| Message ID | 20230307-rust-drm-v1-3-917ff5bc80a8@asahilina.net |
|---|---|
| State | New |
| Headers | show |
| Series | Rust DRM subsystem abstractions (& preview AGX driver) | expand |
On Tue, 2023-03-07 at 23:25 +0900, Asahi Lina wrote: > A DRM File is the DRM counterpart to a kernel file structure, > representing an open DRM file descriptor. Add a Rust abstraction to > allow drivers to implement their own File types that implement the > DriverFile trait. > > Signed-off-by: Asahi Lina <lina@asahilina.net> > --- > rust/bindings/bindings_helper.h | 1 + > rust/kernel/drm/drv.rs | 7 ++- > rust/kernel/drm/file.rs | 113 > ++++++++++++++++++++++++++++++++++++++++ > rust/kernel/drm/mod.rs | 1 + > 4 files changed, 120 insertions(+), 2 deletions(-) > > diff --git a/rust/bindings/bindings_helper.h > b/rust/bindings/bindings_helper.h > index 2a999138c4ae..7d7828faf89c 100644 > --- a/rust/bindings/bindings_helper.h > +++ b/rust/bindings/bindings_helper.h > @@ -8,6 +8,7 @@ > > #include <drm/drm_device.h> > #include <drm/drm_drv.h> > +#include <drm/drm_file.h> > #include <drm/drm_ioctl.h> > #include <linux/delay.h> > #include <linux/device.h> > diff --git a/rust/kernel/drm/drv.rs b/rust/kernel/drm/drv.rs > index 29a465515dc9..1dcb651e1417 100644 > --- a/rust/kernel/drm/drv.rs > +++ b/rust/kernel/drm/drv.rs > @@ -144,6 +144,9 @@ pub trait Driver { > /// Should be either `drm::gem::Object<T>` or > `drm::gem::shmem::Object<T>`. > type Object: AllocImpl; > > + /// The type used to represent a DRM File (client) > + type File: drm::file::DriverFile; > + > /// Driver metadata > const INFO: DriverInfo; > > @@ -213,8 +216,8 @@ macro_rules! drm_device_register { > impl<T: Driver> Registration<T> { > const VTABLE: bindings::drm_driver = drm_legacy_fields! { > load: None, > - open: None, // TODO: File abstraction > - postclose: None, // TODO: File abstraction > + open: Some(drm::file::open_callback::<T::File>), > + postclose: Some(drm::file::postclose_callback::<T::File>), > lastclose: None, > unload: None, > release: None, > diff --git a/rust/kernel/drm/file.rs b/rust/kernel/drm/file.rs > new file mode 100644 > index 000000000000..48751e93c38a > --- /dev/null > +++ b/rust/kernel/drm/file.rs > @@ -0,0 +1,113 @@ > +// SPDX-License-Identifier: GPL-2.0 OR MIT > + > +//! DRM File objects. > +//! > +//! C header: > [`include/linux/drm/drm_file.h`](../../../../include/linux/drm/drm_fi > le.h) > + > +use crate::{bindings, drm, error::Result}; > +use alloc::boxed::Box; > +use core::marker::PhantomData; > +use core::ops::Deref; > + > +/// Trait that must be implemented by DRM drivers to represent a DRM > File (a client instance). > +pub trait DriverFile { > + /// The parent `Driver` implementation for this `DriverFile`. > + type Driver: drm::drv::Driver; > + > + /// Open a new file (called when a client opens the DRM device). > + fn open(device: &drm::device::Device<Self::Driver>) -> > Result<Box<Self>>; > +} > + > +/// An open DRM File. > +/// > +/// # Invariants > +/// `raw` is a valid pointer to a `drm_file` struct. > +#[repr(transparent)] > +pub struct File<T: DriverFile> { > + raw: *mut bindings::drm_file, > + _p: PhantomData<T>, > +} > + > +pub(super) unsafe extern "C" fn open_callback<T: DriverFile>( > + raw_dev: *mut bindings::drm_device, > + raw_file: *mut bindings::drm_file, > +) -> core::ffi::c_int { > + let drm = core::mem::ManuallyDrop::new(unsafe { > drm::device::Device::from_raw(raw_dev) }); Maybe you can help educate me a bit here... This feels like a really sketchy pattern. We're creating a Device from a pointer, an operation which inherently consumes a reference but then marking it