Drivers taking different actions depending on sleep state

[Florian Fainelli]

On 06/21/2017 03:57 PM, Rafael J. Wysocki wrote:
> On Thu, Jun 22, 2017 at 12:48 AM, Florian Fainelli <f.fainelli at gmail.com> wrote:
>> On 06/21/2017 02:59 PM, Rafael J. Wysocki wrote:
>>> On Wed, Jun 21, 2017 at 11:16 PM, Florian Fainelli <f.fainelli at gmail.com> wrote:
>>>> On 06/09/2017 03:53 PM, Rafael J. Wysocki wrote:
>>>>> Hi,
>>>>>
>>>>> On Fri, Jun 9, 2017 at 5:20 PM, Mason <slash.tmp at free.fr> wrote:
>>>>>> Hello,
>>>>>>
>>>>>> I read the "Sleep States" documentation:
>>>>>> https://www.kernel.org/doc/Documentation/power/states.txt
>>>>>>
>>>>>> It mentions /sys/power/mem_sleep but I don't have that in 4.9
>>>>>> # ll /sys/power/
>>>>>> -rw-r--r--    1 root     root          4096 Jan  1 00:31 pm_async
>>>>>> -rw-r--r--    1 root     root          4096 Jan  1 00:31 pm_freeze_timeout
>>>>>> -rw-r--r--    1 root     root          4096 Jan  1 00:31 state
>>>>>> -rw-r--r--    1 root     root          4096 Jan  1 00:31 wakeup_count
>>>>>>
>>>>>> # cat /sys/power/state
>>>>>> freeze mem
>>>>>>
>>>>>> Currently my platform's "mem" is a true suspend-to-RAM trigger,
>>>>>> where drivers are supposed to save their state (register values
>>>>>> will be lost), then Linux hands control over to firmware which
>>>>>> enables RAM self-refresh and powers the chip down. When the system
>>>>>> resumes, drivers restore their state from their copy in memory.
>>>>>>
>>>>>> One driver is responsible for loading/unloading microcode running
>>>>>> on the DSPs. This operation is required only when powering down
>>>>>> the chip, but it should be avoided for "low-latency" sleeps.
>>>>>>
>>>>>> The problem is that, if I understand correctly, drivers have no way
>>>>>> of knowing which sleep state is being entered/exited?
>>>>>>
>>>>>> How can I have the microcode driver take different decisions
>>>>>> based on the sleep state?
>>>>>
>>>>> The cleanest way would be to run that code from one of the platform
>>>>> suspend hooks that receive information on what sleep state is to be
>>>>> entered.
>>>>
>>>> I am not sure this would be cleaner, because that would create a tighter
>>>> dependency between different drivers, each of them having their
>>>> suspend/resume routings and the driver that implements the
>>>> platform_suspend_ops, that could also create some nice layering
>>>> violations and some difficult to solve dependencies.
>>>>
>>>>>
>>>>> Alternatively, those hooks can set/clear flags that can be accessed by
>>>>> drivers, but that of course may your drivers depend on the platform
>>>>> (still, in the microcode case the driver seems to be
>>>>> platform-dependent anyway).
>>>>
>>>> It may be platform dependent, but the actual system-wide suspend/resume
>>>> implementations can vary a lot. For example you may have started with
>>>> some particular CPU architecture on your platforms, with one driver
>>>> implementing an instance of platform_suspend_ops, and then as you moved
>>>> to another CPU architecture, some of that could be managed by a generic
>>>> driver (e.g: ARM SCPI, ACPI etc. etc.).
>>>>
>>>> The same HW blocks are likely to be present on these different SoCs, and
>>>> have the same requirements where they need to see a slightly different
>>>> path taken on suspend/resume. If we have to patch both the "legacy"
>>>> platform_suspend_ops, and the "new" platform_suspend_ops that does not
>>>> really scale.
>>>>
>>>> Would it be that much of a stretch if we reflected e.g:
>>>> PM_SUSPEND_STANDBY, PM_SUSPEND_MEM into the pm_message_t that is
>>>> communicated to platform_driver::suspend and platform_driver::resume?
>>>
>>> I'm not sure what you mean, really.
>>>
>>> The ->suspend callback in struct platform_driver has been long deprecated.
>>
>> What I mean is that we could take advantage of the pm_message_t argument
>> passed to platform_driver::resume and platform_driver::resume to
>> communicate the system sleep state we are about to enter (and conversely
>> exit).
> 
> So the ->suspend and ->resume callbacks in struct platform_driver
> (which I think is what you are referring to) are legacy.
> 
> The majority of drivers I know about use struct dev_pm_ops and the
> callbacks in there do not take the pm_message_t argument.
> 
> Moreover, even if they did take it, the exact meaning of
> PM_SUSPEND_STANDBY and PM_SUSPEND_MEM is platform-specific and drivers
> would need to find out what they actually mean for the given platform
> somehow anyway.
> 
>> This would allow drivers to take a different path whether e.g:
>> pm_message_t == PM_SUSPEND_STANDBY or PM_SUSPEND_MEM.
>>
>> If these are deprecated, then should we introduce a global getter
>> function that reflects which suspend state we are about to enter?  This
>> would allow drivers do to something like (pseudo code):
>>
>> static int drv_suspend(struct device d)
>> {
>>         suspend_state_t state = suspend_get_state();
>>
>>         switch (state) {
>>         case PM_SUSPEND_STANDBY:
>>                 return 0;
>>         case PM_SUSPEND_MEM:
>>                 /* save HW context */
>> }
> 
> That is conceivable, but again, the meaning of STANDBY and MEM is
> platform-specific.  Actions to be taken for, say, STANDBY, may differ
> from one platform to another.

True, though I would only expect drivers for that particular platform to
care about that information and these drivers would only make sense on
that particular platform so the meaning of STANDBY and MEM would be
clear for those drivers. The intent is really to keep the "distributed"
model where individual drivers manage their particular piece of HW,
while utilizing a global piece of information that is platform specific.

If this seems acceptable to you along with proper documentation to
illustrate the platform specific meaning of these states, I will got
ahead and cook a patch.

Thanks!
-- 
Florian