On Thu, 24 Mar 2022 19:02:16 +0100 Halil Pasic pasic@linux.ibm.com wrote:
I'll admit I still never quite grasped the reason for also adding the override to swiotlb_sync_single_for_device() in aa6f8dcbab47, but I think by that point we were increasingly tired and confused and starting to second-guess ourselves (well, I was, at least).
I raised the question, do we need to do the same for swiotlb_sync_single_for_device(). Did that based on my understanding of the DMA API documentation. I had the following scenario in mind
SWIOTLB without the snyc_single: Memory Bounce buffer Owner
start 12345678 xxxxxxxx C dma_map(DMA_FROM_DEVICE) 12345678 -> 12345678 C->D device writes partial data 12345678 12ABC678 <- ABC D sync_for_cpu(DMA_FROM_DEVICE) 12ABC678 <- 12ABC678 D->C cpu modifies buffer 66666666 12ABC678 C sync_for_device(DMA_FROM_DEVICE) 66666666 12ABC678 C->D device writes partial data 66666666 1EFGC678 <-EFG D dma_unmap(DMA_FROM_DEVICE) 1EFGC678 <- 1EFGC678 D->C
Legend: in Owner column C stands for cpu and D for device.
Without swiotlb, I believe we should have arrived at 6EFG6666. To get the same result, IMHO, we need to do a sync in sync_for_device(). And aa6f8dcbab47 is an imperfect solution to that (because of size).
@Robin, Christoph: Do we consider this a valid scenario?
Regards, Halil