Re: [REGRESSION] Recent swiotlb DMA_FROM_DEVICE fixes break ath9k-based AP

[Johannes Berg]

On Sun, 2022-03-27 at 05:15 +0200, Halil Pasic wrote:

> 
> The key here is "sync_sg API, all the parameters must be the same
> as those passed into the single mapping API", but I have to admit,
> I don't understand the *single* in here.
> 

Hah. So I wasn't imagining things after all.

However, as the rest of the thread arrives, this still means it's all
broken ... :)

> The intended meaning of the
> last sentence is that one can do partial sync by choose 
> dma_hande_sync, size_sync such that dma_handle_mapping <= dma_handle_sync
> < dma_handle_mapping + size_mapping and dma_handle_sync + size_sync <=
> dma_handle_mapping + size_mapping. But the direction has to remain the
> same.

Right.

> BTW, the current documented definition of the direction is about the
> data transfer direction between memory and the device, and how the CPU
> is interacting with the memory is not in scope. A quote form the
> documentation.
> 
> """
> ======================= =============================================
> DMA_NONE                no direction (used for debugging)
> DMA_TO_DEVICE           data is going from the memory to the device
> DMA_FROM_DEVICE         data is coming from the device to the memory
> DMA_BIDIRECTIONAL       direction isn't known
> ======================= =============================================
> """
> (Documentation/core-api/dma-api.rst)
> 
> My feeling is, that re-defining the dma direction is not a good idea. But
> I don't think my opinion has much weight here.

However, this basically means that the direction argument to the flush
APIs are completely useless, and we do have to define something
new/else...

johannes