The dmaengine in Linux significantly simplifies writing of drivers for devices using DMA, especially if they support and use scatter-gather (SG) transfers.
However, there is a problem in case if the length of such transfer is not known a priori. This situation is quite common. It may happen in case of USB transfers, or in case of AXI Stream transfers. In the case of an AXI Stream connected device, the transfer should be terminated by setting the tlast signal to '1'. The driver should prepare the buffer for the longest possible transfer, but after the transfer is complete, it should be able to find the number of actually transferred bytes. Unfortunately it seems, that there is no documented way to read the length of the completed transfer.
The single transfer is represented with an SG-table, that is later converted into the dma_async_tx_descriptor, using the dmaengine_prep_slave_sg function , submitted to the DMA layer (e.g. like here), and finally scheduled for execution via dma_async_issue_pending. After that, the transfer descriptor may be accessed only via a dma_cookie returned by the dmaengine_prep_slave_sg function.
The problem with determining the actual length of the transfer was reported for the USB transfers, and a patch adding the transferred field to the dma_async_tx_descriptor was proposed. However that proposal was rejected, and after a discussion a solution based on using the dmaengine_tx_status, and checking the residue field in the returned dma_tx_state structure.
Unfortunately, it seems that the proposed solution does not work neither in the 4.4 kernel (used for Xilinx SoC devices) nor in the newest 4.7 kernel.
The residue field is set to 0 in case of completed transfers regardless of the actual number of transferred bytes.
So my question is: How can I reliably determine the actual number of transferred bytes in the completed SG DMA transfer in a dmaengine compatible driver?
PS. That question with more Xilinx-specific details related to the AXI DMA IP core has been also asked on the Xilinx forum.