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

Madhavan Srinivasan
2026-04-01T04:29:36Z

On 4/1/26 6:35 AM, Ritesh Harjani (IBM) wrote:
> ++CC few people who would be interested in this fix.
>
> Gaurav Batra <gbatra@linux.ibm.com> writes:
>
>> 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 <dan@danny.cz>
>> Closes: https://gitlab.freedesktop.org/drm/amd/-/work_items/5039
> We could even add the lore link so that in future people can find the
> discussion.
> Closes: https://lore.kernel.org/linuxppc-dev/20260313142351.609bc4c3efe1184f64ca5f44@danny.cz/
>
>> Tested-by: Dan Horak <dan@danny.cz>
>> Signed-off-by: Ritesh Harjani <ritesh.list@gmail.com>
> Feel free to change this to the following, I mainly only suggested the fix :)
>
> Suggested-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
>
>
>> Signed-off-by: Gaurav Batra <gbatra@linux.ibm.com>
>> ---
> Generally this info...
>> I am working on testing the patch in an LPAR with AMDGPU. I will update the
>> results soon.
> ... goes below the three dashes in here ^^^
> This is the same place where people also update the patch change log.
>
> But sure, thanks for updating. As for this patch, it looks good to me.
> So, feel free to add:
>
> Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
I will fix the commit message changes and add it to my fixes-test
since we are at the end of -rc7 window, need to get this out soon

Maddy

>
>
>> 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.
> This should go up, as mentioned above.
>
> -ritesh