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=-15.5 required=3.0 tests=BAYES_00,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_SANE_2 autolearn=unavailable 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 6C9B7C433E0 for ; Sun, 21 Feb 2021 15:52:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 42B2B64DA8 for ; Sun, 21 Feb 2021 15:52:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230117AbhBUPwa (ORCPT ); Sun, 21 Feb 2021 10:52:30 -0500 Received: from mail.kernel.org ([198.145.29.99]:44032 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230057AbhBUPw2 (ORCPT ); Sun, 21 Feb 2021 10:52:28 -0500 Received: from archlinux (cpc108967-cmbg20-2-0-cust86.5-4.cable.virginm.net [81.101.6.87]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id BAF9F64DA8; Sun, 21 Feb 2021 15:51:43 +0000 (UTC) Date: Sun, 21 Feb 2021 15:51:40 +0000 From: Jonathan Cameron To: William Breathitt Gray Cc: kernel@pengutronix.de, linux-stm32@st-md-mailman.stormreply.com, a.fatoum@pengutronix.de, kamel.bouhara@bootlin.com, gwendal@chromium.org, alexandre.belloni@bootlin.com, david@lechnology.com, linux-iio@vger.kernel.org, linux-kernel@vger.kernel.org, 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, o.rempel@pengutronix.de Subject: Re: [PATCH v8 20/22] counter: Implement events_queue_size sysfs attribute Message-ID: <20210221155140.3e1ef13c@archlinux> In-Reply-To: References: <013b2b8682ddc3c85038083e6d5567696b6254b3.1613131238.git.vilhelm.gray@gmail.com> <20210214181146.66d43da7@archlinux> X-Mailer: Claws Mail 3.17.8 (GTK+ 2.24.33; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 18 Feb 2021 19:32:16 +0900 William Breathitt Gray wrote: > On Sun, Feb 14, 2021 at 06:11:46PM +0000, Jonathan Cameron wrote: > > On Fri, 12 Feb 2021 21:13:44 +0900 > > William Breathitt Gray wrote: > > > > > The events_queue_size sysfs attribute provides a way for users to > > > dynamically configure the Counter events queue size for the Counter > > > character device interface. The size is in number of struct > > > counter_event data structures. The number of elements will be rounded-up > > > to a power of 2 due to a requirement of the kfifo_alloc function called > > > during reallocation of the queue. > > > > > > Cc: Oleksij Rempel > > > Signed-off-by: William Breathitt Gray > > > --- > > > Documentation/ABI/testing/sysfs-bus-counter | 8 +++++++ > > > drivers/counter/counter-chrdev.c | 23 +++++++++++++++++++ > > > drivers/counter/counter-chrdev.h | 2 ++ > > > drivers/counter/counter-sysfs.c | 25 +++++++++++++++++++++ > > > 4 files changed, 58 insertions(+) > > > > > > diff --git a/Documentation/ABI/testing/sysfs-bus-counter b/Documentation/ABI/testing/sysfs-bus-counter > > > index 847e96f19d19..f6cb2a8b08a7 100644 > > > --- a/Documentation/ABI/testing/sysfs-bus-counter > > > +++ b/Documentation/ABI/testing/sysfs-bus-counter > > > @@ -212,6 +212,14 @@ Description: > > > both edges: > > > Any state transition. > > > > > > +What: /sys/bus/counter/devices/counterX/events_queue_size > > > +KernelVersion: 5.13 > > > +Contact: linux-iio@vger.kernel.org > > > +Description: > > > + Size of the Counter events queue in number of struct > > > + counter_event data structures. The number of elements will be > > > + rounded-up to a power of 2. > > > + > > > What: /sys/bus/counter/devices/counterX/name > > > KernelVersion: 5.2 > > > Contact: linux-iio@vger.kernel.org > > > diff --git a/drivers/counter/counter-chrdev.c b/drivers/counter/counter-chrdev.c > > > index 16f02df7f73d..53eea894e13f 100644 > > > --- a/drivers/counter/counter-chrdev.c > > > +++ b/drivers/counter/counter-chrdev.c > > > @@ -375,6 +375,29 @@ void counter_chrdev_remove(struct counter_device *const counter) > > > cdev_del(&counter->chrdev); > > > } > > > > > > +int counter_chrdev_realloc_queue(struct counter_device *const counter, > > > + size_t queue_size) > > > +{ > > > + int err; > > > + DECLARE_KFIFO_PTR(events, struct counter_event); > > > + unsigned long flags; > > > + > > > + /* Allocate new events queue */ > > > + err = kfifo_alloc(&events, queue_size, GFP_ATOMIC); > > > > Is there any potential for losing events? > > We take the events_list_lock down below so we're safe against missing an > event, but past events currently unread in the queue will be lost. > > Shortening the size of the queue is inherently a destructive process if > we have more events in the current queue than can fit in the new queue. > Because we a liable to lose some events in such a case, I think it's > best to keep the behavior of this reallocation consistent and have it > provide a fresh empty queue every time, as opposed to sometimes dropping > events and sometimes not. > > I also suspect an actual user would be setting the size of their queue > to the required amount before they begin watching events, rather than > adjusting it sporadically during a live operation. > Absolutely agree. As such I wonder if you are better off enforcing this behaviour? If the cdev is open for reading, don't allow the fifo to be resized. Jonathan