[PATCH v5 01/38] gpu: nova-core: fix aux device registration for multi-GPU systems

[John Hubbard <jhubbard@nvidia.com>]

The auxiliary device registration was using a hardcoded ID of 0, which
caused probe() to fail on multi-GPU systems with:

   sysfs: cannot create duplicate filename '/bus/auxiliary/devices/NovaCore.nova-drm.0'

Fix this by using an atomic counter to generate unique IDs for each
GPU's aux device registration. The TODO item to eventually use XArray
for recycling aux device IDs is retained, but for now, this works very
nicely.

This has the side effect of making debugfs[1] work on multi-GPU systems.

[1] https://lore.kernel.org/20260203224757.871729-1-ttabi@nvidia.com

Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 drivers/gpu/nova-core/driver.rs | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/nova-core/driver.rs b/drivers/gpu/nova-core/driver.rs
index 5a4cc047bcfc..fb54f28f6da1 100644
--- a/drivers/gpu/nova-core/driver.rs
+++ b/drivers/gpu/nova-core/driver.rs
@@ -1,5 +1,10 @@
 // SPDX-License-Identifier: GPL-2.0
 
+use core::sync::atomic::{
+    AtomicU32,
+    Ordering, //
+};
+
 use kernel::{
     auxiliary,
     device::Core,
@@ -19,6 +24,9 @@
 
 use crate::gpu::Gpu;
 
+/// Counter for generating unique auxiliary device IDs.
+static AUXILIARY_ID_COUNTER: AtomicU32 = AtomicU32::new(0);
+
 #[pin_data]
 pub(crate) struct NovaCore {
     #[pin]
@@ -85,12 +93,17 @@ fn probe(pdev: &pci::Device<Core>, _info: &Self::IdInfo) -> impl PinInit<Self, E
                 GFP_KERNEL,
             )?;
 
+            // TODO[XARR]: Use XArray for proper ID allocation/recycling. Until then, use a simple
+            // atomic counter which never recycles IDs. A unique ID is required for multi-GPU
+            // systems, because without it, probe() would fail for all but the first GPU.
+            let aux_id = AUXILIARY_ID_COUNTER.fetch_add(1, Ordering::Relaxed);
+
             Ok(try_pin_init!(Self {
                 gpu <- Gpu::new(pdev, bar.clone(), bar.access(pdev.as_ref())?),
                 _reg <- auxiliary::Registration::new(
                     pdev.as_ref(),
                     c"nova-drm",
-                    0, // TODO[XARR]: Once it lands, use XArray; for now we don't use the ID.
+                    aux_id,
                     crate::MODULE_NAME
                 ),
             }))
-- 
2.53.0