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Date:   Thu, 2 Aug 2018 00:51:04 -0600
From:   Lina Iyer <ilina@codeaurora.org>
To:     Marc Zyngier <marc.zyngier@arm.com>
Cc:     swboyd@chromium.org, evgreen@chromium.org,
        linus.walleij@linaro.org, bjorn.andersson@linaro.org,
        rplsssn@codeaurora.org, linux-kernel@vger.kernel.org,
        linux-arm-msm@vger.kernel.org, rnayak@codeaurora.org,
        devicetree@vger.kernel.org
Subject: Re: [PATCH RESEND RFC 1/4] drivers: pinctrl: qcom: add wakeup
 capability to GPIO
Message-ID: <20180802065104.GA27850@codeaurora.org>
References: <20180801020021.9782-1-ilina@codeaurora.org>
 <20180801020021.9782-2-ilina@codeaurora.org>
 <86600uy4vh.wl-marc.zyngier@arm.com>
 <20180801194538.GA6422@codeaurora.org>
 <86wot9wb9u.wl-marc.zyngier@arm.com>
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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@codeaurora.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@codeaurora.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.

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.

>> > 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.

>The question remains: why
>isn't it generating any output if it knows something as fired? The
>output line is level, right? What you describe would make sense if it
>was generating an edge, but that'd really be a terrible design...
>

>As for tripping the summary interrupt, see check_irq_resend(). This
>will only work if this summary interrupt is edge.
>
Will explore this.

Thanks Marc.

-- Lina