linux-kernel - Re: [PATCH RFC 02/14] drivers: irqchip: pdc: Do not toggle IRQ

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Date:   Wed, 11 Sep 2019 10:15:57 -0600
From:   Lina Iyer <ilina@...eaurora.org>
To:     Stephen Boyd <swboyd@...omium.org>
Cc:     evgreen@...omium.org, linus.walleij@...aro.org,
        marc.zyngier@....com, linux-kernel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, bjorn.andersson@...aro.org,
        mkshah@...eaurora.org, linux-gpio@...r.kernel.org,
        rnayak@...eaurora.org
Subject: Re: [PATCH RFC 02/14] drivers: irqchip: pdc: Do not toggle
 IRQ_ENABLE during mask/unmask

On Thu, Sep 05 2019 at 18:39 -0600, Stephen Boyd wrote:
>Quoting Lina Iyer (2019-08-29 11:11:51)
>> When an interrupt is to be serviced, the convention is to mask the
>> interrupt at the chip and unmask after servicing the interrupt. Enabling
>> and disabling the interrupt at the PDC irqchip causes an interrupt storm
>> due to the way dual edge interrupts are handled in hardware.
>>
>> Skip configuring the PDC when the IRQ is masked and unmasked, instead
>> use the irq_enable/irq_disable callbacks to toggle the IRQ_ENABLE
>> register at the PDC. The PDC's IRQ_ENABLE register is only used during
>> the monitoring mode when the system is asleep and is not needed for
>> active mode detection.
>
>I think this is saying that we want to always let the line be sent
>through the PDC to the parent irqchip, in this case GIC, so that we
>don't get an interrupt storm for dual edge interrupts? Why does dual
>edge interrupts cause a problem?
>
I am not sure about the hardware details, but the PDC designers did not
expect enable and disable to be called whenever the interrupt is
handled. This specially becomes a problem for dual edge interrupts which
seems to generate a interrupt storm when enabled/disabled while handling
the interrupt.

--Lina