linux-kernel - Re: [PATCH v3 3/5] gpio: gpiolib: Allow free() callback to be overridden

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Date:   Thu, 12 May 2022 18:55:38 +0100
From:   "Lad, Prabhakar" <prabhakar.csengg@...il.com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Linus Walleij <linus.walleij@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Bartosz Golaszewski <brgl@...ev.pl>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        Phil Edworthy <phil.edworthy@...esas.com>,
        Biju Das <biju.das.jz@...renesas.com>
Subject: Re: [PATCH v3 3/5] gpio: gpiolib: Allow free() callback to be overridden

Hi Marc,

On Thu, May 12, 2022 at 5:26 PM Marc Zyngier <maz@...nel.org> wrote:
>
> On Thu, 12 May 2022 14:50:05 +0100,
> "Lad, Prabhakar" <prabhakar.csengg@...il.com> wrote:
> >
> > Hi Marc,
> >
> > On Thu, May 12, 2022 at 2:24 PM Marc Zyngier <maz@...nel.org> wrote:
> > >
> > > On Thu, 12 May 2022 13:48:53 +0100,
> > > "Lad, Prabhakar" <prabhakar.csengg@...il.com> wrote:
> > > >
> > > > Hi Marc,
> > > >
> > > > Thank you for the review.
> > > >
> > > > On Thu, May 12, 2022 at 12:19 PM Marc Zyngier <maz@...nel.org> wrote:
> > > > >
> > > > > On Wed, 11 May 2022 19:32:08 +0100,
> > > > > Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com> wrote:
> > > > > >
> > > > > > Allow free() callback to be overridden from irq_domain_ops for
> > > > > > hierarchical chips.
> > > > > >
> > > > > > This allows drivers to free any resources which are allocated during
> > > > > > populate_parent_alloc_arg().
> > > > >
> > > > > Do you mean more than the fwspec? I don't see this being used.
> > > > >
> > > > The free callback is used in patch 5/5 where free is overridden by
> > > > rzg2l_gpio_irq_domain_free. I just gave an example there as an
> > > > populate_parent_alloc_arg()  In actual in the child_to_parent_hwirq
> > > > callback I am using a bitmap [0] to get a free tint slot, this bitmap
> > > > needs freeing up when the GPIO interrupt is released from the driver
> > > > that as when overridden free callback frees the allocated tint slot so
> > > > that its available for re-use.
> > >
> > > Right, so that's actually a different life-cycle, and the whole
> > > populate_parent_alloc_arg() is a red herring. What you want is to free
> > > resources that have been allocated via some other paths. It'd be good
> > Is there any other path which I have missed where I can free up resources?
>
> No, that's the only one. It is just that usually, the alloc()
> callback is where you are supposed to perform... allocations.
>
OK.

> It'd be good if you could move your allocation there, as I would
> expect calls to child_to_parent_hwirq() to be idempotent.
>
For now I'll go with the current implementation, as currently a an
array is maintained which is tied with the tint slot and child (which
is obtained from child_to_parent_hwirq)

> >
> > > if your commit message actually reflected this instead of using an
> > > example that doesn't actually exist.
> > >
> > My bad, I will update the commit message.
> >
> > > >
> > > > > There is also the question of why we need to have dynamic allocation
> > > > > for the fwspec itself. Why isn't that a simple stack allocation in the
> > > > > context of gpiochip_hierarchy_irq_domain_alloc()?
> > > > >
> > > > you mean gpio core itself should handle the fwspec
> > > > allocation/freeing?
> > >
> > > Yes. The only reason we resort to dynamic allocation is because
> > > ThunderX is using MSI-based GPIOs, and thus doesn't use a fwspec (no
> > > firmware is involved here).
> > >
> > I see..
> >
> > > If we had a union of the two types, we could just have a stack
> > > variable, and pass that along, completely sidestepping the whole
> > > dynamic allocation/freeing business.
> > >
> > Right agreed.
>
> FWIW, I've just posted a PoC patch[1].
>
I guess I'll have to rebase my changes on top of it now ;)

Cheers,
Prabhakar