linux-kernel - Re: [PATCH v3] PM / core: conditionally skip system pm in device/driver model

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Date: Mon, 26 Feb 2024 10:40:12 -0800
From: Florian Fainelli <f.fainelli@...il.com>
To: Guan-Yu Lin <guanyulin@...gle.com>
Cc: rafael@...nel.org, pavel@....cz, len.brown@...el.com,
 gregkh@...uxfoundation.org, andriy.shevchenko@...ux.intel.com,
 petr.tesarik.ext@...wei.com, rdunlap@...radead.org, james@...iv.tech,
 broonie@...nel.org, james.clark@....com, masahiroy@...nel.org,
 linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org
Subject: Re: [PATCH v3] PM / core: conditionally skip system pm in
 device/driver model

On 2/26/24 02:28, Guan-Yu Lin wrote:
> On Sat, Feb 24, 2024 at 2:20 AM Florian Fainelli <f.fainelli@...il.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 – 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.

OK, so now the question is whether the peripheral is entirely visible to 
Linux, or is it entirely owned by the co-processor, or is there a 
combination of both and the usage of the said device driver is dynamic 
between Linux and your co-processor?

A sysfs entry does not seem like the appropriate way to described which 
states need to be skipped and which ones can remain under control of 
Linux, you would have to use your firmware's description for that (ACPI, 
Device Tree, etc.) such that you have a more comprehensive solution that 
can span a bigger scope.
-- 
Florian