linux-kernel - Re: [PATCH 2/7] gpio: brcmstb: release the bgpio lock during irq handlers

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Date:   Tue, 3 Oct 2017 20:07:32 -0700
From:   Gregory Fong <gregory.0xf0@...il.com>
To:     Doug Berger <opendmb@...il.com>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Brian Norris <computersforpeace@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        bcm-kernel-feedback-list <bcm-kernel-feedback-list@...adcom.com>,
        linux-gpio@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 2/7] gpio: brcmstb: release the bgpio lock during irq handlers

On Tue, Oct 3, 2017 at 7:09 PM, Doug Berger <opendmb@...il.com> wrote:
> On 10/03/2017 06:55 PM, Gregory Fong wrote:
>> Hi Doug,
>>
>> On Fri, Sep 29, 2017 at 8:40 PM, Doug Berger <opendmb@...il.com> wrote:
>>> The basic memory-mapped GPIO controller lock must be released
>>> before calling the registered GPIO interrupt handlers to allow
>>> the interrupt handlers to access the hardware.  Otherwise, the
>>> hardware accesses will deadlock when they attempt to grab the
>>> lock.
>>
>> I was having some trouble understanding exactly what the problem was
>> here, but I think I see it now.  Since this locks the entire bank,
>> where some GPIOs might be set as inputs and some as inputs (and
>> interrupt sources), then an interrupt on a GPIO that is supposed to
>> set another GPIO in the bank would result in deadlock.  Is that
>> correct?  If so, please update the commit message to make that clear,
>> and nice fix.  If not that, it would be nice to know what scenario can
>> cause a problem.
>
> That is an example, but there are really many possibilities.
>
> Basically, if a registered interrupt handler wants to access its GPIO
> you are likely to run into trouble.  Another example might be an
> interrupt that is configured to trigger on either edge transition and
> the handler wants to know whether the input is currently high or low.
>
> I can submit a V2 with a change in the description if you would like,
> but I'm not sure what the clearest example would be.

If you could just mention both of these possible cases, that would be
great! With that change,

Acked-by: Gregory Fong <gregory.0xf0@...il.com>