[PATCH v6 6/7] gpu: nova-core: create debugfs root in module init

[Timur Tabi <ttabi@nvidia.com>]

Create the 'nova_core' root debugfs entry when the driver loads.

Normally, non-const global variables need to be protected by a
mutex.  Instead, we use unsafe code, as we know the entry is never
modified after the driver is loaded.  This solves the lifetime
issue of the mutex guard, which would otherwise have required the
use of `pin_init_scope`.

Signed-off-by: Timur Tabi <ttabi@nvidia.com>
---
 drivers/gpu/nova-core/Kconfig      | 13 +++++++++++++
 drivers/gpu/nova-core/nova_core.rs | 28 ++++++++++++++++++++++++++++
 2 files changed, 41 insertions(+)

diff --git a/drivers/gpu/nova-core/Kconfig b/drivers/gpu/nova-core/Kconfig
index 527920f9c4d3..974c5d08f6de 100644
--- a/drivers/gpu/nova-core/Kconfig
+++ b/drivers/gpu/nova-core/Kconfig
@@ -14,3 +14,16 @@ config NOVA_CORE
 	  This driver is work in progress and may not be functional.
 
 	  If M is selected, the module will be called nova_core.
+
+config NOVA_CORE_DEBUGFS
+	bool "Nova Core debugfs support"
+	depends on NOVA_CORE
+	depends on DEBUG_FS
+	default y
+	help
+	  Enable debugfs support for the Nova Core driver. This exposes
+	  debugging information and log buffers via the debugfs filesystem.
+
+	  Each GPU will have an entry under /sys/kernel/debug/nova_core.
+
+	  If unsure, say Y.
diff --git a/drivers/gpu/nova-core/nova_core.rs b/drivers/gpu/nova-core/nova_core.rs
index 80ecbb50ec82..582b03013ebc 100644
--- a/drivers/gpu/nova-core/nova_core.rs
+++ b/drivers/gpu/nova-core/nova_core.rs
@@ -3,6 +3,7 @@
 //! Nova Core GPU Driver
 
 use kernel::{
+    debugfs,
     driver::Registration,
     pci,
     prelude::*,
@@ -27,16 +28,43 @@
 
 pub(crate) const MODULE_NAME: &kernel::str::CStr = <LocalModule as kernel::ModuleMetadata>::NAME;
 
+// FIXME: Move this into per-module data once that exists
+static mut DEBUGFS_ROOT: Option<debugfs::Dir> = None;
+
+/// Guard that clears DEBUGFS_ROOT when dropped.
+struct DebugfsRootGuard;
+
+impl Drop for DebugfsRootGuard {
+    fn drop(&mut self) {
+        // SAFETY: This guard is dropped after _driver (due to field order),
+        // so the driver is unregistered and no probe() can be running.
+        unsafe { DEBUGFS_ROOT = None };
+    }
+}
+
 #[pin_data]
 struct NovaCoreModule {
+    // Fields are dropped in declaration order, so _driver is dropped first,
+    // then _debugfs_guard clears DEBUGFS_ROOT.
     #[pin]
     _driver: Registration<pci::Adapter<driver::NovaCore>>,
+    _debugfs_guard: DebugfsRootGuard,
 }
 
 impl InPlaceModule for NovaCoreModule {
     fn init(module: &'static kernel::ThisModule) -> impl PinInit<Self, Error> {
+        #[cfg(CONFIG_NOVA_CORE_DEBUGFS)]
+        {
+            let dir = debugfs::Dir::new(kernel::c_str!("nova_core"));
+
+            // SAFETY: We are the only driver code running during init, so there
+            // cannot be any concurrent access to `DEBUGFS_ROOT`.
+            unsafe { DEBUGFS_ROOT = Some(dir) };
+        }
+
         try_pin_init!(Self {
             _driver <- Registration::new(MODULE_NAME, module),
+            _debugfs_guard: DebugfsRootGuard,
         })
     }
 }
-- 
2.52.0