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=-4.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,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 0B96AC4360C for ; Mon, 30 Sep 2019 05:22:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E082220815 for ; Mon, 30 Sep 2019 05:22:43 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1725809AbfI3FWn (ORCPT ); Mon, 30 Sep 2019 01:22:43 -0400 Received: from mga06.intel.com ([134.134.136.31]:62157 "EHLO mga06.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725767AbfI3FWn (ORCPT ); Mon, 30 Sep 2019 01:22:43 -0400 X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga002.jf.intel.com ([10.7.209.21]) by orsmga104.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 29 Sep 2019 22:22:42 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.64,565,1559545200"; d="scan'208";a="202774589" Received: from pipin.fi.intel.com (HELO pipin) ([10.237.72.175]) by orsmga002.jf.intel.com with ESMTP; 29 Sep 2019 22:22:40 -0700 From: Felipe Balbi To: William Breathitt Gray , David Lechner Cc: linux-iio@vger.kernel.org, jic23@jic23.retrosnub.co.uk, Fabien Lahoudere Subject: Re: [RFC/PATCHv2 2/2] counter: introduce support for Intel QEP Encoder In-Reply-To: <20190928213348.GA4693@icarus> References: <20190917114403.GA8368@icarus> <20190919080305.960198-1-felipe.balbi@linux.intel.com> <20190919080305.960198-2-felipe.balbi@linux.intel.com> <20190922233538.GA3119@icarus> <20190928213348.GA4693@icarus> Date: Mon, 30 Sep 2019 08:22:39 +0300 Message-ID: <87muemwe74.fsf@gmail.com> MIME-Version: 1.0 Content-Type: text/plain Sender: linux-iio-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-iio@vger.kernel.org Hi, William Breathitt Gray writes: > On Tue, Sep 24, 2019 at 04:46:57PM -0500, David Lechner wrote: >> On 9/22/19 6:35 PM, William Breathitt Gray wrote: >> > On Thu, Sep 19, 2019 at 11:03:05AM +0300, Felipe Balbi wrote: >> >> Add support for Intel PSE Quadrature Encoder >> >> >> >> Signed-off-by: Felipe Balbi >> >> --- >> >> >> >> Changes since v1: >> >> - Many more private sysfs files converted over to counter interface >> >> >> >> >> >> How do you want me to model this device's Capture Compare Mode (see >> >> below)? >> > >> > Hi Felipe, >> > >> > I'm CCing Fabien and David as they may be interested in the timestamps >> > discussion. See below for some ideas I have on implementing this. >> > >> >> Could be an interesting read (thread from my first counter driver): >> >> https://lore.kernel.org/linux-iio/1b913919-beb9-34e7-d915-6bcc40eeee1d@lechnology.com/ >> >> What would be useful to me is something like the buffer feature in iio >> where a timestamp is associated with a count and stored in a buffer so that >> we can look at a window of all values recorded in the last 20ms. Being able >> to access this via mmap would be very helpful for performance (running on >> 300MHz ARM). Anything to do with timestamps in sysfs is probably not useful >> unless it is a rare event, like a watchdog timeout. > > I'm CCing Jonathan Cameron since I'm not familiar with how IIO handles > timestamps and buffers. I don't want to reinvent something that is > working well, so hopefully we can reuse the IIO timestamp design for the > Counter subsystem. > > I would argue that a human-readable timestamps printout is useful for > certain applications (e.g. a tally counter attached to a fault line: a > human administrator will be able to review previous fault times). > However as you point out, a low latency operation is necessary for > performance critical applications. > > Although you are correct that mmap is a good low latency operation to > get access to a timestamp buffer, I'm afraid giving direct access to > memory like that will lead to many incompatible representations of > timestamp data (e.g. variations in endianness, signedness, data size, > etc.). I would like a standardized representation for this data that > userspace applications can expect to receive and interpret, especially > when time is widely represented as an unsigned integer. > > Felipe suggested the creation of a counter_event structure so that users > can poll on an attribute. This kind of behavior is useful for notifying > users of interrupts and other events, but I think we should restrict the > use of the read call on these sysfs attributes to just human-readable > data. Instead, perhaps ioctl calls can be used to facilitate binary data > transfers. > > For example, we can define a COUNTER_GET_TIMESTAMPS_IOCTL ioctl request > that returns a counter_timestamps structure with a timestamps array > populated: > > struct counter_timestamps{ > size_t num_timestamps; > unsigned int *timestamps; > } > > That would allow quick access to the timestamps data, while also > restricting it to a standard representation that all userspace > applications can follow and interpret. In addition, this won't interfer > with polling, so users can still wait for an interrupt and then decide > whether they want to use the slower human-readable printout (via read) > or the faster binary data access (via ioctl). Seems like we're starting to build the need for a /dev/counter[0123...] representation of the subsystem. If that's the case, then it may very well be that sysfs becomes somewhat optional. I think is makes sense to rely more on character devices specially since I know of devices running linux with so little memory that sysfs (and a bunch of other features) are removed from the kernel. Having a character device representation would allow counter subsystem to be used on such devices. cheers -- balbi