Re: [RFC PATCH] arm64: mm: Add invalidate back in arch_sync_dma_for_device when FROM_DEVICE

[Robin Murphy <robin.murphy@arm.com>]

On 2022-11-17 08:24, Ard Biesheuvel wrote:
> On Thu, 17 Nov 2022 at 07:33, Nanyong Sun <sunnanyong@huawei.com> wrote:
>>
>> The commit c50f11c6196f ("arm64: mm: Don't invalidate FROM_DEVICE
>> buffers at start of DMA transfer") replaces invalidate with clean
>> when DMA_FROM_DEVICE, this changes the behavior of functions like
>> dma_map_single() and dma_sync_single_for_device(*, *, *, DMA_FROM_DEVICE),
>> then it may make some drivers works unwell because the implementation
>> of these DMA APIs lose the original cache invalidation.
>>
>> Situation 1:
> ...
>> Situation 2:
>> After backporting the above commit, we find a network card driver go
>> wrong with cache inconsistency when doing DMA transfer: CPU got the
>> stale data in cache when reading DMA data received from device.
> 
> I suppose this means those drivers may lack dma_sync_single_for_cpu()
> calls after the inbound transfers complete, and are instead relying on
> the cache invalidation performed before the transfer to make the DMA'd
> data visible to the CPU.

It also commonly goes wrong the other way round when the drivers are 
correct but DT/ACPI failed to indicate a coherent device as such. If 
writes from the device actually snoop, they hit the still-present cache 
line, which then gets invalidated by unmap/sync_for_cpu and the new data 
is lost.

Robin.