Re: [PATCH v2 2/3] PM / core: Add IN_BAND_WAKEUP driver flag

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Ulf,

On Mon, Dec 11, 2017 at 11:24 AM, Ulf Hansson <ulf.hansson@linaro.org> wrote:
> On 10 December 2017 at 11:16, Geert Uytterhoeven <geert@linux-m68k.org> wrote:
>> On Sun, Dec 10, 2017 at 3:30 AM, Rafael J. Wysocki <rjw@rjwysocki.net> wrote:
>>> On Monday, November 13, 2017 4:46:42 PM CET Ulf Hansson wrote:
>>>> For some bus types and PM domains, it's not sufficient to only check the
>>>> return value from device_may_wakeup(), to fully understand how to configure
>>>> wakeup settings for the device during system suspend.
>>>>
>>>> In particular, sometimes the device may need to remain in its power state,
>>>> in case the driver has configured it for in band IRQs, as to be able to
>>>> generate wakeup signals. Therefore, define and document an IN_BAND_WAKEUP
>>>> driver flag, to enable drivers to instruct bus types and PM domains about
>>>> this setting.
>>>>
>>>> Of course, in case the bus type and PM domain has additional information
>>>> about wakeup settings of the device, they may override the behaviour.
>>>>
>>>> Using the IN_BAND_WAKEUP driver flag for a device, may also affect how bus
>>>> types and PM domains should treat the device's parent during system
>>>> suspend. Therefore, in __device_suspend(), let the PM core propagate the
>>>> wakeup setting by using a status flag in the struct dev_pm_info for the
>>>> parent. This also makes it consistent with how the existing "wakeup_path"
>>>> status flag is being assigned.
>>>
>>> I've been thinking about this quite a bit recently and my conclusion is that
>>> the flag makes perfect sence (as it covers a valid use case), but I would
>>> define it and design the handling of it a bit differently.
>>
>> I'm also still thinking about this, cfr. my recent silence w.r.t.
>> these matters...
>>
>> On Renesas ARM SoCs (and at least some SH SoCs, but these are stuck in the
>> pre-DT legacy clock domain), a device needs to be kept running if it is a
>> wake-up source (e.g. WoL), or if it's part of the wake-up patch (e.g.
>> irqchip). So in the absence of the GENPD_FLAG_ACTIVE_WAKEUP flag in the PM
>> Domain driver, all individual drivers would need to set the IN_BAND_WAKEUP
>> flag (but see the exception below)
>
> Could you elaborate a bit about the wakeup-path? Let me start and you
> can fill in.
>
> In $subject patch, the PM core propagates the IN_BAND_WAKEUP flag of
> the device to its parent device, via the ->power.wakeup_path_in_band
> flag. That becomes additional new information, as we already has the
> PM core to propagate the value of device_may_wakeup() (if true) to the
> parent device, via the ->power.wakeup_path flag.
>
> I assume that isn't sufficient, because the wakeup path isn't
> reflected in the child/parent hierarchy?

That's correct. Interrupts are not part of the parent/child relationship.
Traditionally, irqchips were not platform devices, but now many irqchips
used on embedded platforms are.

> So, I guess this is about drivers who deals with wakeup enabled
> devices and which are consumers of other resources (irqchips for
> example). As part of the wakup path, these resources also needs to be
> kept running, right?

Correct.

> In your case, what other resources besides the irqchip, can be
> considered as a part of the wakeup path, but also being outside the
> parent/child hierarchy of the wakeup enabled device?

GPIOs used for wake-up? E.g. SDHI GPIO. We don't seem to have that
enabled for the Renesas SDHI drivers, though.

For gpio-keys, this is handled through enable_irq_wake(), so again it
uses an irqchip.

> Note that, an important fact as of today, is that when
> GENPD_FLAG_ACTIVE_WAKEUP is set for the genpd, genpd also checks the
> ->power.wakeup_path flag for the device. Both conditions must be true,
> else genpd powers off the PM domain (and the device).
>
> In other words, all devices being part of the wakeup path must either
> have device_may_wakeup() to return true for it or the
> ->power.wakeup_path set, else genpd will power off the PM domain,
> regardless of whether the GENPD_FLAG_ACTIVE_WAKEUP flag is set or not.

Correct.

>> Hence for this class of SoCs, this would duplicate the existing
>> dev->power.wakeup and dev->power.wakeup_path flags, so that's why I would
>> prefer using GENPD_FLAG_ACTIVE_WAKEUP instead (like we already do for the
>> older SH-Mobile SoCs, but not for newer R-Car and RZ series).
>> For other SoC families, the situation may be different.
>>
>> For us, the only exception are devices where the wakeup-source is not the
>> device itself, but a GPIO, e.g. SDHI card detect. In such a case, only the
>> device driver knows if the device is to be woken up through a dedicated CD
>> line, or through a GPIO CD. So that would call for a method to opt-out,
>> e.g. OUT_BAND_WAKEUP.
>
> The main reason to why I chosen the IN_BAND_WAKEUP method, is because
> genpd's *default* method is to power off the PM domain (and the
> device) even if wakeup is enabled for the device. Hence, genpd treats
> all wakeups as default being out-of-band IRQs.

OK.

> Having an OUT_BAND_WAKEUP flag, would thus only make sense for those
> genpds that has GENPD_FLAG_ACTIVE_WAKEUP set, I guess that is what you
> are saying as well?

Yes.

IMHO this is orthogonal to the device: the device driver knows if the
device itself is generating the wake-up event (e.g. Wake-on-LAN, dedicated
SDHI CD), or some other device is taking care (e.g. GPIO SDHI CD).
The device driver may not know if the device needs to be kept awake to
to handle wake-up events, that may be platform knowledge handled by genpd.

>> To complicate matters, some drivers may be used on SoCs where the device
>> needs to be kept running (clock and/or power domain), and on SoCs where the
>> device is always running. This difference is typically handled by genpd,
>> and the device driver may not even be aware. Of course the driver can just
>> set IN_BAND_WAKEUP regardless, (else it has to check for the presence of
>> clocks and/or power-domains properties itself, duplicating genpd
>> core/driver code).
>>
>> So what about
>>
>>          if (IN_BAND_WAKEUP ||
>>             (GENPD_FLAG_ACTIVE_WAKEUP && !OUT_BAND_WAKEUP)) {
>
> We don't want to suspend the device in case of IN_BAND_WAKEUP, right!?
>
>>                 ... suspend device...
>>         }

Oops, inverted logic. I should not write technical emails on Sunday morning.

Yes, the device must be kept awake if either IN_BAND_WAKEUP is set, or
if GENPD_FLAG_ACTIVE_WAKEUP is set but OUT_BAND_WAKEUP isn't.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds