Re: [PATCH 2/4] arm64: dts: qcom: sc7280: Add dma-coherent property into venus node

[Krzysztof Kozlowski <krzk@kernel.org>]

On 14/08/2026 07:29, Vishnu Reddy wrote:
> 
> On 8/13/2026 2:49 PM, Krzysztof Kozlowski wrote:
>> On Sat, Aug 01, 2026 at 01:07:28PM +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.
>>>
>>> Fixes: 37613aee2179 ("arm64: dts: qcom: sc7280: Add venus DT node")
>> Really does not look like a fix. How skipping a cache sync can fix
>> anything exactly?
> 
> dma-coherent tells the kernel that this device DMA path is hardware-coherent,
> so the SMMU maps its buffers as cacheable. That attribute is what causes the
> coherent interconnect to snoop CPU cache lines on every VPU transaction, so
> hardware keeps CPU caches and VPU-visible memory in sync — which is why the
> dma-coherent property lets us skip the unnecessary cache clean/invalidate.

You just repeated what I wrote. What for?

> 
> If a device doesn't support I/O coherence, its buffers would instead be mapped
> non-cacheable, no snooping would occur, and the driver would need explicit
> dma_sync_*() calls at each transition. Venus on SC7280 does support I/O
> coherence, so dma-coherent describes that existing hardware capability to
> the DMA subsystem.

How does this anyhow explain your commit msg? Are you saying driver is
buggy that it misses sync calls?

Best regards,
Krzysztof