linux-kernel - Re: [RFC PATCH 0/6] leds: Fix pca955x GPIO pin mappings

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Message-ID: <CAHp75Vc2W+WmwNj1AvH6EiT_80c+5gADV9QzK+asHxpd1Ucppw@mail.gmail.com>
Date:   Wed, 28 Jul 2021 12:13:06 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Andrew Jeffery <andrew@...id.au>
Cc:     "linux-leds@...r.kernel.org" <linux-leds@...r.kernel.org>,
        "linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
        Cédric Le Goater <clg@...d.org>,
        Rob Herring <robh+dt@...nel.org>,
        Joel Stanley <joel@....id.au>, Pavel Machek <pavel@....cz>,
        Linus Walleij <linus.walleij@...aro.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-aspeed@...ts.ozlabs.org" <linux-aspeed@...ts.ozlabs.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 0/6] leds: Fix pca955x GPIO pin mappings

On Wed, Jul 28, 2021 at 8:43 AM Andrew Jeffery <andrew@...id.au> wrote:
> On Fri, 23 Jul 2021, at 17:45, Andy Shevchenko wrote:
> > On Friday, July 23, 2021, Andrew Jeffery <andrew@...id.au> wrote:

> > > This series does a bunch of crimes, so it's an RFC. I'm cross-posting to the
> > > pinctrl/GPIO and LEDs lists because the PCA955x devices impact all of them. What
> > > needs fixing is the leds-pca955x driver's failure to map the GPIO numberspace to
> > > the pin numberspace of the PCA955x devices. The series solves that by
> > > implementing pinctrl and pinmux in the leds-pca955x driver.
> > >
> > > Things I'm unsure about:
> > >
> > > 1. Patch 1: The pinctrl_gpio_as_pin() API feels a bit dirty, not sure what
> > >    others thoughts are on that (Linus?).
> > >
> > > 2. Patch 2: I've added a new callback to hook the entirety of the pinctrl map
> > >    parsing rather than supplying a subnode-specific callback. This was necessary
> > >    to handle the PCA955x devicetree binding in a backwards compatible way.
> > >
> > > 3. Patch 4: The PCA955x devices don't actually have any pinmux hardware, but the
> > >    properties of the pinctrl/pinmux subsystems in the kernel map nicely onto the
> > >    problem we have. But it's quite a bit of code...
> > >
> > > 4. Patch 6: I also lost a bunch of time to overlooking the get_group_pins()
> > >    callback for pinctrl, and it seems odd to me that it isn't required.
> > >
> > > Please review!
> >
> >
> > Sounds like a hack.
>
> Yes, possibly. Feedback like this is why I sent the series as an RFC.
>
> > I was briefly looking into patches 1-4 and suddenly
> > realized that the fix can be similar as in PCA9685 (PWM), I.e. we
> > always have chips for the entire pin space and one may map them
> > accordingly, requested in one realm (LED) in the other (GPIO)
> > automatically is BUSY. Or I missed the point?
>
> No, you haven't missed the point. I will look at the PCA9685 driver.
>
> That said, my goal was to implement the behaviour intended by the
> existing binding (i.e. fix a bug).

Okay, so it implies that this used to work at some point. What has
changed from that point? Why can't we simply fix the culprit commit?

> However, userspace would never have
> got the results it expected with the existing driver implementation, so
> I guess you could argue that no such (useful) userspace exists. Given
> that, we could adopt the strategy of always defining a gpiochip
> covering the whole pin space, and parts of the devicetree binding just
> become redundant.

I'm lost now. GPIO has its own userspace ABI, how does it work right
now in application to this chip?

-- 
With Best Regards,
Andy Shevchenko