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From:   Stephen Boyd <swboyd@chromium.org>
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Date:   Fri, 21 May 2021 12:18:33 -0700
Message-ID: <CAE-0n50BOV6UofBzqqb+KzcOR7W=h3VD2g4CzeqB6+a0v-aZUQ@mail.gmail.com>
Subject: Re: [PATCH v4 1/2] drm/msm/dp: handle irq_hpd with sink_count = 0 correctly
To:     khsieh@codeaurora.org
Cc:     agross@kernel.org, bjorn.andersson@linaro.org, robdclark@gmail.com,
        sean@poorly.run, vkoul@kernel.org, abhinavk@codeaurora.org,
        aravindh@codeaurora.org, linux-arm-msm@vger.kernel.org,
        linux-kernel@vger.kernel.org
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Quoting khsieh@codeaurora.org (2021-05-21 08:21:58)
> >
> > Ok. So you're saying that we want to put both events on the queue
> > regardless, and put IRQ_HPD there first because we want to check the
> > status bit? Doesn't reading the status bit require the dongle to be
> > connected though? So if an unplug came in along with an irq_hpd we may
> > queue both the irq_hpd and the unplug, but when it comes time to
> > process
> > the irq_hpd in the kthread the link will be gone and so trying the dpcd
> > read for the link status will fail?
> >
> yes,
> we had a previous bug with this scenarios already.
> https://partnerissuetracker.corp.google.com/issues/170598152
> At this case, dongle produce two interrupts, irq_hpd followed by unplug
> immediately (not presented at isr status register at same time), at the
> time dongle unplugged form DTU.
> But due to dp ctrl reset at handling irq_hpd which cause unplug mask bit
> be cleared so that unplug interrupt got lost.
>

Again, wouldn't that be too late if the hardirq handler is delayed to
the point that the two irqs are pending in the isr status register?