linux-kernel - Re: [PATCH v3 1/3] pinctrl: msm: Really mask level interrupts to prevent latching

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Date:   Thu, 23 Aug 2018 09:57:18 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Doug Anderson <dianders@...omium.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>
Cc:     Stephen Boyd <swboyd@...omium.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH v3 1/3] pinctrl: msm: Really mask level interrupts to
 prevent latching

On Thu, Aug 16, 2018 at 10:49 PM Doug Anderson <dianders@...omium.org> wrote:
> On Thu, Aug 16, 2018 at 1:06 PM, Stephen Boyd <swboyd@...omium.org> wrote:
> > The interrupt controller hardware in this pin controller has two status
> > enable bits. The first "normal" status enable bit enables or disables
> > the summary interrupt line being raised when a gpio interrupt triggers
> > and the "raw" status enable bit allows or prevents the hardware from
> > latching an interrupt into the status register for a gpio interrupt.
> > Currently we just toggle the "normal" status enable bit in the mask and
> > unmask ops so that the summary irq interrupt going to the CPU's
> > interrupt controller doesn't trigger for the masked gpio interrupt.
> >
> > For a level triggered interrupt, the flow would be as follows: the pin
> > controller sees the interrupt, latches the status into the status
> > register, raises the summary irq to the CPU, summary irq handler runs
> > and calls handle_level_irq(), handle_level_irq() masks and acks the gpio
> > interrupt, the interrupt handler runs, and finally unmask the interrupt.
> > When the interrupt handler completes, we expect that the interrupt line
> > level will go back to the deasserted state so the genirq code can unmask
> > the interrupt without it triggering again.
> >
> > If we only mask the interrupt by clearing the "normal" status enable bit
> > then we'll ack the interrupt but it will continue to show up as pending
> > in the status register because the raw status bit is enabled, the
> > hardware hasn't deasserted the line, and thus the asserted state latches
> > into the status register again. When the hardware deasserts the
> > interrupt the pin controller still thinks there is a pending unserviced
> > level interrupt because it latched it earlier. This behavior causes
> > software to see an extra interrupt for level type interrupts each time
> > the interrupt is handled.
> >
> > Let's fix this by clearing the raw status enable bit for level type
> > interrupts so that the hardware stops latching the status of the
> > interrupt after we ack it. We don't do this for edge type interrupts
> > because it seems that toggling the raw status enable bit for edge type
> > interrupts causes spurious edge interrupts.
> >
> > Cc: Bjorn Andersson <bjorn.andersson@...aro.org>
> > Cc: Doug Anderson <dianders@...omium.org>
> > Signed-off-by: Stephen Boyd <swboyd@...omium.org>
> > ---
> >  drivers/pinctrl/qcom/pinctrl-msm.c | 24 ++++++++++++++++++++++++
> >  1 file changed, 24 insertions(+)
>
> Reviewed-by: Douglas Anderson <dianders@...omium.org>
>
> NOTE: IMO we should land this fix even if we continue to have debate
> on patch #2 and #3 since this fixes a definite problem.

OK makes sense, I guess I'll queue this for fixes
once v4.19-rc1 is out.

Would be nice to also get Bjorn's buy-in on it!

Yours,
Linus Walleij