Subject: [PATCH] powerpc/powernv/iommu: iommu incorrectly bypass DMA APIs

From: Gaurav Batra <[email-masked]>
Date: Tue, 31 Mar 2026 17:30:22 -0500

In a PowerNV environment, for devices that supports DMA mask less than
64 bit but larger than 32 bits, iommu is incorrectly bypassing DMA
APIs while allocating and mapping buffers for DMA operations.

Devices are failing with ENOMEN during probe with the following messages

amdgpu 0000:01:00.0: [drm] Detected VRAM RAM=4096M, BAR=4096M
amdgpu 0000:01:00.0: [drm] RAM width 128bits GDDR5
amdgpu 0000:01:00.0: iommu: 64-bit OK but direct DMA is limited by 0
amdgpu 0000:01:00.0: dma_iommu_get_required_mask: returning bypass mask 0xfffffffffffffff
amdgpu 0000:01:00.0:  4096M of VRAM memory ready
amdgpu 0000:01:00.0:  32570M of GTT memory ready.
amdgpu 0000:01:00.0: (-12) failed to allocate kernel bo
amdgpu 0000:01:00.0: [drm] Debug VRAM access will use slowpath MM access
amdgpu 0000:01:00.0: [drm] GART: num cpu pages 4096, num gpu pages 65536
amdgpu 0000:01:00.0: [drm] PCIE GART of 256M enabled (table at 0x000000F4FFF80000).
amdgpu 0000:01:00.0: (-12) failed to allocate kernel bo
amdgpu 0000:01:00.0: (-12) create WB bo failed
amdgpu 0000:01:00.0: amdgpu_device_wb_init failed -12
amdgpu 0000:01:00.0: amdgpu_device_ip_init failed
amdgpu 0000:01:00.0: Fatal error during GPU init
amdgpu 0000:01:00.0: finishing device.
amdgpu 0000:01:00.0: probe with driver amdgpu failed with error -12
amdgpu 0000:01:00.0:  ttm finalized

Fixes: 1471c517cf7d ("powerpc/iommu: bypass DMA APIs for coherent allocations for pre-mapped memory")
Reported-by: Dan Horák <[email-masked]>
Closes: https://gitlab.freedesktop.org/drm/amd/-/work_items/5039
Tested-by: Dan Horak <[email-masked]>
Signed-off-by: Ritesh Harjani <[email-masked]>
Signed-off-by: Gaurav Batra <[email-masked]>
---
 arch/powerpc/kernel/dma-iommu.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/kernel/dma-iommu.c b/arch/powerpc/kernel/dma-iommu.c
index 73e10bd4d56d..8b4de508d2eb 100644
--- a/arch/powerpc/kernel/dma-iommu.c
+++ b/arch/powerpc/kernel/dma-iommu.c
@@ -67,7 +67,7 @@ bool arch_dma_unmap_sg_direct(struct device *dev, struct scatterlist *sg,
 }
 bool arch_dma_alloc_direct(struct device *dev)
 {
-	if (dev->dma_ops_bypass)
+	if (dev->dma_ops_bypass && dev->bus_dma_limit)
 		return true;
 
 	return false;
@@ -75,7 +75,7 @@ bool arch_dma_alloc_direct(struct device *dev)
 
 bool arch_dma_free_direct(struct device *dev, dma_addr_t dma_handle)
 {
-	if (!dev->dma_ops_bypass)
+	if (!dev->dma_ops_bypass || !dev->bus_dma_limit)
 		return false;
 
 	return is_direct_handle(dev, dma_handle);
-- 

I am working on testing the patch in an LPAR with AMDGPU. I will update the
results soon.