Message-ID: <YMXdXY+R4e1m3nbx@latitude>
Date:   Sun, 13 Jun 2021 12:26:37 +0200
From:   Jonathan Neuschäfer <j.neuschaefer@....net>
To:     Linus Walleij <linus.walleij@...aro.org>
Cc:     Jonathan Neuschäfer <j.neuschaefer@....net>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, Rob Herring <robh+dt@...nel.org>,
        OpenBMC Maillist <openbmc@...ts.ozlabs.org>,
        Tomer Maimon <tmaimon77@...il.com>,
        Joel Stanley <joel@....id.au>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/8] pinctrl: nuvoton: Add driver for WPCM450

On Fri, Jun 04, 2021 at 11:31:07AM +0200, Linus Walleij wrote:
> Hi Jonathan!
> 
> thanks for your patch!
> 
> On Wed, Jun 2, 2021 at 2:04 PM Jonathan Neuschäfer
> <j.neuschaefer@....net> wrote:
> >
> > This driver is heavily based on the one for NPCM7xx, because the WPCM450
> > is a predecessor of those SoCs.
> >
> > The biggest difference is in how the GPIO controller works. In the
> > WPCM450, the GPIO registers are not organized in multiple banks, but
> > rather placed continually into the same register block, and the driver
> > reflects this.
> 
> This is unfortunate because now you can't use GPIO_GENERIC anymore.
> 
> > Some functionality implemented in the hardware was (for now) left unused
> > in the driver, specifically blinking and pull-up/down.
> >
> > Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@....net>
> 
> (...)
> 
> > +config PINCTRL_WPCM450
> > +       bool "Pinctrl and GPIO driver for Nuvoton WPCM450"
> > +       depends on (ARCH_WPCM450 || COMPILE_TEST) && OF
> > +       select PINMUX
> > +       select PINCONF
> > +       select GENERIC_PINCONF
> > +       select GPIOLIB
> > +       select GPIO_GENERIC
> 
> You are not using GPIO_GENERIC

I'll remove the this line (depending on the outcome of the rest of the
discussion).

> 
> > +struct wpcm450_port {
> > +       /* Range of GPIOs in this port */
> > +       u8 base;
> > +       u8 length;
> > +
> > +       /* Register offsets (0 = register doesn't exist in this port) */
> > +       u8 cfg0, cfg1, cfg2;
> > +       u8 blink;
> > +       u8 dataout, datain;
> > +};
> 
> If you used to have "GPIO banks" and you now have
> "GPIO ports" what is the difference? Why can't these ports
> just be individula gpio_chip:s with their own device tree
> nodes inside the pin controller node?

The naming difference is a fairly arbitrary choice by me.

The real difference is in how the GPIO registers are laid out.
On NPCM7xx, there are blocks of registers at +0, +0x1000, +0x2000,
etc., and within a block, the registers have the same offsets.
On WPCM450, the registers are all mushed together as tightly as
possible[1], so that (a) the ports/banks don't start at nice addresses,
and (b) the register layout can't be predicted from the offset of the
first register in a port (because not all ports have all registers).

> If you split it up then you can go back to using
> GPIO_GENERIC with bgpio_init() again which is a
> big win.
> 
> All you seem to be doing is setting consecutive
> bits in a register by offset, which is what GPIO_GENERIC
> is for, just that it assumes offset is always from zero.
> If you split it into individual gpio_chips per register
> then you get this nice separation and code reuse.

Indeed, if I keep the wpcm450_ports table but use it to call bgpio_init()
with the right register addresses, I think bgpio_init() can work.


Thanks,
Jonathan Neuschäfer


[1]: https://github.com/neuschaefer/wpcm450/wiki/GPIOs-and-pinmux#gpio

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