linux-kernel - Re: [PATCH v2 1/5] PM: domains: Add GENPD_FLAG_RT_SAFE for PREEMPT

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Message-ID: <9771639f-5172-8f3b-3ce6-8fd195aa95b3@linaro.org>
Date:   Fri, 6 Jan 2023 15:52:57 +0100
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     Ulf Hansson <ulf.hansson@...aro.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kevin Hilman <khilman@...nel.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Sudeep Holla <sudeep.holla@....com>, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Adrien Thierry <athierry@...hat.com>,
        Brian Masney <bmasney@...hat.com>,
        linux-rt-users@...r.kernel.org
Subject: Re: [PATCH v2 1/5] PM: domains: Add GENPD_FLAG_RT_SAFE for PREEMPT_RT

On 04/01/2023 16:45, Ulf Hansson wrote:
> On Mon, 19 Dec 2022 at 16:15, Krzysztof Kozlowski
> <krzysztof.kozlowski@...aro.org> wrote:
>>
>> Realtime kernels with PREEMPT_RT must use raw_spinlock_t for domains
>> which are invoked from CPU idle (thus from atomic section).  Example is
>> cpuidle PSCI domain driver which itself is PREEMPT_RT safe, but is being
>> called as part of cpuidle.
> 
> Just so I don't get this wrong, since the cpuidle-psci also calls
> pm_runtime_* functions so it isn't PREEMPT_RT safe, at least not yet?

You are correct. Patch 3 here addresses it by... just not doing runtime
PM. This is a hacky workaround but:
1. I don't have any other idea,
2. It's not a big problem because RT systems are not supposed to have
any CPU idle (one of first things during RT system tuning is to disable
cpuidle).

> 
>>
>> Add a flag allowing a power domain provider to indicate it is RT safe.
>> The flag is supposed to be used with existing GENPD_FLAG_IRQ_SAFE.
>>
>> Cc: Adrien Thierry <athierry@...hat.com>
>> Cc: Brian Masney <bmasney@...hat.com>
>> Cc: linux-rt-users@...r.kernel.org
>> Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
> 
> For genpd, overall, I think this looks like an okay approach to me.
> Although, let me check the whole series (I need some more time to do
> that) before I give this my blessing.

Sure, we are all have too many mails in inbox. :)

Best regards,
Krzysztof