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=-0.7 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 1F468C47254 for ; Thu, 30 Apr 2020 20:13:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0581E20774 for ; Thu, 30 Apr 2020 20:13:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727865AbgD3UNw (ORCPT ); Thu, 30 Apr 2020 16:13:52 -0400 Received: from relay9-d.mail.gandi.net ([217.70.183.199]:39037 "EHLO relay9-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726626AbgD3UNu (ORCPT ); Thu, 30 Apr 2020 16:13:50 -0400 X-Originating-IP: 86.202.105.35 Received: from localhost (lfbn-lyo-1-9-35.w86-202.abo.wanadoo.fr [86.202.105.35]) (Authenticated sender: alexandre.belloni@bootlin.com) by relay9-d.mail.gandi.net (Postfix) with ESMTPSA id 9F8B7FF80A; Thu, 30 Apr 2020 20:13:45 +0000 (UTC) Date: Thu, 30 Apr 2020 22:13:45 +0200 From: Alexandre Belloni To: William Breathitt Gray Cc: jic23@kernel.org, kamel.bouhara@bootlin.com, gwendal@chromium.org, david@lechnology.com, felipe.balbi@linux.intel.com, fabien.lahoudere@collabora.com, linux-iio@vger.kernel.org, linux-kernel@vger.kernel.org, linux-stm32@st-md-mailman.stormreply.com, linux-arm-kernel@lists.infradead.org, syednwaris@gmail.com, patrick.havelange@essensium.com, fabrice.gasnier@st.com, mcoquelin.stm32@gmail.com, alexandre.torgue@st.com Subject: Re: [PATCH 0/4] Introduce the Counter character device interface Message-ID: <20200430201345.GX51277@piout.net> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, On 29/04/2020 14:11:34-0400, William Breathitt Gray wrote: > Over the past couple years we have noticed some shortcomings with the > Counter sysfs interface. Although useful in the majority of situations, > there are certain use-cases where interacting through sysfs attributes > can become cumbersome and inefficient. A desire to support more advanced > functionality such as timestamps, multi-axis positioning tables, and > other such latency-sensitive applications, has motivated a reevaluation > of the Counter subsystem. I believe a character device interface will be > helpful for this more niche area of counter device use. > > To quell any concerns from the offset: this patchset makes no changes to > the existing Counter sysfs userspace interface -- existing userspace > applications will continue to work with no modifications necessary. I > request that driver maintainers please test their applications to verify > that this is true, and report any discrepancies if they arise. > On that topic, I'm wondering why the counter subsystem uses /sys/bus instead of /sys/class that would be more natural for a class of devices. I can't see how counters would be considered busses. I think you should consider moving it over to /sys/class (even if deprecating /sys/bus/counter will be long). > Interaction with Counter character devices occurs via ioctl commands. > This allows userspace applications to access and set counter data using > native C datatypes rather than working through string translations. > I agree with David that you should consider using read to retrieve the counter data as this will simplify interrupt handling/polling and blocking/non-blocking reads can be used by an application. ABI wise, this can also be a good move as you could always consider having an ioctl requesting a specific format when reading the device so you are not stuck with the initial format you are going to choose. > 2. Should device driver callbacks return int or long? I sometimes see > error values returned as long (e.g. PTR_ERR(), the file_operations > structure's ioctl callbacks, etc.); when is it necessary to return > long as opposed to int? > You should use a long if you ever have to return a point as it is guaranteed to have the correct size. Else, just stick to an int if you are not going to overflow it. > 3. I only implemented the unlocked_ioctl callback. Should I implement a > compat_ioctl callback as well? > The compat_ioctl is to handle 32bit userspace running on a 64bit kernel. If your structures have the same size in both cases, then you don't have to implement compat_ioctl. Have a look at Documentation/driver-api/ioctl.rst -- Alexandre Belloni, Bootlin Embedded Linux and Kernel engineering https://bootlin.com