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 3419BC433F5 for ; Tue, 22 Feb 2022 16:21:59 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233521AbiBVQWW convert rfc822-to-8bit (ORCPT ); Tue, 22 Feb 2022 11:22:22 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:48626 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230429AbiBVQWW (ORCPT ); Tue, 22 Feb 2022 11:22:22 -0500 Received: from frasgout.his.huawei.com (frasgout.his.huawei.com [185.176.79.56]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 11EE66C96A; Tue, 22 Feb 2022 08:21:55 -0800 (PST) Received: from fraeml715-chm.china.huawei.com (unknown [172.18.147.206]) by frasgout.his.huawei.com (SkyGuard) with ESMTP id 4K34Cl08jQz683mZ; Wed, 23 Feb 2022 00:20:51 +0800 (CST) Received: from lhreml710-chm.china.huawei.com (10.201.108.61) by fraeml715-chm.china.huawei.com (10.206.15.34) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2308.21; Tue, 22 Feb 2022 17:21:52 +0100 Received: from localhost (10.47.30.92) 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 15.1.2308.21; Tue, 22 Feb 2022 16:21:51 +0000 Date: Tue, 22 Feb 2022 16:21:50 +0000 From: Jonathan Cameron To: Andy Shevchenko CC: Nuno =?ISO-8859-1?Q?S=E1?= , "Sa, Nuno" , "linux-iio@vger.kernel.org" , "devicetree@vger.kernel.org" , Rob Herring , "Jonathan Cameron" , Lars-Peter Clausen , "Hennerich, Michael" Subject: Re: [PATCH v3 1/3] iio: dac: add support for ltc2688 Message-ID: <20220222162150.0000361f@Huawei.com> In-Reply-To: References: <11bd63bc07fd406bfa31bdc38b597011cc9312cc.camel@gmail.com> <3f2523127eb320a9825e272353afea9673e5d003.camel@gmail.com> <20220221173045.00003969@Huawei.com> Organization: Huawei Technologies Research and Development (UK) Ltd. X-Mailer: Claws Mail 4.0.0 (GTK+ 3.24.29; i686-w64-mingw32) MIME-Version: 1.0 Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 8BIT X-Originating-IP: [10.47.30.92] X-ClientProxiedBy: lhreml713-chm.china.huawei.com (10.201.108.64) To lhreml710-chm.china.huawei.com (10.201.108.61) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: devicetree@vger.kernel.org On Mon, 21 Feb 2022 20:49:48 +0200 Andy Shevchenko wrote: > On Mon, Feb 21, 2022 at 05:30:45PM +0000, Jonathan Cameron wrote: > > On Mon, 21 Feb 2022 19:04:38 +0200 > > Andy Shevchenko wrote: > > > > > On Mon, Feb 21, 2022 at 01:48:12PM +0100, Nuno Sá wrote: > > > > On Sun, 2022-02-20 at 13:32 +0200, Andy Shevchenko wrote: > > > > > On Fri, Feb 18, 2022 at 02:51:28PM +0100, Nuno Sá wrote: > > > > > > On Mon, 2022-02-14 at 15:49 +0200, Andy Shevchenko wrote: > > > > > > > On Mon, Feb 07, 2022 at 09:19:46PM +0100, Nuno Sá wrote: > > > > > > > > On Mon, 2022-02-07 at 13:09 +0200, Andy Shevchenko wrote: > > > > > > > > > On Sun, Feb 06, 2022 at 01:19:59PM +0000, Sa, Nuno wrote: > > > > > > > > > > > From: Andy Shevchenko > > > > > > > > > > > Sent: Saturday, February 5, 2022 6:30 PM > > > > > > > > > > > On Fri, Jan 21, 2022 at 03:24:59PM +0100, Nuno Sá wrote: > > > > > > ... > > > > > > > > > > > > > > > +       ret = kstrtou16(buf, 10, &val); > > > > > > > > > > > > > > > > > > > > > > In other function you have long, here u16. I would expect that > > > > > > > > > > > the types are of the same class, e.g. if here you have u16, > > > > > > > > > > > then there something like s32 / s64. Or here something like > > > > > > > > > > > unsigned short. > > > > > > > > > > > > > > > > > > > > > > A bit of elaboration why u16 is chosen here? > > > > > > > > > > > > > > > > > > > > Well, I never really saw any enforcement here to be honest > > > > > > > > > > (rather than using stdint types...). So I pretty much just use > > > > > > > > > > these in unsigned types because I'm lazy and u16 is faster to > > > > > > > > > > type than unsigned short... In this case, unless Jonathan really > > > > > > > > > > asks for it, I prefer not to go all over the driver and change > > > > > > > > > > this... > > > > > > > > > > > > > > > > > > This is about consistency. It may work as is, but it feels not good > > > > > > > > > when for int (or unsigned int) one uses fixed-width types. Also > > > > > > > > > it's non- written advice to use fixed-width variables when it's > > > > > > > > > about programming registers or so, for the rest, use POD types. > > > > > > > > > > > > Ok, going a