linux-kernel - Re: [PATCH RESEND RFC 1/4] drivers: pinctrl: qcom: add wakeup capability to GPIO

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Date:   Thu, 2 Aug 2018 06:58:27 -0600
From:   Lina Iyer <ilina@...eaurora.org>
To:     Marc Zyngier <marc.zyngier@....com>
Cc:     swboyd@...omium.org, evgreen@...omium.org,
        linus.walleij@...aro.org, bjorn.andersson@...aro.org,
        rplsssn@...eaurora.org, linux-kernel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, rnayak@...eaurora.org,
        devicetree@...r.kernel.org
Subject: Re: [PATCH RESEND RFC 1/4] drivers: pinctrl: qcom: add wakeup
 capability to GPIO

On Thu, Aug 02 2018 at 01:27 -0600, Marc Zyngier wrote:
>On Thu, 02 Aug 2018 07:51:04 +0100,
>Lina Iyer <ilina@...eaurora.org> wrote:
>>
>> On Thu, Aug 02 2018 at 00:08 -0600, Marc Zyngier wrote:
>> > Hi Lina,
>> >
>> > On Wed, 01 Aug 2018 20:45:38 +0100,
>> > Lina Iyer <ilina@...eaurora.org> wrote:
>> >>
>> >> Thanks for the feedback, Marc.
>> >>
>> >> On Wed, Aug 01 2018 at 00:31 -0600, Marc Zyngier wrote:
>> >> > On Wed, 01 Aug 2018 03:00:18 +0100,
>> >> > Lina Iyer <ilina@...eaurora.org> wrote:
>> >> >>
>> >> >> +static irqreturn_t wake_irq_gpio_handler(int irq, void *data)
>> >> >> +{
>> >> >> +	struct irq_data *irqd = data;
>> >> >> +	struct irq_desc *desc = irq_data_to_desc(irqd);
>> >> >> +	struct irq_chip *chip = irq_desc_get_chip(desc);
>> >> >> +	struct gpio_chip *gc = irq_data_get_irq_chip_data(irqd);
>> >> >> +	int irq_pin = irq_find_mapping(gc->irq.domain, irqd->hwirq);
>> >> >> +
>> >> >> +	chained_irq_enter(chip, desc);
>> >> >> +	generic_handle_irq(irq_pin);
>> >> >> +	chained_irq_exit(chip, desc);
>> >> >
>> >> > That's crazy. I'm not even commenting on the irq handler vs chained
>> >> > irqchip thing, but directly calling into a completely different part
>> >> > of the irq hierarchy makes me feel nauseous,
>> >> >
>> >> I know the sentiment; I am not too happy about it myself. Explanation
>> >> below.
>> >>
>> >> > Why isn't the interrupt still pending at the pinctrl level? Looking at
>> >> > the diagram in the cover letter, I'd have hoped that the signal routed
>> >> > to the PDC would wakeup the GIC, but that by virtue of being *also*
>> >> > wired to the TLMM, the interrupt would be handled via the normal path.
>> >> >
>> >> The short answer: TLMM is not active to sense a wakeup interrupt.
>> >
>> > I get that bit. See below for the bit I don't get.
>> >
>> >> When we enter system sleep mode, the TLMM and the GIC are powered off
>> >> and the PDC is the only powered-on interrupt controller. The IRQs
>> >> configured at the PDC are the only ones capable of waking the system.
>> >> Upon sensing the interrupt, the PDC intiates a system wakeup and replays
>> >> the interrupt to the GIC. The GIC relays that to AP. Unfortunately, the
>> >> interrupts from the GPIO do not trigger the TLMM summary line. Therefore
>> >> this handler needs to figure out what GPIO caused the wakeup and notify
>> >> the corresponding driver.
>> >
>> > That's most bizarre. What causes the TLMM output line not to reflect
>> > the fact that an input is asserted?
>> There is a parallel line that is directed from the PIN directly to the
>> PDC in addition to the TLMM. This line does not go through the TLMM.
>
>I got that from the cover letter.
>
>>
>> Active path						_____
>>                 /-------------- > [ TLMM ] --------> |     |
>>            [ Device GPIO ]	   	     summary   | GIC | ==>
>> 		\				       |     |
>> 		  ----------------> [ PDC ] ---------> |_____|
>> Wakeup path	     GPIO as IRQ		IRQ
>>
>> > I understand that it has been
>> > turned off, but surely the PDC wakes it up at the same time as the
>> > GIC, and it should realise that there is something pending...
>> >
>> PDC is always-on and when it detects any interrupt, will wakeup the GIC
>> and then replay the interrupt line to the GIC. This interrupt line is
>> not the summary line, but a separate line from GPIO/PIN to the PDC.
>>
>> Since the TLMM was also in low power mode, when the GIC was powered
>> down, the TLMM never sees the IRQ and therefore will not send the
>> summary line to GIC. The wakeup path is GPIO -> PDC -> GIC.
>
>Sure. But once woken up (GIC *and* TLMM), the gpio line (which I
>assume is level) is still high at the TLMM input. So why isn't it
>registering that state once it has been woken up?
>
>I can understand that it would be missing an edge. But that doesn't
>hold for level signalling.
>
Sure, yes. Sorry for not registering your point in my response.
Once woken up we should see the level interrupt in TLMM.

-- Lina

>>
>> >> > Why isn't that the case? And if that's because the HW is broken and
>> >> > doesn't buffer edge interrupts, why can't you use the resend mechanism
>> >> > instead?
>> >> >
>> >> The PDC hardware can replay the interrupts accurately. However, it will
>> >> replay only the pin and its not the TLMM summary IRQ. The handler here,
>> >> needs to notify the driver that the wakeup interrupt happened and needs
>> >> to take action. If I could trip the summary IRQ in this handler that
>> >> would work too. Can it be done?
>> >
>> > So the TLMM has indeed noticed the interrupt and you can read the TLMM
>> > status registers to find out what fired?
>> I think that's where it is probably confusing. The TLMM will not see the
>> interrupt because it was in low power mode.
>
>See above. I can buy that for edge, but not level.
>
>Thanks,
>
>	M.
>
>--
>Jazz is not dead, it just smell funny.