ManuallyDrop so drm_device_put() never gets called. It took me a while but I think I figured out what you're trying to do: Make it so all the Rust stuff works with Device, not drm_device but it still feels really wrong. It works, it just feels like there's a lot of unsafe abstraction juggling happening here and I expect this operation is going to be pretty common in the Rust abstraction layer. Am I missing something? ~Faith > + // SAFETY: This reference won't escape this function > + let file = unsafe { &mut *raw_file }; > + > + let inner = match T::open(&drm) { > + Err(e) => { > + return e.to_kernel_errno(); > + } > + Ok(i) => i, > + }; > + > + file.driver_priv = Box::into_raw(inner) as *mut _; > + > + 0 > +} > + > +pub(super) unsafe extern "C" fn postclose_callback<T: DriverFile>( > + _dev: *mut bindings::drm_device, > + raw_file: *mut bindings::drm_file, > +) { > + // SAFETY: This reference won't escape this function > + let file = unsafe { &*raw_file }; > + > + // Drop the DriverFile > + unsafe { Box::from_raw(file.driver_priv as *mut T) }; > +} > + > +impl<T: DriverFile> File<T> { > + // Not intended to be called externally, except via > declare_drm_ioctls!() > + #[doc(hidden)] > + pub unsafe fn from_raw(raw_file: *mut bindings::drm_file) -> > File<T> { > + File { > + raw: raw_file, > + _p: PhantomData, > + } > + } > + > + #[allow(dead_code)] > + /// Return the raw pointer to the underlying `drm_file`. > + pub(super) fn raw(&self) -> *const bindings::drm_file { > + self.raw > + } > + > + /// Return an immutable reference to the raw `drm_file` > structure. > + pub(super) fn file(&self) -> &bindings::drm_file { > + unsafe { &*self.raw } > + } > +} > + > +impl<T: DriverFile> Deref for File<T> { > + type Target = T; > + > + fn deref(&self) -> &T { > + unsafe { &*(self.file().driver_priv as *const T) } > + } > +} > + > +impl<T: DriverFile> crate::private::Sealed for File<T> {} > + > +/// Generic trait to allow users that don't care about driver > specifics to accept any File<T>. > +/// > +/// # Safety > +/// Must only be implemented for File<T> and return the pointer, > following the normal invariants > +/// of that type. > +pub unsafe trait GenericFile: crate::private::Sealed { > + /// Returns the raw const pointer to the `struct drm_file` > + fn raw(&self) -> *const bindings::drm_file; > + /// Returns the raw mut pointer to the `struct drm_file` > + fn raw_mut(&mut self) -> *mut bindings::drm_file; > +} > + > +unsafe impl<T: DriverFile> GenericFile for File<T> { > + fn raw(&self) -> *const bindings::drm_file { > + self.raw > + } > + fn raw_mut(&mut self) -> *mut bindings::drm_file { > + self.raw > + } > +} > diff --git a/rust/kernel/drm/mod.rs b/rust/kernel/drm/mod.rs > index 69376b3c6db9..a767942d0b52 100644 > --- a/rust/kernel/drm/mod.rs > +++ b/rust/kernel/drm/mod.rs > @@ -4,4 +4,5 @@ > > pub mod device; > pub mod drv; > +pub mod file; > pub mod ioctl; >
On 10/03/2023 06.16, Faith Ekstrand wrote: > On Tue, 2023-03-07 at 23:25 +0900, Asahi Lina wrote: >> A DRM File is the DRM counterpart to a kernel file structure, >> representing an open DRM file descriptor. Add a Rust abstraction to >> allow drivers to implement their own File types that implement the >> DriverFile trait. >> >> Signed-off-by: Asahi Lina <lina@asahilina.net> >> --- >> rust/bindings/bindings_helper.h | 1 + >> rust/kernel/drm/drv.rs | 7 ++- >> rust/kernel/drm/file.rs | 113 >> ++++++++++++++++++++++++++++++++++++++++ >> rust/kernel/drm/mod.rs | 1 + >> 4 files changed, 120 insertions(+), 2 deletions(-) >> >> diff --git a/rust/bindings/bindings_helper.h >> b/rust/bindings/bindings_helper.h >> index 