[PATCH] gpu: nova-core: replace unsafe pointer cast with pin-init self-reference

[Gary Guo <gary@kernel.org>]

From: Gary Guo <gary@garyguo.net>

Nova core's driver data makes use of a few unsafe self-references for
device resources. Replace them with pin-init's self reference feature
instead.

`auxiliary::Registration` is only allowed to capture a single lifetime, but
it needs to capture `&'gpu Gpu<'bar>`. However, we cannot use coercion here
as `Gpu` is invariant over `'bar`. As we know that `'bar` outlives `'gpu`,
use pin-init's existential lifetime to erase the `'bar` lifetime in
`NovaCoreApi`.

Swap bar1 and bar0's drop order to make pin-init happy.

Signed-off-by: Gary Guo <gary@garyguo.net>
---
 drivers/gpu/nova-core/api.rs    | 15 ++++++++++-----
 drivers/gpu/nova-core/driver.rs | 27 ++++++++-------------------
 2 files changed, 18 insertions(+), 24 deletions(-)

diff --git a/drivers/gpu/nova-core/api.rs b/drivers/gpu/nova-core/api.rs
index 29207c4bb743..62b1c8fe7125 100644
--- a/drivers/gpu/nova-core/api.rs
+++ b/drivers/gpu/nova-core/api.rs
@@ -22,9 +22,14 @@
 };
 
 /// API handle for the auxiliary bus child drivers to interact with nova-core.
-pub struct NovaCoreApi<'bound> {
-    pub(crate) gpu: Pin<&'bound Gpu<'bound>>,
-    pub(crate) pdev: &'bound pci::Device<Bound>,
+#[pin_data]
+pub struct NovaCoreApi<'gpu>
+where
+    exists<'bar>: 'gpu,
+{
+    #[uses('bar: invariant)]
+    pub(crate) gpu: Pin<&'gpu Gpu<'bar>>,
+    pub(crate) pdev: &'gpu pci::Device<Bound>,
 }
 
 impl NovaCoreApi<'_> {
@@ -36,12 +41,12 @@ pub fn of(adev: &auxiliary::Device<Bound>) -> Result<NovaCoreApiHandle<'_>> {
 
     /// Returns the GPU [`GetGspStaticInfoReply`].
     pub fn gsp_static_info(&self) -> &GetGspStaticInfoReply {
-        &self.gpu.gsp_static_info
+        self.with_gpu(|gpu| &gpu.gsp_static_info)
     }
 
     /// Returns the GPU [`Spec`].
     pub fn spec(&self) -> &Spec {
-        &self.gpu.spec
+        self.with_gpu(|gpu| &gpu.spec)
     }
 
     /// Returns the size of the PCIe BAR used for accessing VRAM, typically
diff --git a/drivers/gpu/nova-core/driver.rs b/drivers/gpu/nova-core/driver.rs
index 560a70c4f5a7..7ddde686879f 100644
--- a/drivers/gpu/nova-core/driver.rs
+++ b/drivers/gpu/nova-core/driver.rs
@@ -33,11 +33,13 @@
 #[pin_data]
 pub(crate) struct NovaCore<'bound> {
     #[allow(clippy::type_complexity)]
-    _reg: auxiliary::Registration<'bound, ForLt!(NovaCoreApi<'_>)>,
+    #[uses('gpu: invariant)]
+    _reg: auxiliary::Registration<'gpu, ForLt!(NovaCoreApi<'_>)>,
     #[pin]
-    pub(crate) gpu: Gpu<'bound>,
-    bar: pci::Bar<'bound, BAR0_SIZE>,
+    #[uses('bar1: invariant, 'bar: invariant)]
+    pub(crate) gpu: Gpu<'bar1>,
     bar1: pci::Bar<'bound>,
+    bar: pci::Bar<'bound, BAR0_SIZE>,
 }
 
 pub(crate) struct NovaCoreDriver;
@@ -129,13 +131,10 @@ fn probe<'bound>(
                 crate::irq::doorbell_test::run_selftest(pdev, bar, spec.chipset)?;
             },
 
-            // TODO: Use self-referential pin-init syntax once available.
             gpu <- Gpu::new(
                 pdev,
-                // SAFETY: `bar` is initialized above, pinned, and outlives `gpu`.
-                unsafe { &*core::ptr::from_ref(bar) },
-                // SAFETY: `bar1` is initialized above, pinned, and outlives `gpu`.
-                unsafe { &*core::ptr::from_ref(bar1) },
+                bar,
+                bar1,
             ).pin_chain(|_gpu| {
                 #[cfg(CONFIG_NOVA_CORE_SELFTESTS)]
                 _gpu.run_selftests(pdev);
@@ -143,16 +142,6 @@ fn probe<'bound>(
             }),
 
             _reg: {
-                // TODO: Use `&gpu` self-referential pin-init syntax once available.
-                //
-                // SAFETY: `gpu` is initialized before this expression is evaluated
-                // (`try_pin_init!()` initializes fields in initializer order), lives at
-                // a pinned stable address, and is dropped after `_reg` (struct field
-                // drop order).
-                let gpu = unsafe {
-                    Pin::new_unchecked(&*core::ptr::from_ref(gpu.as_ref().get_ref()))
-                };
-
                 // SAFETY: `NovaCore` is dropped when the device is unbound;
                 // i.e. `mem::forget()` is never called on it.
                 unsafe {
@@ -164,7 +153,7 @@ fn probe<'bound>(
                         // never recycles IDs.
                         AUXILIARY_ID_COUNTER.fetch_add(1, Relaxed),
                         crate::MODULE_NAME,
-                        NovaCoreApi { gpu, pdev },
+                        pin_init!(NovaCoreApi { gpu, pdev }),
                     )?
                 }
             },

base-commit: 2efb7a11f1aa4d2dc285fe2680cca22cb9f72f64
-- 
2.54.0