linux-kernel - Re: [PATCH v2 05/11] pinctrl: mediatek: avoid virtual gpio trying to set reg

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Date:   Tue, 7 Jan 2020 11:20:10 +0100
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Hanks Chen <hanks.chen@...iatek.com>
Cc:     Mars Cheng <mars.cheng@...iatek.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Rob Herring <robh@...nel.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Stephen Boyd <sboyd@...nel.org>,
        Sean Wang <sean.wang@...nel.org>,
        CC Hwang <cc.hwang@...iatek.com>,
        Loda Chou <loda.chou@...iatek.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "moderated list:ARM/Mediatek SoC support" 
        <linux-mediatek@...ts.infradead.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, wsd_upstream@...iatek.com,
        mtk01761 <wendell.lin@...iatek.com>,
        linux-clk <linux-clk@...r.kernel.org>
Subject: Re: [PATCH v2 05/11] pinctrl: mediatek: avoid virtual gpio trying to
 set reg

On Mon, Dec 23, 2019 at 4:11 AM Hanks Chen <hanks.chen@...iatek.com> wrote:
> On Fri, 2019-08-23 at 10:57 +0200, Linus Walleij wrote:
> > On Mon, Aug 19, 2019 at 11:22 AM Mars Cheng <mars.cheng@...iatek.com> wrote:

> > This does not explain what a "virtual GPIO" is in this
> > context, so please elaborate. What is this? Why does
> > it exist? What is it used for?
> >
> > GPIO is "general purpose input/output" and it is a
> > pretty rubbery category already as it is, so we need
> > to define our terms pretty strictly.
> >
> Virtual GPIO only used inside SOC and not being exported to outside SOC
> in MTK platform. Some modules use virtual GPIO as eint (e.g. pmic or
> usb).

I would call that internal GPIOs, those are very real rails inside
the chip made with polysilicone so there is nothing "virtual"
about them. If the documentation for the chip calls them virtual
then explain in the driver that these are SoC-internal
lines so that everyone will get it.

Is the PMIC inside the SoC? I thought that was usually outside of it
in its own chip.

But I suppose there could be some interface to it in the SoC and
then that interface has this EINT?

> In MTK platform, external interrupt (EINT) and GPIO is 1-1 mapping and
> we can set GPIO as eint.
> But some modules use specific eint which doesn't have real GPIO pin.
> So we use virtual GPIO to map it.

OK I get it I think... just put these comments into the code as well
so we understand when reading the code what is going on.

> > > +       if (mtk_is_virt_gpio(hw, gpio))
> > > +               return 1;
> >
> > Why are "virtual GPIOs" always inputs?
>
> We set virtual GPIO as eint.
> It mean virtual GPIO only used inside SOC and not being exported to
> outside SOC.

Are you saying that:
- "Virtual" GPIOs are always and only used for interrupts
- Since they are only used for interrupts, they are always inputs

Then write that in a comment to the above change so we know
this context.

Yours,
Linus Walleij