linux-kernel - Re: [PATCH] mmc: meson-gx: remove IRQF

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Date:   Mon, 05 Oct 2020 10:55:54 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Ulf Hansson <ulf.hansson@...aro.org>
Cc:     Jerome Brunet <jbrunet@...libre.com>,
        Kevin Hilman <khilman@...libre.com>,
        "open list\:ARM\/Amlogic Meson..." 
        <linux-amlogic@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "linux-mmc\@vger.kernel.org" <linux-mmc@...r.kernel.org>,
        Brad Harper <bjharper@...il.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [PATCH] mmc: meson-gx: remove IRQF_ONESHOT

On Mon, Oct 05 2020 at 10:22, Ulf Hansson wrote:
> On Fri, 2 Oct 2020 at 18:49, Jerome Brunet <jbrunet@...libre.com> wrote:
>>
>> IRQF_ONESHOT was added to this driver to make sure the irq was not enabled
>> again until the thread part of the irq had finished doing its job.
>>
>> Doing so upsets RT because, under RT, the hardirq part of the irq handler
>> is not migrated to a thread if the irq is claimed with IRQF_ONESHOT.
>> In this case, it has been reported to eventually trigger a deadlock with
>> the led subsystem.
>>
>> Preventing RT from doing this migration was certainly not the intent, the
>> description of IRQF_ONESHOT does not really reflect this constraint:
>>
>>  > IRQF_ONESHOT - Interrupt is not reenabled after the hardirq handler finished.
>>  >              Used by threaded interrupts which need to keep the
>>  >              irq line disabled until the threaded handler has been run.
>>
>> This is exactly what this driver was trying to acheive so I'm still a bit
>> confused whether this is a driver or an RT issue.
>>
>> Anyway, this can be solved driver side by manually disabling the IRQs
>> instead of the relying on the IRQF_ONESHOT. IRQF_ONESHOT may then be removed
>> while still making sure the irq won't trigger until the threaded part of
>> the handler is done.
>
> Thomas, may I have your opinion on this one.
>
> I have no problem to apply $subject patch, but as Jerome also
> highlights above - this kind of makes me wonder if this is an RT
> issue, that perhaps deserves to be solved in a generic way.
>
> What do you think?

Let me stare at the core code. Something smells fishy.