Re: [PATCH v2 09/15] accel/qda: Add DMA-backed GEM objects and memory manager integration

[Ekansh Gupta <ekansh.gupta@oss.qualcomm.com>]

On 24-08-2026 20:37, Rob Clark wrote:
>>> qda_memory_dma.c / qda_memory_dma.h
>>>   DMA coherent allocation backend. qda_dma_alloc() calls
>>>   dma_alloc_coherent() on the CB device and encodes the stream ID
>>>   (SID) in the upper 32 bits of the returned DMA address, following
>>>   the Qualcomm FastRPC convention for IOMMU address space tagging.
>>>   qda_dma_free() strips the SID prefix before calling
>>>   dma_free_coherent().
>>>
>>> qda_memory_manager.c
>>>   Adds process-to-device assignment: each DRM file (process) is
>>>   assigned one IOMMU context bank device for the lifetime of the
>>>   session. qda_memory_manager_assign_device() first checks whether
>>>   the process already has a device (reusing it with a refcount
>>>   increment), then falls back to claiming an unassigned device.
>>>   qda_memory_manager_alloc() and qda_memory_manager_free() delegate
>>>   to the DMA backend after resolving the correct CB device for the
>>>   calling process.
>> Oh, stuff like that is usually a pretty big NO-GO now.
>>
>> AMD has made the same mistake with KFD and it resulted in a massive chaos.
>>
>> The DRM file is the driver context your process uses and even if there are multiple DRM files for the same PID you should absolute *NOT* share anything between them.
>>
>> The general rule of thumb is to not attach anything to the process using the DRM file descriptor.
> A couple other things to keep in mind:
> 
> 1. drm device fd can be passed across processes
> 2. virtgpu/virglrenderer drm native ctx is a single process that opens
> the drm device file N times on behalf of N guest userspace processes.
> 
> All that to say, Christian is right here.. drm_file is your context,
> pgtables should be attached to the drm_file (potentially via
> drm_gpuvm.. which you perhaps want to use).
> 
> BR,
> -R
> 
>> Regards,
>> Christian.
>>

Hi Christian, Rob,

Thank you for highlighting this.

I'll rework the CB assignment to be per-drm_file. Each open() will get a
CB assigned from the available pool (find-first-available, not
find-by-PID). The assigned_pid field and PID-matching logic will be
removed entirely.

The hardware has a limited number of context banks that serve many
sessions (on fastrpc, via the DT "qcom,nsessions" property), so multiple
drm_files may end up sharing the same underlying CB due to hardware
constraints, but while implementing that also, I'll ensure that the
association goes through drm_file->driver_priv, never through a PID scan.

I'll also look into drm_gpuvm for the longer-term address space
management as Rob suggested.

Thanks,
Ekansh