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

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.

> Best regards,
> Krzysztof
>