linux-kernel - Re: [PATCH] drm/panic: fix compilation issue on ARM

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Message-ID: <b2c043e1-b27f-4a3c-ae11-4d354685d079@ryhl.io>
Date: Mon, 20 Jan 2025 23:09:37 +0100
From: Alice Ryhl <alice@...l.io>
To: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
 Emmanuel Gil Peyrot <linkmauve@...kmauve.fr>
Cc: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
 Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>,
 Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>,
 Benno Lossin <benno.lossin@...ton.me>,
 Andreas Hindborg <a.hindborg@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>,
 Trevor Gross <tmgross@...ch.edu>, dri-devel@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org,
 Jocelyn Falempe <jfalempe@...hat.com>
Subject: Re: [PATCH] drm/panic: fix compilation issue on ARM

On 1/20/25 10:52 PM, Miguel Ojeda wrote:
> Hi Emmanuel,
> 
> On Mon, Jan 20, 2025 at 1:45 PM Emmanuel Gil Peyrot
> <linkmauve@...kmauve.fr> wrote:
>>
>> In C, the char type is specified with “The implementation shall define char to
>> have the same range, representation, and behavior as either signed char or
>> unsigned char.”
>>
>> On x86 it defaults to signed char, and on ARM it defaults to unsigned char.
>> This carries over to Rust’s FFI, which aliases its c_char type to i8 on x86,
>> and to u8 on ARM.
> 
> In the kernel `-funsigned-char` is used, see commit 3bc753c06dd0
> ("kbuild: treat char as always unsigned").
> 
> In any case, the change is fine, because we want to use the proper
> type, but this is already in the Rust PR for this cycle, which should
> land in mainline in some days, see commit 27c7518e7f1c ("rust: finish
> using custom FFI integer types") in rust-next.
> 
> However, I am nevertheless confused, because in mainline
> `crate::ffi::c_char` is `core::ffi::c_char` which is `i8` in both
> arm64 and x86_64 and thus there is no build issue there.
Isn't core::ffi::c_char an u8 on arm64?

Alice