2a999138c4ae..7d7828faf89c 100644 >> --- a/rust/bindings/bindings_helper.h >> +++ b/rust/bindings/bindings_helper.h >> @@ -8,6 +8,7 @@ >> >> #include <drm/drm_device.h> >> #include <drm/drm_drv.h> >> +#include <drm/drm_file.h> >> #include <drm/drm_ioctl.h> >> #include <linux/delay.h> >> #include <linux/device.h> >> diff --git a/rust/kernel/drm/drv.rs b/rust/kernel/drm/drv.rs >> index 29a465515dc9..1dcb651e1417 100644 >> --- a/rust/kernel/drm/drv.rs >> +++ b/rust/kernel/drm/drv.rs >> @@ -144,6 +144,9 @@ pub trait Driver { >> /// Should be either `drm::gem::Object<T>` or >> `drm::gem::shmem::Object<T>`. >> type Object: AllocImpl; >> >> + /// The type used to represent a DRM File (client) >> + type File: drm::file::DriverFile; >> + >> /// Driver metadata >> const INFO: DriverInfo; >> >> @@ -213,8 +216,8 @@ macro_rules! drm_device_register { >> impl<T: Driver> Registration<T> { >> const VTABLE: bindings::drm_driver = drm_legacy_fields! { >> load: None, >> - open: None, // TODO: File abstraction >> - postclose: None, // TODO: File abstraction >> + open: Some(drm::file::open_callback::<T::File>), >> + postclose: Some(drm::file::postclose_callback::<T::File>), >> lastclose: None, >> unload: None, >> release: None, >> diff --git a/rust/kernel/drm/file.rs b/rust/kernel/drm/file.rs >> new file mode 100644 >> index 000000000000..48751e93c38a >> --- /dev/null >> +++ b/rust/kernel/drm/file.rs >> @@ -0,0 +1,113 @@ >> +// SPDX-License-Identifier: GPL-2.0 OR MIT >> + >> +//! DRM File objects. >> +//! >> +//! C header: >> [`include/linux/drm/drm_file.h`](../../../../include/linux/drm/drm_fi >> le.h) >> + >> +use crate::{bindings, drm, error::Result}; >> +use alloc::boxed::Box; >> +use core::marker::PhantomData; >> +use core::ops::Deref; >> + >> +/// Trait that must be implemented by DRM drivers to represent a DRM >> File (a client instance). >> +pub trait DriverFile { >> + /// The parent `Driver` implementation for this `DriverFile`. >> + type Driver: drm::drv::Driver; >> + >> + /// Open a new file (called when a client opens the DRM device). >> + fn open(device: &drm::device::Device<Self::Driver>) -> >> Result<Box<Self>>; >> +} >> + >> +/// An open DRM File. >> +/// >> +/// # Invariants >> +/// `raw` is a valid pointer to a `drm_file` struct. >> +#[repr(transparent)] >> +pub struct File<T: DriverFile> { >> + raw: *mut bindings::drm_file, >> + _p: PhantomData<T>, >> +} >> + >> +pub(super) unsafe extern "C" fn open_callback<T: DriverFile>( >> + raw_dev: *mut bindings::drm_device, >> + raw_file: *mut bindings::drm_file, >> +) -> core::ffi::c_int { >> + let drm = core::mem::ManuallyDrop::new(unsafe { >> drm::device::Device::from_raw(raw_dev) }); > > Maybe you can help educate me a bit here... This feels like a really > sketchy pattern. We're creating a Device from a pointer, an operation > which inherently consumes a reference but then marking it ManuallyDrop > so drm_device_put() never gets called. It took me a while but I think > I figured out what you're trying to do: Make it so all the Rust stuff > works with Device, not drm_device but it still feels really wrong. It > works, it just feels like there's a lot of unsafe abstraction juggling > happening here and I expect this operation is going to be pretty common > in the Rust abstraction layer. So I think this is going to be a pretty common pattern in this kind of abstraction. The problem is that, of course, in C there is no distinction between an owned reference and a borrowed one. Here we have a borrowed reference to a struct drm_device, and we need to turn it into a &Device (which is the Rust equivalent type). But for &Device to exist we need a Device