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From: Guan-Yu Lin <guanyulin@google.com>
Date: Mon, 26 Feb 2024 18:28:00 +0800
Message-ID: <CAOuDEK3wP6zhEwgUn5zSedtwTYVFaJeBfeXkSg897EhpGP9=ig@mail.gmail.com>
Subject: Re: [PATCH v3] PM / core: conditionally skip system pm in
 device/driver model
To: Florian Fainelli <f.fainelli@gmail.com>
Cc: rafael@kernel.org, pavel@ucw.cz, len.brown@intel.com, 
	gregkh@linuxfoundation.org, andriy.shevchenko@linux.intel.com, 
	petr.tesarik.ext@huawei.com, rdunlap@infradead.org, james@equiv.tech, 
	broonie@kernel.org, james.clark@arm.com, masahiroy@kernel.org, 
	linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org
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On Sat, Feb 24, 2024 at 2:20=E2=80=AFAM Florian Fainelli <f.fainelli@gmail.=
com> wrote:
>
> On 2/23/24 06:38, Guan-Yu Lin wrote:
> > In systems with a main processor and a co-processor, asynchronous
> > controller management can lead to conflicts.  One example is the main
> > processor attempting to suspend a device while the co-processor is
> > actively using it. To address this, we introduce a new sysfs entry
> > called "conditional_skip". This entry allows the system to selectively
> > skip certain device power management state transitions. To use this
> > feature, set the value in "conditional_skip" to indicate the type of
> > state transition you want to avoid.  Please review /Documentation/ABI/
> > testing/sysfs-devices-power for more detailed information.
>
> This looks like a poor way of dealing with a lack of adequate resource
> tracking from Linux on behalf of the co-processor(s) and I really do not
> understand how someone is supposed to use that in a way that works.
>
> Cannot you use a HW maintained spinlock between your host processor and
> the co-processor such that they can each claim exclusive access to the
> hardware and you can busy-wait until one or the other is done using the
> device? How is your partitioning between host processor owned blocks and
> co-processor(s) owned blocks? Is it static or is it dynamic?
> --
> Florian
>

This patch enables devices to selectively participate in system power
transitions. This is crucial when multiple processors, managed by
different operating system kernels, share the same controller. One
processor shouldn't enforce the same power transition procedures on
the controller =E2=80=93 another processor might be using it at that moment=
.
While a spinlock is necessary for synchronizing controller access, we
still need to add the flexibility to dynamically customize power
transition behavior for each device. And that's what this patch is
trying to do.
In our use case, the host processor and co-processor are managed by
separate operating system kernels. This arrangement is static.