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Message-ID: <Z1LD42UsCvtAhluY@pollux>
Date: Fri, 6 Dec 2024 10:29:07 +0100
From: Danilo Krummrich <dakr@...nel.org>
To: Lee Jones <lee@...nel.org>
Cc: linux-kernel@...r.kernel.org, arnd@...db.de, gregkh@...uxfoundation.org,
ojeda@...nel.org, alex.gaynor@...il.com, boqun.feng@...il.com,
gary@...yguo.net, bjorn3_gh@...tonmail.com, benno.lossin@...ton.me,
a.hindborg@...nel.org, aliceryhl@...gle.com, tmgross@...ch.edu,
rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v4 2/4] samples: rust: Provide example using the new Rust
MiscDevice abstraction
On Fri, Dec 06, 2024 at 09:05:06AM +0000, Lee Jones wrote:
> This sample driver demonstrates the following basic operations:
>
> * Register a Misc Device
> * Create /dev/rust-misc-device
> * Provide open call-back for the aforementioned character device
> * Operate on the character device via a simple ioctl()
> * Provide close call-back for the character device
>
> Signed-off-by: Lee Jones <lee@...nel.org>
> ---
> samples/rust/Kconfig | 10 ++++
> samples/rust/Makefile | 1 +
> samples/rust/rust_misc_device.rs | 80 ++++++++++++++++++++++++++++++++
> 3 files changed, 91 insertions(+)
> create mode 100644 samples/rust/rust_misc_device.rs
>
> diff --git a/samples/rust/Kconfig b/samples/rust/Kconfig
> index b0f74a81c8f9..df384e679901 100644
> --- a/samples/rust/Kconfig
> +++ b/samples/rust/Kconfig
> @@ -20,6 +20,16 @@ config SAMPLE_RUST_MINIMAL
>
> If unsure, say N.
>
> +config SAMPLE_RUST_MISC_DEVICE
> + tristate "Misc device"
> + help
> + This option builds the Rust misc device.
> +
> + To compile this as a module, choose M here:
> + the module will be called rust_misc_device.
> +
> + If unsure, say N.
> +
> config SAMPLE_RUST_PRINT
> tristate "Printing macros"
> help
> diff --git a/samples/rust/Makefile b/samples/rust/Makefile
> index c1a5c1655395..ad4b97a98580 100644
> --- a/samples/rust/Makefile
> +++ b/samples/rust/Makefile
> @@ -2,6 +2,7 @@
> ccflags-y += -I$(src) # needed for trace events
>
> obj-$(CONFIG_SAMPLE_RUST_MINIMAL) += rust_minimal.o
> +obj-$(CONFIG_SAMPLE_RUST_MISC_DEVICE) += rust_misc_device.o
> obj-$(CONFIG_SAMPLE_RUST_PRINT) += rust_print.o
>
> rust_print-y := rust_print_main.o rust_print_events.o
> diff --git a/samples/rust/rust_misc_device.rs b/samples/rust/rust_misc_device.rs
> new file mode 100644
> index 000000000000..3837532d259e
> --- /dev/null
> +++ b/samples/rust/rust_misc_device.rs
> @@ -0,0 +1,80 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +// Copyright (C) 2024 Google LLC.
> +
> +//! Rust misc device sample.
> +
> +use kernel::{
> + c_str,
> + ioctl::_IO,
> + miscdevice::{MiscDevice, MiscDeviceOptions, MiscDeviceRegistration},
> + prelude::*,
> +};
> +
> +const RUST_MISC_DEV_HELLO: u32 = _IO('|' as u32, 9);
> +
> +module! {
> + type: RustMiscDeviceModule,
> + name: "rust_misc_device",
> + author: "Lee Jones",
> + description: "Rust misc device sample",
> + license: "GPL",
> +}
> +
> +struct RustMiscDeviceModule {
> + _miscdev: Pin<KBox<MiscDeviceRegistration<RustMiscDevice>>>,
> +}
> +
> +impl kernel::Module for RustMiscDeviceModule {
> + fn init(_module: &'static ThisModule) -> Result<Self> {
> + pr_info!("Initialising Rust Misc Device Sample\n");
> +
> + let options = MiscDeviceOptions {
> + name: c_str!("rust-misc-device"),
> + };
> +
> + Ok(Self {
> + _miscdev: KBox::pin_init(
> + MiscDeviceRegistration::<RustMiscDevice>::register(options),
> + GFP_KERNEL,
> + )?,
> + })
Since this v4 was sent just when I was commenting on v3:
Why do we add examples where we ask people to allocate those structures with
kmalloc()?
`MiscDevice` should switch to the generic `Registration` type in [1] and use
`InPlaceModule`, such that those structures land in the .data section of the
binary.
[1] https://lore.kernel.org/rust-for-linux/20241205141533.111830-3-dakr@kernel.org/
> + }
> +}
> +
> +struct RustMiscDevice;
> +
> +#[vtable]
> +impl MiscDevice for RustMiscDevice {
> + type Ptr = KBox<Self>;
> +
> + fn open() -> Result<KBox<Self>> {
> + pr_info!("Opening Rust Misc Device Sample\n");
This should be `dev_info!`, but I see why you don't have access to the device
structure here...
@Greg: How do miscdev drivers do this in C? I looked at a couple of them, but
all of those seem to use pr_* macros. They can't get the device pointer from
the inode or file pointer.
However, C drivers could refer to the struct miscdevice directly since it's
almost always a static variable in the file scope.
In Rust we do the static allocation part with `InPlaceModule` as mentioned
above. However, this still doesn't let us refer to the underlying struct
miscdevice.
This all would be much cleaner if there'd be a "fake" probe() callback for a
struct miscdevice. This way we could perfectly align the miscdevice abstraction
with the abstractions for all other drivers, such as PCI, platform, etc.
- Danilo
> +
> + Ok(KBox::new(RustMiscDevice, GFP_KERNEL)?)
> + }
> +
> + fn ioctl(
> + _device: <Self::Ptr as kernel::types::ForeignOwnable>::Borrowed<'_>,
> + cmd: u32,
> + _arg: usize,
> + ) -> Result<isize> {
> + pr_info!("IOCTLing Rust Misc Device Sample\n");
> +
> + match cmd {
> + RUST_MISC_DEV_HELLO => pr_info!("Hello from the Rust Misc Device\n"),
> + _ => {
> + pr_err!("IOCTL not recognised: {}\n", cmd);
> + return Err(ENOTTY);
> + }
> + }
> +
> + Ok(0)
> + }
> +}
> +
> +impl Drop for RustMiscDevice {
> + fn drop(&mut self) {
> + pr_info!("Exiting the Rust Misc Device Sample\n");
> + }
> +}
> --
> 2.47.0.338.g60cca15819-goog
>
>
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