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 8B912C433EF for ; Thu, 2 Dec 2021 16:18:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1359393AbhLBQWD (ORCPT ); Thu, 2 Dec 2021 11:22:03 -0500 Received: from mail-ot1-f53.google.com ([209.85.210.53]:36487 "EHLO mail-ot1-f53.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1359376AbhLBQWA (ORCPT ); Thu, 2 Dec 2021 11:22:00 -0500 Received: by mail-ot1-f53.google.com with SMTP id w6-20020a9d77c6000000b0055e804fa524so328674otl.3; Thu, 02 Dec 2021 08:18:38 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=PGr6jOv41SrTwtlqda0X2UyFg0226rw2ZkBazfb+o5k=; b=gBwyYvNphxIuXkfh8JioOYQUXLmSrGfR5EPZSuWWfxbVZzRcOde8AOucdEXFJxL2nh +vJRRk/N/nX6t6BcFf39SNG28hl0oEDs1XrYn842SM30ALh2LdMWIfzdRdKZuaReD8vo pKHGoYFClXoTiZu7/C8wUQgJyja409d9WKqkfTIsL3dKAPEIZS8QyR0uYXNl3GhynnhX Eck3KUT+Hx5DZQzlxzz48+d382R1E5oY9GMh2tFxNMockxJ0HawwVE2OnZ37XGqpGb0k UFIFhHBQ5//qV/+YPYoWuvUI7vxaZvzgZGqBPLhrytpgQbzGJ8KiN9LL+Uu0apTqQPoe Ojaw== X-Gm-Message-State: AOAM531bwG8xw2oWX2m/kPzKFHzeQf+fLZlagJB2JWr3gH5jEnN14wpa L/n8LXPbe5D/NnIbs8sEzEcUmf3/OLh0K0fgwv4= X-Google-Smtp-Source: ABdhPJzHtVZSjjV45oLOos1CN7dzyFhbiQ1Y0F1gcAsRPvCm96dSQKmyAPmC6g2VeVAP6kg8JB2XLavoRt1oU+ES53E= X-Received: by 2002:a9d:4c10:: with SMTP id l16mr12078971otf.198.1638461917691; Thu, 02 Dec 2021 08:18:37 -0800 (PST) MIME-Version: 1.0 References: <20211026222626.39222-1-ulf.hansson@linaro.org> <5794197.lOV4Wx5bFT@kreacher> In-Reply-To: From: "Rafael J. Wysocki" Date: Thu, 2 Dec 2021 17:18:26 +0100 Message-ID: Subject: Re: [PATCH] PM: runtime: Allow rpm_resume() to succeed when runtime PM is disabled To: Ulf Hansson Cc: "Rafael J. Wysocki" , "Rafael J. Wysocki" , Alan Stern , Linux PM , Kevin Hilman , Maulik Shah , Linux ARM , Linux Kernel Mailing List Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Dec 2, 2021 at 12:29 PM Ulf Hansson wrote: > > On Wed, 1 Dec 2021 at 21:11, Rafael J. Wysocki wrote: > > > > On Wednesday, December 1, 2021 6:44:08 PM CET Rafael J. Wysocki wrote: > > > On Wed, Dec 1, 2021 at 4:23 PM Ulf Hansson wrote: > > > > > > > > On Wed, 1 Dec 2021 at 14:49, Rafael J. Wysocki wrote: > > > > > > > > > > On Wed, Dec 1, 2021 at 10:02 AM Ulf Hansson wrote: > > > > > > > > > > > > On Tue, 30 Nov 2021 at 18:26, Rafael J. Wysocki wrote: > > > > > > > > > > > > > > On Tue, Nov 30, 2021 at 5:41 PM Ulf Hansson wrote: > > > > > > > > > > > > > > > > On Tue, 30 Nov 2021 at 14:02, Rafael J. Wysocki wrote: > > > > > > > > > > > > > > > > > > On Tue, Nov 30, 2021 at 12:58 PM Ulf Hansson wrote: > > > > > > > > > > > > > > > > > > > > [...] > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Am I thinking correctly that this is mostly about working around the > > > > > > > > > > > > > > > > limitations of pm_runtime_force_suspend()? > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > No, this isn't related at all. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > The cpuidle-psci driver doesn't have PM callbacks, thus using > > > > > > > > > > > > > > > pm_runtime_force_suspend() would not work here. > > > > > > > > > > > > > > > > > > > > > > > > > > > > Just wanted to send a ping on this to see if we can come to a > > > > > > > > > > > > > > conclusion. Or maybe we did? :-) > > > > > > > > > > > > > > > > > > > > > > > > > > > > I think in the end, what slightly bothers me, is that the behavior is > > > > > > > > > > > > > > a bit inconsistent. Although, maybe it's the best we can do. > > > > > > > > > > > > > > > > > > > > > > > > > > I've been thinking about this and it looks like we can do better, but > > > > > > > > > > > > > instead of talking about this I'd rather send a