linux-kernel - Re: [PATCH v5 1/2] usb: host: plat: Enable xhci plat runtime PM

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Date:   Wed, 22 Mar 2017 14:43:36 +0200
From:   Mathias Nyman <mathias.nyman@...ux.intel.com>
To:     Baolin Wang <baolin.wang@...aro.org>,
        Felipe Balbi <balbi@...nel.org>
Cc:     Mathias Nyman <mathias.nyman@...el.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Mark Brown <broonie@...nel.org>,
        USB <linux-usb@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Alan Stern <stern@...land.harvard.edu>
Subject: Re: [PATCH v5 1/2] usb: host: plat: Enable xhci plat runtime PM

On 22.03.2017 12:40, Baolin Wang wrote:
> Hi,
>
> On 22 March 2017 at 17:00, Felipe Balbi <balbi@...nel.org> wrote:
>>
>> Hi,
>>
>> Baolin Wang <baolin.wang@...aro.org> writes:
>>>>>>> I don't yet understand why we can't just keep runtime pm disabled as a
>>>>>>> default for xhci platform devices.
>>>>>>> It could be enabled by whatever creates the platform device by setting some
>>>>>>> device property
>>>>>>> (or equivalent), which would be checked in xhci_plat_probe() before enabling
>>>>>>> runtime pm. It
>>>>>>> could then optionally be set in sysfs via power/control entry.
>>>>>>
>>>>>> I think runtime pm is not one hardware property, is it suitable if we
>>>>>> introduce one device property to enable/disable runtime pm?
>>>>
>>>> we already this functionality exposed on sysfs.
>>>
>>>  From my understanding, Mathias suggested me to add one device property
>>> (name like "usb-host-runtimePM") by platform_device_add_properties()
>>> to enable/disable runtime PM when creating platform device, like
>>> usb3_lpm_capable:
>>>

It was more of generic pondering how to automatically enable runtime PM for platforms
that know their xhci can runtime suspend/resume. But all this can be skipped for now and just
force users to manually enable xhci platform runtime pm in sysfs for now.

For me, and for xhci point of view only, patch 1/2 is quite ok.
Only some minor adjustments like calling runtime_get in the beginning of probe, and runtime_put
at the end end after both hcd's are added.

Probe could additionally call pm_runtime_forbid() to prevent runtime pm frpm being on as
default, (increases usage count, modifies runtime_auto)

This would force the user to explicitly enable runtime pm usin power/control in sysfs.

In my opinion that patch could be a separate one, and how dwc3 deals with it can be a separate
topic.

>>
>> yeah, that's silly. We already have means for doing that:
>>
>>          my_probe()
>>          {
>>                  [...]
>>
>>                  pm_runtime_enable(dev);
>>                  pm_runtime_forbid(dev);
>
> That's same with getting the usage counter.
>
>>
>>                  [...]
>>
>>                  return 0;
>>          }
>>
>>>>> Secondly, we only can resume the xhci platform device by getting the
>>>>> xhci usage counter from gadget driver, since the cable plug in/out
>>>>> events only can be notified to glue layer of gadget driver(like dwc3
>>>>> glue layer). That means if we want to suspend xhci platform device, we
>>>>
>>>> this is a problem with the glue layer, IMO. It should be something like
>>>> so:
>>>>
>>>> static irqreturn_t dwc3_foobar_wakeup(int irq, void *_glue)
>>>> {
>>>>          struct dwc3_foobar_glue *glue = _glue;
>>>>          u32 reg;
>>>>
>>>>          reg = real(glue->base, OFFSET);
>>>>          if (reg & CONNECT)
>>>>                  pm_runtime_resume(&glue->dwc3);
>>>>
>>>>          return IRQ_HANDLED;
>>>> }
>>>>
>>>> then dwc3's ->runtime_resume() should check if the event is supposed to
>>>> be handled by host or peripheral by checking which mode it was before
>>>> suspend and making the assumption that we don't change modes while
>>>> suspended. Something like:
>>>>
>>>> static int dwc3_runtime_resume(struct device *dev)
>>>> {
>>>>          struct dwc3 *dwc = dev_get_drvdata(dev);
>>>>
>>>>          [...]
>>>>
>>>>          if (dwc->is_host)
>>>>                  pm_runtime_resume(dwc->xhci.dev);
>>>>
>>>>          [...]
>>>>
>>>>          return 0;
>>>> }
>>>
>>> Yeah, if we don't need to get xhci usage counter in xhci_plat_probe(
>>> to avoid affecting other controller's runtime PM, we can do like this
>>> and do not need to get/put counter.
>>
>> why do you need to get xhci's usage counter in xhci_plat_probe() ?
>>
>> And why would one xhci affect the other? They are different struct
>> device instances and, thus, have different pm usage counter. How would
>> one xhci's pm_runtime affect another?
>
> What I mean is if another USB controller's driver did not implement
> runtime pm callbacks but system enables CONFIG_PM, that will make xhci
> device auto-suspended when after probing xhci-plat if we did not get
> xhci device usage counter, but gadget driver can not resume xhci
> without implementing runtime PM callbacks.
>
> If we want to implement xhci-plat runtime resume/suspend without
> getting usage counter, we should assume every driver using xhci-plat
> should implement their runtime PM callbacks. Is this right?

That is my understanding as well, Basically the xhci parents driver
should end up calling  xhci_plat_runtime_resume() one way or another.
Just like Felipe's glue driver fix above does.

So lets keep pm_runtime_forbid() as default for xhci-plat for now.

-Mathias