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From: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Date: Mon, 19 Jul 2021 17:43:40 +0200
Message-ID: <CANiq72nzWrs2RoLGYVy3s=bXCxywtOvQLMV7w6mnBgeMK8uwNQ@mail.gmail.com>
Subject: Re: [TECH TOPIC] Rust for Linux
To: Andrew Lunn <andrew@lunn.ch>
Cc: Linus Walleij <linus.walleij@linaro.org>, Wedson Almeida Filho <wedsonaf@google.com>, 
	Greg KH <greg@kroah.com>, Bartosz Golaszewski <bgolaszewski@baylibre.com>, 
	Kees Cook <keescook@chromium.org>, Jan Kara <jack@suse.cz>, 
	James Bottomley <James.Bottomley@hansenpartnership.com>, 
	Julia Lawall <julia.lawall@inria.fr>, 
	Laurent Pinchart <laurent.pinchart@ideasonboard.com>, Roland Dreier <roland@kernel.org>, 
	ksummit@lists.linux.dev, Viresh Kumar <viresh.kumar@linaro.org>
Content-Type: text/plain; charset="UTF-8"

On Mon, Jul 19, 2021 at 5:15 PM Andrew Lunn <andrew@lunn.ch> wrote:
>
> Can you express endianness here as well? There are often cases where
> the hardware is always big endian independent of the CPU
> endianness. Some of the readl/writel macros handle this, adding a swap
> when the two don't match.

Yes, of course!

Querying the endianness can be done as usual, similar to the C preprocessor:

    pub fn f(n: i32) -> i32 {
        #[cfg(target_endian = "little")]
        return n + 42;

        #[cfg(target_endian = "big")]
        return n;
    }

As well as with the `cfg!` macro, more ergonomic for small things like this:

    pub fn f(n: i32) -> i32 {
        if cfg!(target_endian = "little") {
            n + 42
        } else {
            n
        }
    }

As well in macros (like the `memory_map!` shown above).

If you just need the most common case, i.e. converting from/to a given
endianness, the primitive types come with functions for that, e.g. the
following would include a `bswap` in x86:

    pub fn f(n: i32) -> i32 {
        i32::from_be(n)
    }

See `{from,to}_{be,le}` in https://doc.rust-lang.org/std/primitive.i32.html

Cheers,
Miguel