drm/asahi: mmu: Fix deadlock on remap ops

Signed-off-by: Asahi Lina <lina@asahilina.net>

Author: hoshinolina

drivers/gpu/drm/asahi/mmu.rs

@@ -231,6 +231,8 @@ struct StepContext {
     prev_va: Option<Pin<KBox<gpuvm::GpuVa<VmInner>>>>,
     next_va: Option<Pin<KBox<gpuvm::GpuVa<VmInner>>>>,
     vm_bo: Option<ARef<gpuvm::GpuVmBo<VmInner>>>,
+    prev_vm_bo_1: Option<ARef<gpuvm::GpuVmBo<VmInner>>>,
+    prev_vm_bo_2: Option<ARef<gpuvm::GpuVmBo<VmInner>>>,
     prot: u32,
 }
 
@@ -412,6 +414,17 @@ impl gpuvm::DriverGpuVm for VmInner {
             }
         }
 
+        if ctx.prev_vm_bo_1.is_none() {
+            assert!(ctx.prev_vm_bo_1.replace(vm_bo).is_none());
+        } else if ctx.prev_vm_bo_2.is_none() {
+            assert!(ctx.prev_vm_bo_2.replace(vm_bo).is_none());
+        } else {
+            dev_crit!(
+                self.dev.as_ref(),
+                "step_remap: too many remap ops, deadlock expected"
+            );
+        }
+
         Ok(())
     }
 }