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From:   Linus Walleij <linus.walleij@linaro.org>
Date:   Fri, 22 Apr 2022 23:14:15 +0200
Message-ID: <CACRpkda=SNKAeBpfdA2seRgZwmb0-SMxq-_c0je6gdR-8m+p_A@mail.gmail.com>
Subject: Re: [PATCH v1 2/6] gpio: realtek-otto: Support reversed port layouts
To:     Sander Vanheule <sander@svanheule.net>
Cc:     linux-gpio@vger.kernel.org, devicetree@vger.kernel.org,
        Bartosz Golaszewski <brgl@bgdev.pl>,
        Rob Herring <robh+dt@kernel.org>,
        Krzysztof Kozlowski <krzk+dt@kernel.org>,
        Bert Vermeulen <bert@biot.com>, linux-kernel@vger.kernel.org,
        andy.shevchenko@gmail.com
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On Thu, Apr 21, 2022 at 9:55 AM Sander Vanheule <sander@svanheule.net> wrote:

> The kernel for RTL930x SoC is built with CONFIG_CPU_BIG_ENDIAN=y, just like the
> older SoCs that were previously supported. The SoC's IRQ controller is also the
> same across RTL930x/RTL839x/RTL838x, even though 32-bit registers are used
> there.
>
> On RTL838x/RTL839x the GPIO IRQ control registers have byte layout:
>         [H1] [L1] [H2] [L2]
>         [H3] [L3] [H4] [L4]
>
> On RTL930x, the GPIO IRQ control registers are:
>         [H2] [L2] [H1] [L1]
>         [H4] [L4] [H3] [L3]
> which is the reverse of:
>         [L1] [H1] [L2] [H2]
>         [L3] [H3] [L4] [H4]
>
>
> Same for the GPIO registers:
>         On RTL83xx: [P1] [P2] [P3] [P4] (four 8b ports)
>         On RTL930x: [P4] [P3] [P2] [P1] (one BE32 port)
>
> It looks like the RTL930x could use a little-endian interpretation of the 32b
> registers, followed by a little-endian interpretation of the contained port
> values. That would mean two reorderings for every 16b read or write operation,
> and manual manipulation of the register values. Although I have to say that the
> current offset calculation is not too pretty either.

I'm happy.

It's not very invasive and the bulk of the problem is addressed by
simply using the GPIO MMIO library, so:
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>

If someone knows a more elegant way, they can send a patch,
this works so we should merge it.

Yours,
Linus Walleij