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

[Krzysztof Kozlowski <krzk@kernel.org>]

On 14/08/2026 11:49, Vishnu Reddy wrote:
> 
> On 8/14/2026 12:01 PM, Krzysztof Kozlowski wrote:
>> 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?
> 
> No, driver is not buggy here. dma-coherent flag is missed in the dt node.

Again, no... or explain why. Missing dma-coherent is not a fix, but
rather optimisation, but you claimed it is a fix. Explain why.

Best regards,
Krzysztof