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=-10.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_2 autolearn=ham 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 9EED4C4361B for ; Mon, 14 Dec 2020 16:35:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 530BE21D7F for ; Mon, 14 Dec 2020 16:35:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728826AbgLNQfe (ORCPT ); Mon, 14 Dec 2020 11:35:34 -0500 Received: from frasgout.his.huawei.com ([185.176.79.56]:2258 "EHLO frasgout.his.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727956AbgLNQfd (ORCPT ); Mon, 14 Dec 2020 11:35:33 -0500 Received: from fraeml714-chm.china.huawei.com (unknown [172.18.147.226]) by frasgout.his.huawei.com (SkyGuard) with ESMTP id 4Cvn460FbYz67QJj; Tue, 15 Dec 2020 00:32:38 +0800 (CST) Received: from lhreml710-chm.china.huawei.com (10.201.108.61) by fraeml714-chm.china.huawei.com (10.206.15.33) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2106.2; Mon, 14 Dec 2020 17:34:51 +0100 Received: from localhost (10.47.77.193) by lhreml710-chm.china.huawei.com (10.201.108.61) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2106.2; Mon, 14 Dec 2020 16:34:51 +0000 Date: Mon, 14 Dec 2020 16:34:23 +0000 From: Jonathan Cameron To: Peter Rosin CC: Jonathan Cameron , Linus Walleij , linux-iio , "Hartmut Knaack" , Lars-Peter Clausen , "Peter Meerwald-Stadler" Subject: Re: [PATCH] iio: afe: iio-rescale: Support processed channels Message-ID: <20201214163423.00005e6c@Huawei.com> In-Reply-To: References: <20201101232211.1194304-1-linus.walleij@linaro.org> <435ebb1b-431c-fdeb-023e-39c6f6102e22@axentia.se> <20201213121615.55a86f77@archlinux> <20201214150728.00001fa7@Huawei.com> Organization: Huawei Technologies Research and Development (UK) Ltd. X-Mailer: Claws Mail 3.17.4 (GTK+ 2.24.32; i686-w64-mingw32) MIME-Version: 1.0 Content-Type: text/plain; charset="US-ASCII" Content-Transfer-Encoding: 7bit X-Originating-IP: [10.47.77.193] X-ClientProxiedBy: lhreml718-chm.china.huawei.com (10.201.108.69) To lhreml710-chm.china.huawei.com (10.201.108.61) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-iio@vger.kernel.org On Mon, 14 Dec 2020 16:30:22 +0100 Peter Rosin wrote: > On 2020-12-14 16:07, Jonathan Cameron wrote: > > On Mon, 14 Dec 2020 09:34:40 +0100 > > Peter Rosin wrote: > > > >> On 2020-12-13 13:16, Jonathan Cameron wrote: > >>> On Sun, 13 Dec 2020 00:22:17 +0100 > >>> Peter Rosin wrote: > >>> > >>>> On 2020-12-12 13:26, Linus Walleij wrote: > >>>>> On Mon, Nov 2, 2020 at 12:22 AM Linus Walleij wrote: > >>>>> > >>>>>> It happens that an ADC will only provide raw or processed > >>>>>> voltage conversion channels. (adc/ab8500-gpadc.c). > >>>>>> On the Samsung GT-I9070 this is used for a light sensor > >>>>>> and current sense amplifier so we need to think of something. > >>>>>> > >>>>>> The idea is to allow processed channels and scale them > >>>>>> with 1/1 and then the rescaler can modify the result > >>>>>> on top. > >>>>>> > >>>>>> Cc: Peter Rosin > >>>>>> Signed-off-by: Linus Walleij > >>>>> > >>>>> Did we reach any conclusion on this? I really need to use > >>>>> the rescaler on an ADC that only handles processed channels... > >>>>> > >>>>> I'm sorry that I can't make this ADC disappear :D > >>>> > >>>> Hi! > >>>> > >>>> My conclusion was that the patch is buggy since it presents inconsistent > >>>> information. That needs to be fixed one way or the other. If the offending > >>>> information cannot be filtered out for some reason, I don't know what to > >>>> do. Details in my previous comment [1]. BTW, I still do not know the answer > >>>> to the .read_avail question at the end of that message, and I don't have > >>>> time to dig into it. Sorry. > >>> > >>> Unless I'm missing something, I think it presents no information unless > >>> we strangely have a driver providing read_avail for _RAW but only > >>> _PROCESSED channels which is a bug. I'm not that bothered about > >>> missing information in this particular, somewhat obscure, corner case. > >>> > >>> So I think we should take the patch as it stands. It's missed the > >>> merge window now anyway unfortunately. So Peter, I would suggest we > >>> take this and perhaps revisit to tidy up loose corners when we all have > >>> more time. > >> > >> My concern was a driver with a raw channel, including read_avail, providing > >> raw sample values but that no easy conversion existed to get from that to > >> the processed values. One option for the driver in that case would be to > >> provide these raw values, but then have no scaling info. > > > > Generally I resist this a lot. The reason is that it is impossible to write > > generic userspace software against it. The one time we did let this happen > > was with some of the heart rate sensors (pulse oximeters) where the algorithm > > to derive the eventual value is both complex - based on published literature, > > and proprietary (what was actually readily usable). What the measurement being > > provided to userspace was is well documented, but not how on earth you get from > > that to something useable for what the sensor is designed to measure. > > > >> I.e. the way I see > >> it, it is perfectly reasonable for a driver to provide raw with read_avail, > >> no scaling but also processed values. > > > > Why? What use would the raw values actually be? There are a couple of historical > > drivers where they evolved to this state, but it is not one we would normally accept. > > We go to a lot of effort to try and avoid this. > > Drivers that have eveloved over time is exactly one such reason. E.g. a driver > starts out by not caring about wrong measurements at one end of the spectrum > because it is "linear enough" for the first use, someone comes along and fixes > that. But by that time it's impossible to completely remove the raw channel > because that would be a regression for some reason. And there you are. A > driver with raw plus read_avail, no scaling but a processed channel. Or > something like that... Yup, that's pretty much what tends to happen. I've gotten a lot stricter on checking datasheets to try and stop this happening, but still possible more will slip through (particularly as can't always get the datasheet) > > >> And that gets transformed by the > >> rescaler into the processed values being presented as raw, with rescaling > >> added on top, but with the read_avail info for this new raw channel being > >> completely wrong. > >> > >> For the intended driver (ab8500-gpadc) this is not the case (it has no > >> read_avail for its raw channel). But it does have a raw channel, so adding > >> read_avail seems easy and I can easily see other drivers already doing it. > >> Haven't checked that though... > > > > Drat. I'd failed to register this is one of those corner cases. > > I'm not sure, I just browsed the code. Maybe I misread it? It's doing both - you were right. I think there are only a small number of drivers that have that history. Looks superficially like it's easy enough to catch this corner case and block it - so lets do that. Jonathan > > Cheers, > Peter > > >> But if you say that this never happens, fine. Otherwise, since it's too > >> late for the merge window anyway, the patch might as well be updated such > >> that the rescaler blocks the read_avail channel in this situation, if it > >> exists. > > > > That's fair enough. A sanity check and then suitable warning message to explain > > why it is blocked makes sense. >