patch. > > > > > > > > > > > > > > > > > > > > > > > > Alright. > > > > > > > > > > > > > > > > > > > > > > > > I was thinking along the lines of make similar changes for > > > > > > > > > > > > rpm_idle|suspend(). That would make the behaviour even more > > > > > > > > > > > > consistent, I think. > > > > > > > > > > > > > > > > > > > > > > > > Perhaps that's what you have in mind? :-) > > > > > > > > > > > > > > > > > > > > > > Well, not exactly. > > > > > > > > > > > > > > > > > > > > > > The idea is to add another counter (called restrain_depth in the patch) > > > > > > > > > > > to prevent rpm_resume() from running the callback when that is potentially > > > > > > > > > > > problematic. With that, it is possible to actually distinguish devices > > > > > > > > > > > with PM-runtime enabled and it allows the PM-runtime status to be checked > > > > > > > > > > > when it is still known to be meaningful. > > > > > > > > > > > > > > > > > > > > Hmm, I don't quite understand the benefit of introducing a new flag > > > > > > > > > > for this. rpm_resume() already checks the disable_depth to understand > > > > > > > > > > when it's safe to invoke the callback. Maybe there is a reason why > > > > > > > > > > that isn't sufficient? > > > > > > > > > > > > > > > > > > The problem is that disable_depth > 0 may very well mean that runtime > > > > > > > > > PM has not been enabled at all for the given device which IMO is a > > > > > > > > > problem. > > > > > > > > > > > > > > > > > > As it stands, it is necessary to make assumptions, like disable_depth > > > > > > > > > == 1 meaning that runtime PM is really enabled, but the PM core has > > > > > > > > > disabled it temporarily, which is somewhat questionable. > > > > > > > > > > > > > > > > > > Another problem with disabling is that it causes rpm_resume() to fail > > > > > > > > > even if the status is RPM_ACTIVE and it has to do that exactly because > > > > > > > > > it cannot know why runtime PM has been disabled. If it has never been > > > > > > > > > enabled, rpm_resume() must fail, but if it has been disabled > > > > > > > > > temporarily, rpm_resume() may return 1 when the status is RPM_ACTIVE. > > > > > > > > > > > > > > > > > > The new count allows the "enabled in general, but temporarily disabled > > > > > > > > > at the moment" to be handled cleanly. > > > > > > > > > > > > > > > > My overall comment is that I fail to understand why we need to > > > > > > > > distinguish between these two cases. To me, it shouldn't really > > > > > > > > matter, *why* runtime PM is (or have been) disabled for the device. > > > > > > > > > > > > > > It matters if you want to trust the status, because "disabled" means > > > > > > > "the status doesn't matter". > > > > > > > > > > > > Well, that doesn't really match how the runtime PM interface is being > > > > > > used today. > > > > > > > > > > Well, I clearly disagree. > > > > > > > > Alright, then we can agree to disagree. :-) > > > > > > > > > > > > > > > For example, we have a whole bunch of helper functions, allowing us to > > > > > > update and check the runtime PM state of the device, even when the > > > > > > disable_depth > 0. Some functions, like pm_runtime_set_active() for > > > > > > example, even take parents and device-links into account. > > > > > > > > > > That's true, but that's for a purpose. > > > > > > > > > > If runtime PM becomes enabled after using pm_runtime_set_active(), the > > > > > status should better be consistent with the settings of the parent > > > > > etc. > > > > > > > > > > > > > > > > > > > If you want the status to stay meaningful, but prevent callbacks from > > > > > > > running, you need something else. > > > > > > > > > > > > > > > The