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 mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8E8FEC433EF for ; Wed, 27 Oct 2021 14:35:09 +0000 (UTC) Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 5E0D160F56 for ; Wed, 27 Oct 2021 14:35:09 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.1 mail.kernel.org 5E0D160F56 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=rowland.harvard.edu Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References: Message-ID:Subject:Cc:To:From:Date:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=W9sNx5Rx8hAFWzfJc0JkkMWgrQ2QMeuI3/52YIro1A4=; b=TjvsJ1FShuMGbp cImXKhdzrLWUu1b1vcIFcLTi7cERN0hcrq57LuJzbhXcuRLZTEAa3laI6aja7m6hlWYOwigGoy5YR 4yGrtaMH1vZOPQJseFlqPZd9J9FtF2EbsT/HGDTNPFT/i2L472Ao1QEbkoPFzwX8t9aWii6KpVXFs rJCJmUDF22qMwh7O9qEYBA81affm8c7khY+A3LzpARUGvgpHgYSvjxyV+fWNOTds/VYJSCkrxwXR9 OeWUVwL7my4/OT9xPhS6O3zpYM1dn/VyfT478/PVSYsNvGtOjdiGay6dUHWrD+dNwLJiZ3Z6BS23u FzGV1EJBkvT//f8aIqfw==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1mfjzx-00583r-Ot; Wed, 27 Oct 2021 14:33:49 +0000 Received: from netrider.rowland.org ([192.131.102.5]) by bombadil.infradead.org with smtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1mfjzs-005837-VH for linux-arm-kernel@lists.infradead.org; Wed, 27 Oct 2021 14:33:46 +0000 Received: (qmail 1321012 invoked by uid 1000); 27 Oct 2021 10:33:43 -0400 Date: Wed, 27 Oct 2021 10:33:43 -0400 From: Alan Stern To: Ulf Hansson Cc: "Rafael J . Wysocki" , "Rafael J . Wysocki" , linux-pm@vger.kernel.org, Kevin Hilman , Maulik Shah , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] PM: runtime: Allow rpm_resume() to succeed when runtime PM is disabled Message-ID: <20211027143343.GC1319606@rowland.harvard.edu> References: <20211026222626.39222-1-ulf.hansson@linaro.org> <20211027020235.GA1306582@rowland.harvard.edu> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20211027_073345_176658_FA2294D7 X-CRM114-Status: GOOD ( 48.68 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Wed, Oct 27, 2021 at 12:55:43PM +0200, Ulf Hansson wrote: > On Wed, 27 Oct 2021 at 04:02, Alan Stern wrote: > > > > On Wed, Oct 27, 2021 at 12:26:26AM +0200, Ulf Hansson wrote: > > > During system suspend, the PM core sets dev->power.is_suspended for the > > > device that is being suspended. This flag is also being used in > > > rpm_resume(), to allow it to succeed by returning 1, assuming that runtime > > > PM has been disabled and the runtime PM status is RPM_ACTIVE, for the > > > device. > > > > > > To make this behaviour a bit more useful, let's drop the check for the > > > dev->power.is_suspended flag in rpm_resume(), as it doesn't really need to > > > be limited to this anyway. > > > > > > Signed-off-by: Ulf Hansson > > > --- > > > drivers/base/power/runtime.c | 4 ++-- > > > 1 file changed, 2 insertions(+), 2 deletions(-) > > > > > > diff --git a/drivers/base/power/runtime.c b/drivers/base/power/runtime.c > > > index ec94049442b9..fadc278e3a66 100644 > > > --- a/drivers/base/power/runtime.c > > > +++ b/drivers/base/power/runtime.c > > > @@ -742,8 +742,8 @@ static int rpm_resume(struct device *dev, int rpmflags) > > > repeat: > > > if (dev->power.runtime_error) > > > retval = -EINVAL; > > > - else if (dev->power.disable_depth == 1 && dev->power.is_suspended > > > - && dev->power.runtime_status == RPM_ACTIVE) > > > + else if (dev->power.disable_depth > 0 && > > > + dev->power.runtime_status == RPM_ACTIVE) > > > > IIRC there was a good reason why the original code checked for > > disable_depth == 1 rather than > 0. But I don't remember exactly what > > the reason was. Maybe it had something to do with the fact that during > > a system sleep __device_suspend_late calls __pm_runtime_disable, and the > > code was checking that there were no other disables in effect. > > The check was introduced in the below commit: > > Commit 6f3c77b040fc > Author: Kevin Hilman > Date: Fri Sep 21 22:47:34 2012 +0000 > PM / Runtime: let rpm_resume() succeed if RPM_ACTIVE, even when disabled, v2 > > By reading the commit message it's pretty clear to me that the check > was added to cover only one specific use case, during system suspend. > > That is, that a driver may want to call pm_runtime_get_sync() from a > late/noirq callback (when the PM core has disabled runtime PM), to > understand whether the device is still powered on and accessible. > > > This is > > related to the documented behavior of rpm_resume (it's supposed to fail > > with -EACCES if the device is disabled for runtime PM, no matter what > > power state the device is in). > > > > That probably is also the explanation for why dev->power.is_suspended > > gets checked: It's how the code tells whether a system sleep is in > > progress. > > Yes, you are certainly correct about the current behaviour. It's there > for a reason. > > On the other hand I would be greatly surprised if this change would > cause any issues. Of course, I can't make guarantees, but I am, of > course, willing to help to fix problems if those happen. > > As a matter of fact, I think the current behaviour looks quite > inconsistent, as it depends on whether the device is being system > suspended. > > Moreover, for syscore devices (dev->power.syscore is set for them), > the PM core doesn't set the "is_suspended" flag. Those can benefit > from a common behaviour. > > Finally, I think the "is_suspended" flag actually needs to be > protected by a lock when set by the PM core, as it's being used in two > separate execution paths. Although, rather than adding a lock for > protection, we can just rely on the "disable_depth" in rpm_resume(). > It would be easier and makes the behaviour consistent too. As long as is_suspended isn't _written_ in two separate execution paths, we're probably okay without a lock -- provided the code doesn't mind getting an indefinite result when a read races with a write. > > So overall, I suspect this change should not be made. But some other > > improvement (like a nice comment) might be in order. > > > > Alan Stern > > Thanks for reviewing! You're welcome. Whatever you eventually decide to do should be okay with me. I just wanted to make sure that you understood the deeper issue here and had given it some thought. For example, it may turn out that you can resolve matters simply by updating the documentation. Alan Stern _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel