Re: [WIP RFC v2 10/35] rust: drm/kms: Add DriverConnector::get_mode callback

[Lyude Paul <lyude@redhat.com>]

On Wed, 2024-11-27 at 12:03 -0300, Daniel Almeida wrote:
> Hi Lyude,
> 
> > On 30 Sep 2024, at 20:09, Lyude Paul <lyude@redhat.com> wrote:
> > 
> > Next up is filling out some of the basic connector hotplugging callbacks -
> > which we'll need for setting up the fbdev helpers for KMS devices. Note
> > that connector hotplugging in DRM follows a BFL scheme: pretty much all
> 
> A what scheme?

"Big fucking lock".

It's a bit of crude terminology, but it's the most commonly used term I've
seen to describe this sort of thing in the kernel. In the old days of Linux
before I even worked on the kernel we just had one big lock for SMP. As that
was removed and replaced with other locks, there were still quite a number of
subsystems that basically shared a big lock that protected most state - and
DRM was one of them. To this day there's still a lot of various misc.
connector state that's stored under this lock.

> 
> > probing is protected under the mighty drm_device->mode_config.lock, which
> > of course is a bit counter-intuitive to rust's locking schemes where data
> > is always associated with its lock.
> > 
> > Since that lock is embedded in an FFI type and not a rust type, we need to
> > introduce our own wrapper type that acts as a lock acquisition for this.
> > This brings us to introducing a few new types:
> > 
> > * ModeConfigGuard - the most basic lock guard, as long as this object is
> >  alive we are guaranteed to be holding drm_device->mode_config.lock. This
> >  object doesn't do much else on its own currently.
> > * ConnectorGuard - an object which corresponds to a specific typed DRM
> >  connector. This can only be acquired with a ModeConfigGuard, and will be
> >  used to allow calling methods that are only safe to call with
> >  drm_device->mode_config.lock held. Since it implements
> >  Deref<Target=Connector<T>> as well, it can also be used for any other
> >  operations that would normally be available on a DRM connector.
> > 
> > And finally, we add the DriverConnector::get_modes() trait method which
> > drivers can use to implement the drm_connector_helper_funcs.get_modes
> > callback. Note that while we make this trait method mandatory, we only do
> > so for the time being since VKMS doesn't do very much with DRM connectors -
> > and as such we have no need yet to implement alternative connector probing
> > schemes outside of get_modes().
> > 
> > Signed-off-by: Lyude Paul <lyude@redhat.com>
> > ---
> > rust/kernel/drm/kms.rs           | 70 ++++++++++++++++++++++++++++++++
> > rust/kernel/drm/kms/connector.rs | 57 ++++++++++++++++++++++++--
> > 2 files changed, 124 insertions(+), 3 deletions(-)
> > 
> > diff --git a/rust/kernel/drm/kms.rs b/rust/kernel/drm/kms.rs
> > index d5cad598f016f..d74267c78864f 100644
> > --- a/rust/kernel/drm/kms.rs
> > +++ b/rust/kernel/drm/kms.rs
> > @@ -18,6 +18,8 @@
> >     types::*,
> >     error::to_result,
> >     private::Sealed,
> > +    sync::{Mutex, MutexGuard},
> > +    container_of
> > };
> > use core::{
> >     ops::Deref,
> > @@ -233,6 +235,21 @@ impl<T, K> KmsDriver for T
> >     T: Driver<Kms = K>,
> >     K: Kms<Driver = T> {}
> > 
> > +impl<T: KmsDriver> Device<T> {
> > +    /// Retrieve a pointer to the mode_config mutex
> > +    #[inline]
> > +    pub(crate) fn mode_config_mutex(&self) -> &Mutex<()> {
> > +        // SAFETY: This lock is initialized for as long as `Device<T>` is exposed to users
> > +        unsafe { Mutex::from_raw(&mut (*self.as_raw()).mode_config.mutex) }
> 
> Again, a bit hard to understand what’s going on here, since everything is on a single line.
> 
> > +    }
> > +
> > +    /// Acquire the [`mode_config.mutex`] for this [`Device`].
> > +    #[inline]
> > +    pub fn mode_config_lock(&self) -> ModeConfigGuard<'_, T> {
> > +        ModeConfigGuard(self.mode_config_mutex().lock(), PhantomData)
> > +    }
> > +}
> > +
> > /// A modesetting object in DRM.
> > ///
> > /// This is any type of object where the underlying C object contains a [`struct drm_mode_object`].
> > @@ -339,3 +356,56 @@ unsafe fn dec_ref(obj: ptr::NonNull<Self>) {
> >         unsafe { bindings::drm_mode_object_put(obj.as_ref().raw_mode_obj()) }
> >     }
> > }
> > +
> > +/// A mode config guard.
> > +///
> > +/// This is an exclusive primitive that represents when [`drm_device.mode_config.mutex`] is held - as
> > +/// some modesetting operations (particularly ones related to [`connectors`](connector)) are still
> > +/// protected under this single lock. The lock will be dropped once this object is dropped.
> > +///
> > +/// # Invariants
> > +///
> > +/// - `self.0` is contained within a [`struct drm_mode_config`], which is contained within a
> > +///   [`struct drm_device`].
> > +/// - The [`KmsDriver`] implementation of that [`struct drm_device`] is always `T`.
> > +/// - This type proves that [`drm_device.mode_config.mutex`] is acquired.
> > +///
> > +/// [`struct drm_mode_config`]: (srctree/include/drm/drm_device.h)
> > +/// [`drm_device.mode_config.mutex`]: (srctree/include/drm/drm_device.h)
> > +/// [`struct drm_device`]: (srctree/include/drm/drm_device.h)
> > +pub struct ModeConfigGuard<'a, T: KmsDriver>(MutexGuard<'a, ()>, PhantomData<T>);
> > +
> > +impl<'a, T: KmsDriver> ModeConfigGuard<'a, T> {
> > +    /// Construct a new [`ModeConfigGuard`].
> > +    ///
> > +    /// # Safety
> > +    ///
> > +    /// The caller must ensure that [`drm_device.mode_config.mutex`] is acquired.
> > +    ///
> > +    /// [`drm_device.mode_config.mutex`]: (srctree/include/drm/drm_device.h)
> > +    pub(crate) unsafe fn new(drm: &'a Device<T>) -> Self {
> > +        // SAFETY: Our safety contract fulfills the requirements of `MutexGuard::new()`
> > +        Self(unsafe { MutexGuard::new(drm.mode_config_mutex(), ()) }, PhantomData)
> > +    }
> > +
> > +    /// Return the [`Device`] that this [`ModeConfigGuard`] belongs to.
> > +    pub fn drm_dev(&self) -> &'a Device<T> {
> > +        // SAFETY:
> > +        // - `self` is embedded within a `drm_mode_config` via our type invariants
> > +        // - `self.0.lock` has an equivalent data type to `mutex` via its type invariants.
> > +        let mode_config = unsafe { container_of!(self.0.lock, bindings::drm_mode_config, mutex) };
> > +
> > +        // SAFETY: And that `drm_mode_config` lives in a `drm_device` via type invariants.
> > +        unsafe { Device::borrow(container_of!(mode_config, bindings::drm_device, mode_config)) }
> > +    }
> > +
> > +    /// Assert that the given device is the owner of this mode config guard.
> > +    ///
> > +    /// # Panics
> > +    ///
> > +    /// Panics if `dev` is different from the owning device for this mode config guard.
> > +    #[inline]
> > +    pub(crate) fn assert_owner(&self, dev: &Device<T>) {
> > +        assert!(ptr::eq(self.drm_dev(), dev));
> > +    }
> > +}
> > diff --git a/rust/kernel/drm/kms/connector.rs b/rust/kernel/drm/kms/connector.rs
> > index 54457b327c365..57ab29473c344 100644
> > --- a/rust/kernel/drm/kms/connector.rs
> > +++ b/rust/kernel/drm/kms/connector.rs
> > @@ -19,7 +19,7 @@
> > use core::{
> >     marker::*,
> >     ptr::null_mut,
> > -    mem,
> > +    mem::{self, ManuallyDrop},
> >     ptr::{self, NonNull, addr_of_mut},
> >     ffi::*,
> >     ops::*,
> > @@ -28,6 +28,7 @@
> > use super::{
> >     ModeObject,
> >     RcModeObject,
> > +    ModeConfigGuard,
> >     encoder::*,
> >     KmsDriver,
> > };
> > @@ -93,7 +94,7 @@ pub trait DriverConnector: Send + Sync + Sized {
> >             destroy: Some(connector_destroy_callback::<Self>),
> >             force: None,
> >             detect: None,
> > -            fill_modes: None,
> > +            fill_modes: Some(bindings::drm_helper_probe_single_connector_modes),
> >             debugfs_init: None,
> >             oob_hotplug_event: None,
> >             atomic_duplicate_state: Some(atomic_duplicate_state_callback::<Self::State>),
> > @@ -101,7 +102,7 @@ pub trait DriverConnector: Send + Sync + Sized {
> >         helper_funcs: bindings::drm_connector_helper_funcs {
> >             mode_valid: None,
> >             atomic_check: None,
> > -            get_modes: None,
> > +            get_modes: Some(get_modes_callback::<Self>),
> >             detect_ctx: None,
> >             enable_hpd: None,
> >             disable_hpd: None,
> > @@ -132,6 +133,12 @@ pub trait DriverConnector: Send + Sync + Sized {
> >     ///
> >     /// Drivers may use this to instantiate their [`DriverConnector`] object.
> >     fn new(device: &Device<Self::Driver>, args: Self::Args) -> impl PinInit<Self, Error>;
> > +
> > +    /// Retrieve a list of available display modes for this [`Connector`].
> > +    fn get_modes<'a>(
> > +        connector: ConnectorGuard<'a, Self>,
> > +        guard: &ModeConfigGuard<'a, Self::Driver>
> > +    ) -> i32;
> > }
> > 
> > /// The generated C vtable for a [`DriverConnector`].
> > @@ -229,6 +236,19 @@ pub fn new(
> >         })
> >     }
> > 
> > +    /// Acquire a [`ConnectorGuard`] for this connector from a [`ModeConfigGuard`].
> > +    ///
> > +    /// This verifies using the provided reference that the given guard is actually for the same
> > +    /// device as this connector's parent.
> > +    ///
> > +    /// # Panics
> > +    ///
> > +    /// Panics if `guard` is not a [`ModeConfigGuard`] for this connector's parent [`Device`].
> > +    pub fn guard<'a>(&'a self, guard: &ModeConfigGuard<'a, T::Driver>) -> ConnectorGuard<'a, T> {
> > +        guard.assert_owner(self.drm_dev());
> > +        ConnectorGuard(self)
> > +    }
> > +
> >     /// Attach an encoder to this [`Connector`].
> >     ///
> >     /// TODO: Move this to an `UnregisteredConnector` interface somehow…
> > @@ -327,6 +347,37 @@ unsafe fn from_raw<'a>(ptr: *mut bindings::drm_connector) -> &'a Self {
> >     drop(unsafe { Box::from_raw(connector as *mut Connector<T>) });
> > }
> > 
> > +unsafe extern "C" fn get_modes_callback<T: DriverConnector>(
> > +    connector: *mut bindings::drm_connector,
> > +) -> c_int {
> > +    // SAFETY: This is safe via `DriverConnector`s type invariants.
> > +    let connector = unsafe { Connector::<T>::from_raw(connector) };
> > +
> > +    // SAFETY: This FFI callback is only called while `mode_config.lock` is held
> > +    let guard = ManuallyDrop::new(unsafe { ModeConfigGuard::new(connector.drm_dev()) });
> 
> I’m confused. Can you explain what this ManuallyDrop is being used for?

So there's two different ways the mode_config lock usually gets acquired:

* Explicitly by the driver, to modify some sort of state that's protected by
it
* Implicitly by DRM, for the duration of a callback like get_modes().

Since we want to be able to support both cases in rust, we want to make sure
that our object for exposing this lock can be manually acquired - but also
provide a reference to the lock type for callbacks like get_modes() where the
lock is already acquired and we need to perform actions that work on state
protected by said lock. IMO, the most sensible way of doing this is to just
use ManuallyDrop so that we can basically promise "the lock is held for the
duration of this call", and avoid mistakenly unlocking it at the end of the
callback (something that should be handled by DRM and not us).

> 
> > +
> > +    T::get_modes(connector.guard(&guard), &guard)
> > +}
> > +
> > +/// A privileged [`Connector`] obtained while holding a [`ModeConfigGuard`].
> > +///
> > +/// This provides access to various methods for [`Connector`] that must happen under lock, such as
> > +/// setting resolution preferences and adding display modes.
> > +///
> > +/// # Invariants
> > +///
> > +/// Shares the invariants of [`ModeConfigGuard`].
> > +#[derive(Copy, Clone)]
> > +pub struct ConnectorGuard<'a, T: DriverConnector>(&'a Connector<T>);
> > +
> > +impl<T: DriverConnector> Deref for ConnectorGuard<'_, T> {
> > +    type Target = Connector<T>;
> > +
> > +    fn deref(&self) -> &Self::Target {
> > +        self.0
> > +    }
> > +}
> > +
> > // SAFETY: DRM expects this struct to be zero-initialized
> > unsafe impl Zeroable for bindings::drm_connector_state {}
> > 
> > -- 
> > 2.46.1
> > 
> 
> — Daniel
> 

-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat

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