important point is that the default state for a device is > > > > > > > > RPM_SUSPENDED and someone has moved into RPM_ACTIVE, for whatever > > > > > > > > reason. That should be sufficient to allow rpm_resume() to return '1' > > > > > > > > when disable_depth > 0, shouldn't it? > > > > > > > > > > > > > > No, because there is no rule by which the status of devices with > > > > > > > PM-runtime disabled must be RPM_SUSPENDED. > > > > > > > > > > > > That's not what I was trying to say. > > > > > > > > > > > > The initial/default runtime PM state for a device is RPM_SUSPENDED, > > > > > > which is being set in pm_runtime_init(). Although, I agree that it > > > > > > can't be trusted that this state actually reflects the state of the > > > > > > HW, it's still a valid state for the device from a runtime PM point of > > > > > > view. > > > > > > > > > > No, it is not. It's just the default. > > > > > > > > > > > However, and more importantly, if the state has moved to RPM_ACTIVE, > > > > > > someone must have deliberately moved the device into that state. > > > > > > > > > > Sure, but it cannot be regarded as an indication on whether or not > > > > > runtime PM is supported and has ever been enabled for the given > > > > > device. > > > > > > > > > > Again, there is no rule regarding the status value for devices with > > > > > runtime PM disabled, either way. > > > > > > > > If I understand correctly, that means you think the > > > > pm_runtime_status_suspended() should really be converted to an > > > > internal runtime PM interface, not being exported to users outside. > > > > Right? > > > > > > Not really. > > > > > > I'm just saying that its usefulness is limited. > > > > > > My basic concern is that system-wide PM transitions must always invoke > > > callbacks for devices with PM-runtime disabled, because they may (or > > > may not) be functional regardless of the PM-runtime status and if they > > > are functional, they must be suspended. And note that supporting > > > system-wide PM is not optional and the only way to kind of disable it > > > is to return an error from a device suspend callback (but that's nasty > > > for some use cases). > > > > > > So the "Has PM-runtime been enabled?" question is really fundamental > > > for system-wide PM and it is not sufficient to look at the PM-runtime > > > status to find out, but if the PM-core itself disables PM-runtime > > > (which is has to do at one point to prevent PM-runtime from racing > > > with system-wide PM), it is hard to answer definitely in general. > > > > > > IMO the only way to make it possible to find that out in all cases is > > > to make the PM core retain the power.disable_depth value and that can > > > be done by making it use a different mechanism to prevent PM-runtime > > > callbacks from being run. > > > > > > Alternatively, the current PM-runtime status could be "latched" during > > > the PM-runtime disable operation if power.disable_depth is 0 (and that > > > "latched" value would be initialized to "invalid" in case PM-runtime > > > is never enabled). > > > > Which would be something like the patch below (which additionally cleans up > > pm_runtime_enable() while at it). > > > > The idea being that if the status was RPM_ACTIVE last time when > > power.disable_depth was changing from 0 to 1 and it is still RPM_ACTIVE, it > > can be assumed to reflect what happened to the device last time when it was > > using PM-runtime. > > Alright, this sounds reasonable to me. I have also looked at the code > below and it looks good to me. > > Do you intend to post a formal patch? In any case, feel free to add my > reviewed-by tag. I will, thank you!