linux-kernel - Re: [RFC PATH 2/2] gpio: starfive-jh7100: Add StarFive JH7100 GPIO driver

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Message-ID: <a15823a88515f944cad6d77bdd65555c@walle.cc>
Date:   Wed, 28 Jul 2021 13:19:14 +0200
From:   Michael Walle <michael@...le.cc>
To:     Emil Renner Berthing <kernel@...il.dk>
Cc:     Drew Fustini <drew@...gleboard.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Michael Zhu <michael.zhu@...rfivetech.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Fu Wei <tekkamanninja@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        Huan Feng <huan.feng@...rfivetech.com>
Subject: Re: [RFC PATH 2/2] gpio: starfive-jh7100: Add StarFive JH7100 GPIO
 driver

Am 2021-07-28 12:59, schrieb Emil Renner Berthing:
> On Wed, 28 Jul 2021 at 11:49, Michael Walle <michael@...le.cc> wrote:
>> Hi Drew,
>> Am 2021-07-27 07:28, schrieb Drew Fustini:
>> [..]
>> >> > > Drew please look at drivers/gpio/gpio-ftgpio010.c for an example
>> >> > > of GPIO_GENERIC calling bgpio_init() in probe().
>> >> >
>> >> > Thank you for the suggestion. However, I am not sure that will work for
>> >> > this SoC.
>> >> >
>> >> > The GPIO registers are described in section 12 of JH7100 datasheet [1]
>> >> > and I don't think they fit the expectation of gpio-mmio.c because there
>> >> > is a seperate register for each GPIO line for output data value and
>> >> > output enable.
>> >> >
>> >> > There are 64 output data config registers which are 4 bytes wide. There
>> >> > are 64 output enable config registers which are 4 bytes wide too. Output
>> >> > data and output enable registers for a given GPIO pad are contiguous.
>> >> > GPIO0_DOUT_CFG is 0x50 and GPIO0_DOEN_CFG is 0x54 while GPIO1_DOUT_CFG
>> >> > is 0x58 and GPIO1_DOEN_CFG is 0x5C. The stride between GPIO pads is
>> >> > effectively 8, which yields the formula: GPIOn_DOUT_CFG is 0x50+8n.
>> >> > Similarly, GPIO0_DOEN_CFG is 0x54 and thus GPIOn_DOEN_CFG is 0x54+8n.
>> >> >
>> >> > However, GPIO input data does use just one bit for each line. GPIODIN_0
>> >> > at 0x48 covers GPIO[31:0] and GPIODIN_1 at 0x4c covers GPIO[63:32].
>> 
>> Mh, I'm not sure I'm understanding the datasheet/registers. _DOUT_CFG
>> and _DOEN_CFG seem to specify the pad where this GPIO is mapped to.
>> Shouldn't this be some kind of pinctrl then? Apparently you can map
>> any GPIO number to any output pad, no? Or at least to all pads
>> which are described in Table 11-2. What happens if two different GPIOs
>> are mapped to the same pad? Bit 31 in these _CFG seems to be an invert
>> bit, but what does it invert?
>> 
>> Similar, the input GPIOs are connected to an output pad by all the
>> GPI_*_CFG registers.
>> 
>> To me it seems, that there two multiplexers for each GPIO, where
>> you can connect any GPIOn to any input pad and output pad. Sound
>> like a huge overkill. I must be missing something here.
>> 
>> But what puzzles me the most, where do I set the actual GPIO output
>> value?
> 
> Yeah, it's a little confusing. The DOUT registers choose between a 
> number of
> signals from various peripherals to control the output value of the
> pin. Similarly
> the DOEN registers chose between a number of signals to control the 
> output
> enable of the pin. However, two of those signals are special in that 
> they are
> constant 0 or constant 1. This is how you control the output value and 
> output
> enable from software like a regular GPIO.
> 
> You're completely right though. This ought to be managed by a proper 
> pinctrl
> driver, and I'm working on one here:
> https://github.com/esmil/linux/commits/beaglev-pinctrl

Ahh, I see. So for the non-gpio function you have to set a value other
than 0 or 1, correct?

And as an implementation detail you have to set the corresponding OE
pin if the non-gpio function will need a tristate pin (or whatever).

So, the _DOUT_CFG will actually be shared between the pinctrl and the
gpio driver, right? (I haven't done anything with pinctrl, so this might
be a stupid question).

-michael