Date: Tue, 4 Jun 2024 10:22:39 -0700
From: Nikunj Kela <quic_nkela@...cinc.com>
To: Ulf Hansson <ulf.hansson@...aro.org>
CC: "Rafael J . Wysocki" <rafael@...nel.org>,
        Sudeep Holla
	<sudeep.holla@....com>, <linux-pm@...r.kernel.org>,
        Lorenzo Pieralisi
	<Lorenzo.Pieralisi@....com>,
        Nikunj Kela <nkela@...cinc.com>,
        Prasad Sodagudi
	<psodagud@...cinc.com>,
        Maulik Shah <quic_mkshah@...cinc.com>,
        Daniel Lezcano
	<daniel.lezcano@...aro.org>,
        Krzysztof Kozlowski
	<krzysztof.kozlowski@...aro.org>,
        <linux-rt-users@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/7] pmdomain: core: Enable s2idle for CPU PM domains
 on PREEMPT_RT


On 6/3/2024 6:58 AM, Ulf Hansson wrote:
> On Thu, 30 May 2024 at 16:23, Nikunj Kela <quic_nkela@...cinc.com> wrote:
>>
>> On 5/30/2024 1:15 AM, Ulf Hansson wrote:
>>> On Tue, 28 May 2024 at 21:56, Nikunj Kela <quic_nkela@...cinc.com> wrote:
>>>> On 5/27/2024 7:25 AM, Ulf Hansson wrote:
>>>>> To allow a genpd provider for a CPU PM domain to enter a domain-idle-state
>>>>> during s2idle on a PREEMPT_RT based configuration, we can't use the regular
>>>>> spinlock, as they are turned into sleepable locks on PREEMPT_RT.
>>>>>
>>>>> To address this problem, let's convert into using the raw spinlock, but
>>>>> only for genpd providers that have the GENPD_FLAG_CPU_DOMAIN bit set. In
>>>>> this way, the lock can still be acquired/released in atomic context, which
>>>>> is needed in the idle-path for PREEMPT_RT.
>>>>>
>>>>> Do note that the genpd power-on/off notifiers may also be fired during
>>>>> s2idle, but these are already prepared for PREEMPT_RT as they are based on
>>>>> the raw notifiers. However, consumers of them may need to adopt accordingly
>>>>> to work properly on PREEMPT_RT.
>>>>>
>>>>> Signed-off-by: Ulf Hansson <ulf.hansson@...aro.org>
>>>>> ---
>>>>>
>>>>> Changes in v2:
>>>>>       - None.
>>>>>
>>>>> ---
>>>>>  drivers/pmdomain/core.c   | 47 ++++++++++++++++++++++++++++++++++++++-
>>>>>  include/linux/pm_domain.h |  5 ++++-
>>>>>  2 files changed, 50 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/drivers/pmdomain/core.c b/drivers/pmdomain/core.c
>>>>> index 623d15b68707..072e6bdb6ee6 100644
>>>>> --- a/drivers/pmdomain/core.c
>>>>> +++ b/drivers/pmdomain/core.c
>>>>> @@ -117,6 +117,48 @@ static const struct genpd_lock_ops genpd_spin_ops = {
>>>>>       .unlock = genpd_unlock_spin,
>>>>>  };
>>>>>
>>>>> +static void genpd_lock_raw_spin(struct generic_pm_domain *genpd)
>>>>> +     __acquires(&genpd->raw_slock)
>>>>> +{
>>>>> +     unsigned long flags;
>>>>> +
>>>>> +     raw_spin_lock_irqsave(&genpd->raw_slock, flags);
>>>>> +     genpd->raw_lock_flags = flags;
>>>>> +}
>>>>> +
>>>>> +static void genpd_lock_nested_raw_spin(struct generic_pm_domain *genpd,
>>>>> +                                     int depth)
>>>>> +     __acquires(&genpd->raw_slock)
>>>>> +{
>>>>> +     unsigned long flags;
>>>>> +
>>>>> +     raw_spin_lock_irqsave_nested(&genpd->raw_slock, flags, depth);
>>>>> +     genpd->raw_lock_flags = flags;
>>>>> +}
>>>>> +
>>>>> +static int genpd_lock_interruptible_raw_spin(struct generic_pm_domain *genpd)
>>>>> +     __acquires(&genpd->raw_slock)
>>>>> +{
>>>>> +     unsigned long flags;
>>>>> +
>>>>> +     raw_spin_lock_irqsave(&genpd->raw_slock, flags);
>>>>> +     genpd->raw_lock_flags = flags;
>>>>> +     return 0;
>>>>> +}
>>>>> +
>>>>> +static void genpd_unlock_raw_spin(struct generic_pm_domain *genpd)
>>>>> +     __releases(&genpd->raw_slock)
>>>>> +{
>>>>> +     raw_spin_unlock_irqrestore(&genpd->raw_slock, genpd->raw_lock_flags);
>>>>> +}
>>>>> +
>>>>> +static const struct genpd_lock_ops genpd_raw_spin_ops = {
>>>>> +     .lock = genpd_lock_raw_spin,
>>>>> +     .lock_nested = genpd_lock_nested_raw_spin,
>>>>> +     .lock_interruptible = genpd_lock_interruptible_raw_spin,
>>>>> +     .unlock = genpd_unlock_raw_spin,
>>>>> +};
>>>>> +
>>>>>  #define genpd_lock(p)                        p->lock_ops->lock(p)
>>>>>  #define genpd_lock_nested(p, d)              p->lock_ops->lock_nested(p, d)
>>>>>  #define genpd_lock_interruptible(p)  p->lock_ops->lock_interruptible(p)
>>>>> @@ -2079,7 +2121,10 @@ static void genpd_free_data(struct generic_pm_domain *genpd)
>>>>>
>>>>>  static void genpd_lock_init(struct generic_pm_domain *genpd)
>>>>>  {
>>>>> -     if (genpd->flags & GENPD_FLAG_IRQ_SAFE) {
>>>>> +     if (genpd->flags & GENPD_FLAG_CPU_DOMAIN) {
>>>>> +             raw_spin_lock_init(&genpd->raw_slock);
>>>>> +             genpd->lock_ops = &genpd_raw_spin_ops;
>>>>> +     } else if (genpd->flags & GENPD_FLAG_IRQ_SAFE) {
>>>> Hi Ulf, though you are targeting only CPU domains for now, I wonder if
>>>> FLAG_IRQ_SAFE will be a better choice?  The description of the flag says
>>>> it is safe for atomic context which won't be the case for PREEMPT_RT?
>>> You have a point!
>>>
>>> However, we also need to limit the use of raw spinlocks, from
>>> PREEMPT_RT point of view. In other words, just because a genpd
>>> provider is capable of executing its callbacks in atomic context,
>>> doesn't always mean that it should use raw spinlocks too.
>> Got it! Thanks. Maybe in future, if there is a need, a new GENPD FLAG
>> for RT, something like GENPD_FLAG_IRQ_SAFE_RT, can be added to address this.
> Yes, I agree, something along those lines would make sense.
>
> BTW, did you manage to get some time to test the series on your end?

I haven't been able spend time testing it but I have requested Maulik to
test it and update you. Thanks


> Kind regards
> Uffe

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