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 36860C00140 for ; Sun, 31 Jul 2022 14:17:17 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237093AbiGaORO (ORCPT ); Sun, 31 Jul 2022 10:17:14 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33666 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229639AbiGaORL (ORCPT ); Sun, 31 Jul 2022 10:17:11 -0400 Received: from netrider.rowland.org (netrider.rowland.org [192.131.102.5]) by lindbergh.monkeyblade.net (Postfix) with SMTP id 91482DEC9 for ; Sun, 31 Jul 2022 07:17:09 -0700 (PDT) Received: (qmail 544533 invoked by uid 1000); 31 Jul 2022 10:17:08 -0400 Date: Sun, 31 Jul 2022 10:17:08 -0400 From: Alan Stern To: Guenter Roeck Cc: Alexey Klimov , Oliver Neukum , Greg KH , linux-watchdog@vger.kernel.org, wim@linux-watchdog.org, USB list , Linux Kernel Mailing List , atishp@rivosinc.com, atishp@atishpatra.org, Yury Norov , Alexey Klimov , Aaron Tomlin Subject: Re: [PATCH v5] watchdog: add driver for StreamLabs USB watchdog device Message-ID: References: <20220725030605.1808710-1-klimov.linux@gmail.com> <7770401d-fe3d-bda4-a2e2-55cd004a2d07@suse.com> <20220731082055.GA4008925@roeck-us.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220731082055.GA4008925@roeck-us.net> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Jul 31, 2022 at 01:20:55AM -0700, Guenter Roeck wrote: > On Sun, Jul 31, 2022 at 03:34:16AM +0100, Alexey Klimov wrote: > > On Tue, Jul 26, 2022 at 8:48 AM Oliver Neukum wrote: > > > > > > On 26.07.22 02:21, Alexey Klimov wrote: > > > > On Mon, Jul 25, 2022 at 9:51 AM Greg KH wrote: > > > >> > > > >> On Mon, Jul 25, 2022 at 04:06:05AM +0100, Alexey Klimov wrote: > > > > > > > > [..] > > > > > > > >> Anyway, driver looks good to me, nice work! > > > >> > > > >> Reviewed-by: Greg Kroah-Hartman > > > > > > > > Thanks, Greg. If you don't mind I'll use your tag in next version > > > > after making changes suggested by Guenter since there will be no > > > > significant functional changes. If code will change a lot, then the > > > > process (Documentation/process/submitting-patches.rst) will require me > > > > to drop the tag. > > > > > > Hi, > > > > > > while thinking about this a question arose. How does this > > > device react to a USB reset? A watchdog that can be disabled > > > by a simple reset does not like very reliable to me. > > > Do you need to implement pre/post_reset() ? > > > > You're right. Upon reset the watchdog is disabled even if it was active before. > > Adding empty ->pre_reset() and ->post_reset() helps to avoid that, but > > looking at Documentation and other drivers it seems that I need to do: > > in pre_reset(): > > mutex_lock() to block any other I/O to the usb device; > > __usb_streamlabs_wdt_cmd(STOP) to stop the watchdog; > > and do not unlock the mutex; > > > > in post_reset(): > > if (watchdog_active()) > > __usb_streamlabs_wdt_cmd(START); > > mutex_unlock() to allow other's I/O to the usb deivce. > > > > Seems right? > > > Not necessarily. Is other code doing something similar ? > Using a mutex like this creates the risk for hung tasks. Are mutexes intended to be used in situations where one function acquires the lock, then returns, and then a different function releases the lock? I'm not sure about this. Perhaps a good old semaphore would be more appropriate. But it's clear that I/O to the device does need to be mutually exclusive with resets, one way or another. Alan Stern