Re: [PATCH 1/4] dt-bindings: media: qcom,sc7280-venus: Add dma-coherent property

[Krzysztof Kozlowski]

On Sat, Aug 01, 2026 at 01:07:27PM +0530, Vishnu Reddy wrote:
> While testing with some higher resolution clips, the venus hardware
> triggers a fault due to wrong input data being received. Corruption
> was also observed in the captured output when the client dumped it
> to a file.
> 
> On debugging, this was traced to the venus node not declaring
> dma-coherent. As a result, DMA buffers shared between the CPU and the
> venus video hardware/controller are not guaranteed to be I/O coherent:
> CPU writes to an input buffer can remain in CPU caches without being
> visible to the video hardware when it reads the same buffer, so the
> hardware receives input data that does not match what the CPU wrote.
> Likewise, on the capture path, data written by the video hardware to
> the output buffer may not be visible to the CPU, so the client reads
> stale or partial data, resulting in corruption.
> 
> Add the dma-coherent property to the venus node so that DMA buffers
> shared between the CPU and the video hardware and controller remain
> coherent.

This is confusing - dma-coherent saves you from syncing the buffers, so
how could it solve the issue? You basically skip certain cache sync
operations, not add them.

Best regards,
Krzysztof