linux-kernel - Re: [PATCH v2 7/9] gpio: 104-idi-48: make use of raw

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Date:   Tue, 28 Mar 2017 07:40:26 -0400
From:   William Breathitt Gray <vilhelm.gray@...il.com>
To:     Linus Walleij <linus.walleij@...aro.org>
Cc:     Julia Cartwright <julia@...com>,
        Alexandre Courbot <gnurou@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-rt-users@...r.kernel.org,
        "linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>
Subject: Re: [PATCH v2 7/9] gpio: 104-idi-48: make use of raw_spinlock
 variants

On Tue, Mar 28, 2017 at 11:11:59AM +0200, Linus Walleij wrote:
>On Wed, Mar 22, 2017 at 1:44 PM, William Breathitt Gray
><vilhelm.gray@...il.com> wrote:
>> On Tue, Mar 21, 2017 at 05:43:07PM -0500, Julia Cartwright wrote:
>>>The 104-idi-48 gpio driver currently implements an irq_chip for handling
>>>interrupts; due to how irq_chip handling is done, it's necessary for the
>>>irq_chip methods to be invoked from hardirq context, even on a a
>>>real-time kernel.  Because the spinlock_t type becomes a "sleeping"
>>>spinlock w/ RT kernels, it is not suitable to be used with irq_chips.
>>>
>>>A quick audit of the operations under the lock reveal that they do only
>>>minimal, bounded work, and are therefore safe to do under a raw spinlock.
>>>
>>>Signed-off-by: Julia Cartwright <julia@...com>
>>
>> Hi Julia,
>>
>> This driver also uses a second spinlock_t, called ack_lock, to prevent
>> reentrance into the idi_48_irq_handler function. Should ack_lock also be
>> implemented as a raw_spinlock_t?
>
>Hm, can I apply this one patch or not?
>
>Linus Walleij

Oops, sorry for missing this reply. Julia is correct that ack_lock does
not need to be implemented as raw_spinlock_t. For reference, ack_lock is
used to prevent a race condition on the device hardware itself related
to how the 104-IDI-48 acknowledges IRQ (check out the commit description
for it for a more in-depth explanation if you're curious).

Long story short: Julia's patch is prefectly acceptable as is.

Acked-by: William Breathitt Gray <vilhelm.gray@...il.com>

William Breathitt Gray