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

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Date:   Mon, 05 Oct 2020 14:31:06 +0200
From:   Jerome Brunet <jbrunet@...libre.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ulf Hansson <ulf.hansson@...aro.org>
Cc:     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 05 Oct 2020 at 10:55, Thomas Gleixner <tglx@...utronix.de> wrote:

> 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.

FYI: initial discussion can be found here
https://lore.kernel.org/linux-amlogic/24a844c3-c2e0-c735-ccb7-83736218b548@gmail.com/