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=-5.0 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 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 8292BC433DF for ; Tue, 13 Oct 2020 19:08:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 22911212CC for ; Tue, 13 Oct 2020 19:08:53 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=lechnology.com header.i=@lechnology.com header.b="kTfO6894" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727033AbgJMTIw (ORCPT ); Tue, 13 Oct 2020 15:08:52 -0400 Received: from vern.gendns.com ([98.142.107.122]:57108 "EHLO vern.gendns.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726395AbgJMTIw (ORCPT ); Tue, 13 Oct 2020 15:08:52 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lechnology.com; s=default; h=Content-Transfer-Encoding:Content-Type: In-Reply-To:MIME-Version:Date:Message-ID:From:References:Cc:To:Subject:Sender :Reply-To:Content-ID:Content-Description:Resent-Date:Resent-From: Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id:List-Help: List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=jZT/uATnG2o72Hv+NB/ghQjoMMH5tGI4CdsZ0nvbYXk=; b=kTfO6894m4jP1mJIN/flhtoo/a g1XiQfEPqz80vbKG6Ek+ynL5NI+k2zab9L+m8isH4nWfGnfXzd1KDmtpQKzFX8Acl+xH8BYq5i9yW lgf6u9u9o1EhfQBL89Ysofjg/8Q9a0P+obRGTgPbKEJPkD5KswaytWNV6uppa+cEaSND3zWnWsGRT vYzUvOwI9NcxfPHNqYMEF8BEKRPb79I1MWzDmSudVLkNA5aHpfn5VNemgra9u2omslowqwLxsyWp9 wf2QDnIFcu5sI7dU764T4K1bRqqPoAHZ4X9CdfclQzFkEtvSquFuchm8/OA8LiIKI3HiSp9B5183C gpGLHsog==; Received: from [2600:1700:4830:165f::19e] (port=47662) by vern.gendns.com with esmtpsa (TLS1.2) tls TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 (Exim 4.93) (envelope-from ) id 1kSPfD-0001IG-Hd; Tue, 13 Oct 2020 15:08:47 -0400 Subject: Re: [PATCH v5 4/5] docs: counter: Document character device interface To: William Breathitt Gray Cc: Pavel Machek , jic23@kernel.org, kamel.bouhara@bootlin.com, gwendal@chromium.org, alexandre.belloni@bootlin.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 References: <54190f9875b81b6aa5483a7710b084053a44abb8.1601170670.git.vilhelm.gray@gmail.com> <20201008080909.GA31561@amd> <20201008122845.GA3314@shinobu> <20201013185851.GB32722@shinobu> From: David Lechner Message-ID: <09949c56-19da-1937-418b-99a1a9260c4c@lechnology.com> Date: Tue, 13 Oct 2020 14:08:45 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20201013185851.GB32722@shinobu> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-AntiAbuse: This header was added to track abuse, please include it with any abuse report X-AntiAbuse: Primary Hostname - vern.gendns.com X-AntiAbuse: Original Domain - vger.kernel.org X-AntiAbuse: Originator/Caller UID/GID - [47 12] / [47 12] X-AntiAbuse: Sender Address Domain - lechnology.com X-Get-Message-Sender-Via: vern.gendns.com: authenticated_id: davidmain+lechnology.com/only user confirmed/virtual account not confirmed X-Authenticated-Sender: vern.gendns.com: davidmain@lechnology.com X-Source: X-Source-Args: X-Source-Dir: Precedence: bulk List-ID: X-Mailing-List: linux-iio@vger.kernel.org On 10/13/20 1:58 PM, William Breathitt Gray wrote: > On Mon, Oct 12, 2020 at 12:04:10PM -0500, David Lechner wrote: >> On 10/8/20 7:28 AM, William Breathitt Gray wrote: >>> On Thu, Oct 08, 2020 at 10:09:09AM +0200, Pavel Machek wrote: >>>> Hi! >>>> >>>>> + int main(void) >>>>> + { >>>>> + struct pollfd pfd = { .events = POLLIN }; >>>>> + struct counter_event event_data[2]; >>>>> + >>>>> + pfd.fd = open("/dev/counter0", O_RDWR); >>>>> + >>>>> + ioctl(pfd.fd, COUNTER_SET_WATCH_IOCTL, watches); >>>>> + ioctl(pfd.fd, COUNTER_SET_WATCH_IOCTL, watches + 1); >>>>> + ioctl(pfd.fd, COUNTER_LOAD_WATCHES_IOCTL); >>>>> + >>>>> + for (;;) { >>>>> + poll(&pfd, 1, -1); >>>> >>>> Why do poll, when you are doing blocking read? >>>> >>>>> + read(pfd.fd, event_data, sizeof(event_data)); >>>> >>>> Does your new chrdev always guarantee returning complete buffer? >>>> >>>> If so, should it behave like that? >>>> >>>> Best regards, >>>> Pavel >>>> -- >>>> (english) http://www.livejournal.com/~pavelmachek >>>> (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html >>> >>> I suppose you're right: a poll() should be redundant now with this >>> version of the character device implementation because buffers will >>> always return complete; so a blocking read() should achieve the same >>> behavior that a poll() with read() would. >>> >>> I'll give some more time for additional feedback to come in for this >>> version of the patchset, and then likely remove support for poll() in >>> the v6 submission. >>> >>> William Breathitt Gray >>> >> >> I hope that you mean that you will just remove it from the example >> and not from the chardev. Otherwise it won't be possible to >> integrate this with an event loop. > > Would you elaborate a bit further on this? My thought process is that > because users must set the Counter Events they want to watch, and only > those Counter Events show up in the character device node, a blocking > read() would effectively behave the same as poll() with read(); if none > of the Counter Events occur, the read() just blocks until one does, thus > making the use of a poll() call redundant. > > William Breathitt Gray > If the counter device was the only file descriptor being read, then yes it wouldn't matter. But if we are using this in combination with other file descriptors, then it is common to poll all of the file descriptors using a single syscall to see which one is ready to read rather than doing a non-blocking read on all of the file descriptors, which would result in many unnecessary syscalls.