bit back in the discussion, you argued that in one place I > > > > > > was using long while here u16. Well, in the place I'm using long, that > > > > > > was on purpose because that value is to be compared against an array of > > > > > > longs (which has to be long because it depends on CCF rates). I guess I > > > > > > can als0 use s64, but there is also a reason why long was used. > > > > > > > > > > > > In the u16 case, we really want to have 2 bytes because I'm going to use > > > > > > that value to write the dac code which is 2 bytes. > > > > > > > > > > Okay, that's what I want to hear. If it's indeed goes to be a value to the > > > > > register, then it's fine. > > > > > > > > > > Perhaps a comment? > > > > > > > > I guess you mean to have a comment to state that here we have fixed > > > > size type (as opposed to long, used in another place), because we > > > > directly use the value on a register write? > > > > > > > > Asking it because I'm not planning to add comments in all the places > > > > where I have fixed size types for register read/writes... > > > > > > Thinking more about it and now I'm convinced that using the value that goes to > > > the register in ABI is bad idea (means that user space must not care about the > > > size or contents of the hardware register and should be abstract representation > > > of the HW). > > > > > > OTOH this seems to be "raw" value of something. So, I maybe missed the convention > > > in IIO about this kind of values WRT the variable types used on ABI side. > > > > > > That said, I leave it to Jonathan since I'm not convinced that u16 is a proper > > > choice here. > > > > From a userspace point of view it doesn't care as it's writing a string. > > In this particular case the string only has valid values that from 0-(2^16-1) > > (i.e. 16 bits). So if it writes outside of that range it is an error. > > You could read it into an unsigned long and then check against the range, > > but there is little point given you'd still return an error if it was out of > > range. The fact that kstrto16() does that for you really just a shortcut > > though it will return -ERANGE rather than perhaps -EINVAL which might be used > > for a more generic "not this value". > > > > Userspace can also read the range that is acceptable from > > out_voltage0_raw_available [0 1 2^16-1] and hence not write an invalid value > > in the first place - which is obviously preferred to getting an error. > > Scaling etc is also expressed to userspace so it it wants to write a particular > > voltage it can perform the appropriate scaling. Note that moving linear scaling > > like this to userspace allows easy use of floating point + may be a significant > > performance advantage if using the chrdev interface which uses the same > > approach (and values) as the sysfs interface. > > With the same logic it can be unsigned short, no? It could be any integer as long as it is at least as large as a u16. But it it is larger than a u16 you'll need an additional check on the maximum. > > The point is to use u16 when it's indeed fixed-width value that goes to > hardware or being used as part of a protocol. And thus mentioning of the > IOCTL protocols may justify the choice. Then the question to the other > values, shouldn't they be also fixed-width ones? If we had a fixed width type that took the values 0-4 sure using such a magic type would make sense, but we don't. Note that internally kstrtou16 is just strtoull and a range check. The one other case we have here does pretty much the same thing. Jonathan > > > > > > > > > I can understand your reasoning but again this is something that I > > > > > > > > never really saw being enforced. So, I'm more than ok to change it if > > > > > > > > it really becomes something that we will try to "enforce" in IIO. > > > > > > > > Otherwise it just feels as a random nitpick :). > > > > > > > > > > > > > > No, this is about consistency and common sense. If you define type uXX, > > > > > > > we have an API for that exact type. It's confusing why POD type APIs > > > > > > > are used with fixed-width types or vise versa. > > > > > > > > > > > > > > Moreover (which is pure theoretical, though) some architectures might > > > > > > > have no (mutual) equivalency between these types. >