2C transfers while the device is in use. > > You are spot-on here :-) Now I use IMX 290 and 462. > > OTOH I wonder if such issues are limited to those sensors only. Hmm, yes. I know no other I2C device that has these timeout issues. (*) > The problem is I use Sony IMX290 and IMX462 image sensors, and they have > an apparently hard-coded timeout of about 2^18 their master clock cycles > (= ca. 7 ms with my setup). After the timeout they simply disconnect > from the I2C bus. Of course, this isn't mentioned in the docs. hmm. I have no idea about this sensor and your setup. So I can just give hints: This timeout seems strange. If this 7 ms timeout is required, it would mean that I2C masters require to fullfill real-time/deadline requirements. For "small" I2C master in microcontrolles this seems ok-ish, but for general operating systems real-time requirements are hard. The real-time patches for linux just landed recently and it still requires fine tuning the system for the required deadlines. Maybe you just hit a corner case or a bug, that can be avoid, in the I2C device. Maybe check with the manufacturer directly? > Unfortunately, "normal" I2C accesses take frequently more than those > 7 ms (mostly due to scheduling when all CPU cores are in use). Yes, correctly. There are multiple cases in which I2C transactions to the same device can be preempted/delayed: A busy system, as you said, or when some other driver in the kernel accesses another I2C device on the same bus. This will lock the bus/I2C adapter for the duration of its transfer. Do you know the I2C repeated start feature [1]? This allows to batch together multiple I2C read/writes in a single transfer. And in the best case, this transfer is executed in one go without a delay in between. At least in the kernel it's guaranteed that no other driver can go in between with another transfer. Kind regards, Stefan [1]: https://www.i2c-bus.org/repeated-start-condition/ (*) Fun answer: Actually external watchdogs have timeouts. But the timeout duration is in the range of seconds, not milliseconds. And timeout expiration is expected (in error cases ;-).