to exist in the first place, and Device normally implies ownership of the underlying drm_device. We could just acquire a reference here, but then we're needlessly grabbing a ref only to drop it at the end of the function, which is pointless when the caller is holding another reference for us while the callback runs. And of course Rust likes to claim to offer zero-cost abstractions, so it would be kind of sad to have to do that... ^^ Just doing drm::device::Device::from_raw(raw_dev) is a ticking time bomb, because we haven't acquired a reference (which would normally be required). If that Device ever gets dropped, we've messed up the refcounting and stolen the caller's reference. We could try to ensure it gets passed to core::mem::forget in all paths out, but that gets error-prone very quickly when trying to cover error paths. So instead, we put it into a ManuallyDrop. That takes care of neutering the ref drop, so we don't have to worry about messing that up. Then the only remaining safety requirement is that that the ManuallyDrop<Device> never escape the callback function, and that's easy to ensure: we only pass a &ref to the user (which via auto-deref ends up being a &Device), and then nothing bad can happen. If the user wants an owned reference to the device to keep around, they can call .clone() on it and that's when the incref happens. Basically, ManuallyDrop<T> where T is a reference counted type represents a borrowed reference to a T coming from the C side. You can see another use of this pattern in gem::Object, which contains a ManuallyDrop<Device> that represents a borrowed reference to the device that owns that object. The DRM core (as far as I know!) guarantees that DRM devices outlive all of their GEM objects, so we can materialize a borrowed reference and as long as it never leaves the GEM object, it will be sound. Then we can take &Device references from it whenever we want, and the usual Rust borrow checker rules ensure we can't do something illegal. ~~ Lina
diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h index 2a999138c4ae..7d7828faf89c 100644 --- a/rust/bindings/bindings_helper.h +++ b/rust/bindings/bindings_helper.h @@ -8,6 +8,7 @@ #include <drm/drm_device.h> #include <drm/drm_drv.h> +#include <drm/drm_file.h> #include <drm/drm_ioctl.h> #include <linux/delay.h> #include <linux/device.h> diff --git a/rust/kernel/drm/drv.rs b/rust/kernel/drm/drv.rs index 29a465515dc9..1dcb651e1417 100644 --- a/rust/kernel/drm/drv.rs +++ b/rust/kernel/drm/drv.rs @@ -144,6 +144,9 @@ pub trait Driver { /// Should be either `drm::gem::Object<T>` or `drm::gem::shmem::Object<T>`. type Object: AllocImpl; + /// The type used to represent a DRM File (client) + type File: drm::file::DriverFile; + /// Driver metadata const INFO: DriverInfo; @@ -213,8 +216,8 @@ macro_rules! drm_device_register { impl<T: Driver> Registration<T> { const VTABLE: bindings::drm_driver = drm_legacy_fields! { load: None, - open: None, // TODO: File abstraction - postclose: None, // TODO: File abstraction + open: Some(drm::file::open_callback::<T::File>), + postclose: Some(drm::file::postclose_callback::<T::File>), lastclose: None, unload: None, release: None, diff --git a/rust/kernel/drm/file.rs b/rust/kernel/drm/file.rs new file mode 100644 index 000000000000..48751e93c38a --- /dev/null +++ b/rust/kernel/drm/file.rs @@ -0,0 +1,113 @@ +// SPDX-License-Identifier: GPL-2.0 OR MIT + +//! DRM File objects. +//! +//! C header: [`include/linux/drm/drm_file.h`](../../../../include/linux/drm/drm_file.h) + +use crate::{bindings, drm, error::Result}; +use alloc::boxed::Box; +use core::marker::PhantomData; +use