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 X-Spam-Level: X-Spam-Status: No, score=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id D893CC433C1 for ; Sat, 27 Mar 2021 16:06:35 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 95C79619C7 for ; Sat, 27 Mar 2021 16:06:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230202AbhC0QGC (ORCPT ); Sat, 27 Mar 2021 12:06:02 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41436 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229582AbhC0QFi (ORCPT ); Sat, 27 Mar 2021 12:05:38 -0400 Received: from mail.pqgruber.com (mail.pqgruber.com [IPv6:2a05:d014:575:f70b:4f2c:8f1d:40c4:b13e]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 5D56FC0613B1; Sat, 27 Mar 2021 09:05:38 -0700 (PDT) Received: from workstation.tuxnet (213-47-165-233.cable.dynamic.surfer.at [213.47.165.233]) by mail.pqgruber.com (Postfix) with ESMTPSA id B4276CB478B; Sat, 27 Mar 2021 17:05:36 +0100 (CET) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=pqgruber.com; s=mail; t=1616861136; bh=QPLsMrmELyZRDmLYX2w/l2qB+1PgXZX0xEoCvX+7rAI=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=ubbkyCUxq+dbxTxzPZutEONWDIfQy/30EdpvM+WbbyOjAPZQu94hbgxi+SOJYbEkn fMAaHv+evBuvnddCXj+HTQlX2O7d4caQ0CXCVdMG0OOH+6TxIBVynAlse95+1V+nVl kO59XJZXU20/CJdDsAjsklhei5bILbg4hZaqTJN8= Date: Sat, 27 Mar 2021 17:05:35 +0100 From: Clemens Gruber To: Thierry Reding Cc: Sven Van Asbroeck , Uwe =?iso-8859-1?Q?Kleine-K=F6nig?= , Linux Kernel Mailing List , linux-pwm@vger.kernel.org Subject: Re: [PATCH v5 2/7] pwm: pca9685: Support hardware readout Message-ID: References: <20201216125320.5277-1-clemens.gruber@pqgruber.com> <20201216125320.5277-2-clemens.gruber@pqgruber.com> <20210111203532.m3yvq6e5bcpjs7mc@pengutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Thierry, On Mon, Mar 22, 2021 at 10:19:22AM +0100, Thierry Reding wrote: > On Fri, Jan 29, 2021 at 09:37:47PM +0100, Clemens Gruber wrote: > > Hi Sven, > > > > On Fri, Jan 29, 2021 at 01:05:14PM -0500, Sven Van Asbroeck wrote: > > > Hi Clemens, > > > > > > On Fri, Jan 29, 2021 at 11:31 AM Clemens Gruber > > > wrote: > > > > > > > > Ok, so you suggest we extend our get_state logic to deal with cases > > > > like the following: > > > > > > Kind of. We can't control how other actors (bootloaders etc) program the > > > chip. As far as I know, there are many, many different register settings that > > > result in the same physical chip outputs. So if .probe() wants to preserve the > > > existing chip settings, .get_state() has to be able to deal with every possible > > > setting. Even invalid ones. > > > > Is the driver really responsible for bootloaders that program the chip > > with invalid values? > > The chip comes out of PoR with sane default values. If the bootloader of > > a user messes them up, isn't that a bootloader problem instead of a > > Linux kernel driver problem? > > It is ultimately a problem of the bootloader and where possible the > bootloader should be fixed. However, fixing bootloaders sometimes isn't > possible, or impractical, so the kernel has to be able to deal with > hardware that's been badly programmed by the bootloader. Within reason, > of course. Sometimes this can't be done in any other way than forcing a > hard reset of the chip, but it should always be a last resort. > > > > In addition, .apply() cannot make any assumptions as to which bits are > > > already set/cleared on the chip. Including preserved, invalid settings. > > > > > > This might get quite complex. > > > > > > However if we reset the chip in .probe() to a known state (a normalized state, > > > in the mathematical sense), then both .get_state() and .apply() become > > > much simpler. because they only need to deal with known, normalized states. > > > > Yes, I agree. This would however make it impossible to do a flicker-free > > transition from bootloader to kernel, but that's not really a usecase I > > have so I can live without it. > > > > Another point in favor of resetting is that the driver already does it. > > Removing the reset of the OFF register may break some boards who rely on > > that behaviour. > > My version only extended the reset to include the ON register. > > > > > > > > In short, it's a tradeoff between code complexity, and user friendliness/ > > > features. > > > > > > Sven > > > > Thierry, Uwe, what's your take on this? > > > > Thierry: Would you accept it if we continue to reset the registers in > > .probe? > > Yes, I think it's fine to continue to reset the registers since that's > basically what the driver already does. It'd be great if you could > follow up with a patch that removes the reset and leaves the hardware in > whatever state the bootloader has set up. Then we can take that patch > for a ride and see if there are any complains about it breaking. If > there are we can always try to fix them, but as a last resort we can > also revert, which then may be something we have to live with. But I > think we should at least try to make this consistent with how other > drivers do this so that people don't stumble over this particular > driver's behaviour. Thanks for your input! Sounds good to me. I am currently preparing a new revision of the series. As soon as that is reviewed and good to go, I will look into removing the resets. Clemens