drm/amdkfd: fix build error with AMD_IOMMU_V2=m

Using 'imply AMD_IOMMU_V2' does not guarantee that the driver can link
against the exported functions. If the GPU driver is built-in but the
IOMMU driver is a loadable module, the kfd_iommu.c file is indeed
built but does not work:

x86_64-linux-ld: drivers/gpu/drm/amd/amdkfd/kfd_iommu.o: in function `kfd_iommu_bind_process_to_device':
kfd_iommu.c:(.text+0x516): undefined reference to `amd_iommu_bind_pasid'
x86_64-linux-ld: drivers/gpu/drm/amd/amdkfd/kfd_iommu.o: in function `kfd_iommu_unbind_process':
kfd_iommu.c:(.text+0x691): undefined reference to `amd_iommu_unbind_pasid'
x86_64-linux-ld: drivers/gpu/drm/amd/amdkfd/kfd_iommu.o: in function `kfd_iommu_suspend':
kfd_iommu.c:(.text+0x966): undefined reference to `amd_iommu_set_invalidate_ctx_cb'
x86_64-linux-ld: kfd_iommu.c:(.text+0x97f): undefined reference to `amd_iommu_set_invalid_ppr_cb'
x86_64-linux-ld: kfd_iommu.c:(.text+0x9a4): undefined reference to `amd_iommu_free_device'
x86_64-linux-ld: drivers/gpu/drm/amd/amdkfd/kfd_iommu.o: in function `kfd_iommu_resume':
kfd_iommu.c:(.text+0xa9a): undefined reference to `amd_iommu_init_device'
x86_64-linux-ld: kfd_iommu.c:(.text+0xadc): undefined reference to `amd_iommu_set_invalidate_ctx_cb'
x86_64-linux-ld: kfd_iommu.c:(.text+0xaff): undefined reference to `amd_iommu_set_invalid_ppr_cb'
x86_64-linux-ld: kfd_iommu.c:(.text+0xc72): undefined reference to `amd_iommu_bind_pasid'
x86_64-linux-ld: kfd_iommu.c:(.text+0xe08): undefined reference to `amd_iommu_set_invalidate_ctx_cb'
x86_64-linux-ld: kfd_iommu.c:(.text+0xe26): undefined reference to `amd_iommu_set_invalid_ppr_cb'
x86_64-linux-ld: kfd_iommu.c:(.text+0xe42): undefined reference to `amd_iommu_free_device'

Use IS_REACHABLE to only build IOMMU-V2 support if the amd_iommu symbols
are reachable by the amdkfd driver. Output a warning if they are not,
because that may not be what the user was expecting.

Fixes: 64d1c3a4 ("drm/amdkfd: Centralize IOMMUv2 code and make it conditional")
Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdkfd/kfd_iommu.c

@@ -20,6 +20,10 @@
  * OTHER DEALINGS IN THE SOFTWARE.
  */
 
+#include <linux/kconfig.h>
+
+#if IS_REACHABLE(CONFIG_AMD_IOMMU_V2)
+
 #include <linux/printk.h>
 #include <linux/device.h>
 #include <linux/slab.h>
@@ -355,3 +359,5 @@ int kfd_iommu_add_perf_counters(struct kfd_topology_device *kdev)
 
 	return 0;
 }
+
+#endif

drivers/gpu/drm/amd/amdkfd/kfd_iommu.h

@@ -23,7 +23,9 @@
 #ifndef __KFD_IOMMU_H__
 #define __KFD_IOMMU_H__
 
-#if defined(CONFIG_AMD_IOMMU_V2_MODULE) || defined(CONFIG_AMD_IOMMU_V2)
+#include <linux/kconfig.h>
+
+#if IS_REACHABLE(CONFIG_AMD_IOMMU_V2)
 
 #define KFD_SUPPORT_IOMMU_V2
 
@@ -46,6 +48,9 @@ static inline int kfd_iommu_check_device(struct kfd_dev *kfd)
 }
 static inline int kfd_iommu_device_init(struct kfd_dev *kfd)
 {
+#if IS_MODULE(CONFIG_AMD_IOMMU_V2)
+	WARN_ONCE(1, "iommu_v2 module is not usable by built-in KFD");
+#endif
 	return 0;
 }
 
@@ -73,6 +78,6 @@ static inline int kfd_iommu_add_perf_counters(struct kfd_topology_device *kdev)
 	return 0;
 }
 
-#endif /* defined(CONFIG_AMD_IOMMU_V2) */
+#endif /* IS_REACHABLE(CONFIG_AMD_IOMMU_V2) */
 
 #endif /* __KFD_IOMMU_H__ */