core::ops::Deref; + +/// Trait that must be implemented by DRM drivers to represent a DRM File (a client instance). +pub trait DriverFile { + /// The parent `Driver` implementation for this `DriverFile`. + type Driver: drm::drv::Driver; + + /// Open a new file (called when a client opens the DRM device). + fn open(device: &drm::device::Device<Self::Driver>) -> Result<Box<Self>>; +} + +/// An open DRM File. +/// +/// # Invariants +/// `raw` is a valid pointer to a `drm_file` struct. +#[repr(transparent)] +pub struct File<T: DriverFile> { + raw: *mut bindings::drm_file, + _p: PhantomData<T>, +} + +pub(super) unsafe extern "C" fn open_callback<T: DriverFile>( + raw_dev: *mut bindings::drm_device, + raw_file: *mut bindings::drm_file, +) -> core::ffi::c_int { + let drm = core::mem::ManuallyDrop::new(unsafe { drm::device::Device::from_raw(raw_dev) }); + // SAFETY: This reference won't escape this function + let file = unsafe { &mut *raw_file }; + + let inner = match T::open(&drm) { + Err(e) => { + return e.to_kernel_errno(); + } + Ok(i) => i, + }; + + file.driver_priv = Box::into_raw(inner) as *mut _; + + 0 +} + +pub(super) unsafe extern "C" fn postclose_callback<T: DriverFile>( + _dev: *mut bindings::drm_device, + raw_file: *mut bindings::drm_file, +) { + // SAFETY: This reference won't escape this function + let file = unsafe { &*raw_file }; + + // Drop the DriverFile + unsafe { Box::from_raw(file.driver_priv as *mut T) }; +} + +impl<T: DriverFile> File<T> { + // Not intended to be called externally, except via declare_drm_ioctls!() + #[doc(hidden)] + pub unsafe fn from_raw(raw_file: *mut bindings::drm_file) -> File<T> { + File { + raw: raw_file, + _p: PhantomData, + } + } + + #[allow(dead_code)] + /// Return the raw pointer to the underlying `drm_file`. + pub(super) fn raw(&self) -> *const bindings::drm_file { + self.raw + } + + /// Return an immutable reference to the raw `drm_file` structure. + pub(super) fn file(&self) -> &bindings::drm_file { + unsafe { &*self.raw } + } +} + +impl<T: DriverFile> Deref for File<T> { + type Target = T; + + fn deref(&self) -> &T { + unsafe { &*(self.file().driver_priv as *const T) } + } +} + +impl<T: DriverFile> crate::private::Sealed for File<T> {} + +/// Generic trait to allow users that don't care about driver specifics to accept any File<T>. +/// +/// # Safety +/// Must only be implemented for File<T> and return the pointer, following the normal invariants +/// of that type. +pub unsafe trait GenericFile: crate::private::Sealed { + /// Returns the raw const pointer to the `struct drm_file` + fn raw(&self) -> *const bindings::drm_file; + /// Returns the raw mut pointer to the `struct drm_file` + fn raw_mut(&mut self) -> *mut bindings::drm_file; +} + +unsafe impl<T: DriverFile> GenericFile for File<T> { + fn raw(&self) -> *const bindings::drm_file { + self.raw + } + fn raw_mut(&mut self) -> *mut bindings::drm_file { + self.raw + } +} diff --git a/rust/kernel/drm/mod.rs b/rust/kernel/drm/mod.rs index 69376b3c6db9..a767942d0b52 100644 --- a/rust/kernel/drm/mod.rs +++ b/rust/kernel/drm/mod.rs @@ -4,4 +4,5 @@ pub mod device; pub mod drv; +pub mod file; pub mod ioctl;
A DRM File is the DRM counterpart to a kernel file structure, representing an open DRM file descriptor. Add a Rust abstraction to allow drivers to implement their own File types that implement the DriverFile trait. Signed-off-by: Asahi Lina <lina@asahilina.net> --- rust/bindings/bindings_helper.h | 1 + rust/kernel/drm/drv.rs | 7 ++- rust/kernel/drm/file.rs | 113 ++++++++++++++++++++++++++++++++++++++++ rust/kernel/drm/mod.rs | 1 + 4 files changed, 120 insertions(+), 2 deletions(-)