linux-kernel - Re: [PATCH] PM: domains: Prevent power off for parent unless child is in deepest state

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Message-ID: <b33ceac4-506a-65c8-7c80-b1b0a67ce65e@gmail.com>
Date:   Mon, 31 Jan 2022 21:29:06 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Ulf Hansson <ulf.hansson@...aro.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>, linux-pm@...r.kernel.org
Cc:     Kevin Hilman <khilman@...nel.org>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Rajendra Nayak <rnayak@...eaurora.org>,
        Dong Aisheng <aisheng.dong@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] PM: domains: Prevent power off for parent unless child is
 in deepest state

31.01.2022 14:37, Ulf Hansson пишет:
> A PM domain managed by genpd may support multiple idlestates. During
> genpd_power_off() a genpd governor may be asked to select one of the
> idlestates based upon the dev PM QoS constraints, for example.
> 
> However, there is a problem with the behaviour around this in genpd. More
> precisely, a parent-domain is allowed to be powered off, no matter of what
> idlestate that has been selected for the child-domain.
> 
> So far, we have not received any reports about errors, possibly because
> there might not be platform with this hierarchical configuration, yet.
> Nevertheless, it seems reasonable to change the behaviour into preventing
> the parent-domain from being powered off, unless the deepest idlestate has
> been selected for the child-domain, so let's do that.
> 
> Signed-off-by: Ulf Hansson <ulf.hansson@...aro.org>
> ---
>  drivers/base/power/domain.c | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/drivers/base/power/domain.c b/drivers/base/power/domain.c
> index 5db704f02e71..7f97c5cabdc2 100644
> --- a/drivers/base/power/domain.c
> +++ b/drivers/base/power/domain.c
> @@ -636,6 +636,17 @@ static int genpd_power_off(struct generic_pm_domain *genpd, bool one_dev_on,
>  			atomic_read(&genpd->sd_count) > 0)
>  		return -EBUSY;
>  
> +	/*
> +	 * The children must be in their deepest states to allow the parent to
> +	 * be powered off. Note that, there's no need for additional locking, as
> +	 * powering on a child, requires the parent's lock to be acquired first.
> +	 */
> +	list_for_each_entry(link, &genpd->parent_links, parent_node) {
> +		struct generic_pm_domain *child = link->child;
> +		if (child->state_idx < child->state_count - 1)
> +			return -EBUSY;
> +	}
> +
>  	list_for_each_entry(pdd, &genpd->dev_list, list_node) {
>  		enum pm_qos_flags_status stat;
>  
> @@ -1073,6 +1084,13 @@ static void genpd_sync_power_off(struct generic_pm_domain *genpd, bool use_lock,
>  	    || atomic_read(&genpd->sd_count) > 0)
>  		return;
>  
> +	/* Check that the children are in their deepest state. */
> +	list_for_each_entry(link, &genpd->parent_links, parent_node) {
> +		struct generic_pm_domain *child = link->child;
> +		if (child->state_idx < child->state_count - 1)
> +			return;
> +	}
> +
>  	/* Choose the deepest state when suspending */
>  	genpd->state_idx = genpd->state_count - 1;
>  	if (_genpd_power_off(genpd, false))

Hello Ulf,

Is this needed by a concrete SoC? It needs to be clarified in the commit
message, otherwise looks like this patch wasn't tested and it's unclear
whether this change is really needed.