Hey all, I sent nearly this same message to the "kernelnewbies" mailing list (kernelnewbies@xxxxxxxxxxxxxxxxx) in hopes to get some response there. I haven't heard anything back and I see this mailing list referenced in various documentation. I hope this message is appropriate for this audience. As a newbie, I'm trying to figure out how to properly deal with timeouts after dma_engine_submit(). My intent is to build a new "device" driver using Xilinx's AXIDMA driver linked below. Xilinx provides a couple references called "dma-proxy" and "axidmatest" that exercise the DMA Engine interop with their driver. I think I understand this layering correctly, but I'm pretty new to the DMA Engine framework. xilinx dma driver: https://github.com/Xilinx/linux-xlnx/blob/master/drivers/dma/xilinx/xilinx_dma.c dma-proxy driver: https://github.com/Xilinx-Wiki-Projects/software-prototypes/blob/master/linux-user-space-dma/Software/Kernel/dma-proxy.c xilinx dmatest driver: https://github.com/Xilinx/linux-xlnx/blob/master/drivers/dma/xilinx/axidmatest.c Using the referenced "dma-proxy" as an example, I'm tracing the case where there is a timeout. In my "device" case, I want some data from a DMA slave (DEV_TO_MEM) but it may never come. I'm mentally treating this as a "socket" but I understand I may have retool my mental model... First, this chain eventually resolves into `dma_engine_submit()` via dma_device->device_prep_slave_sg(): ----------------------------------------------------------------- (~ https://github.com/Xilinx-Wiki-Projects/software-prototypes/blob/master/linux-user-space-dma/Software/Kernel/dma-proxy.c#L198) sg_init_table(..., 1); sg_dma_address(... ) = foo.dma_handle; sg_dma_len(...) = foo.length; chan_desc = dma_device->device_prep_slave_sg(..., ..., 1, ..., ..., NULL); if (! chan_desc) { printk(KERN_ERR "dmaengine_prep*() error\n"); else { ... } Then, the driver waits for the completion and prints an error if it cannot complete: ------------------------------------------------------------------------------------------------------------------- unsigned long timeout = msecs_to_jiffies(3000); timeout = wait_for_completion_timeout(foo.cmp, timeout); status = dma_async_is_tx_complete(..., ..., NULL, NULL); if (timeout == 0) { printk(KERN_ERR "DMA timed out\n"); } else { ... } ====== I cannot figure out what to do in the case of a timeout. It appears descriptors (`chan_desc`) are being leaked when completion cannot be completed. I see some patches to make the list of descriptors larger but it appears the default/configured is 255. So if I sit and timeout for 3sec * 255, I run out of descriptors and that DMA engine instance is no longer usable. Both this dma-proxy and dmatest driver don't seem to handle any sort of failure cleanup. I hope someone can point me in the right direction so I can timeout nicely. Maybe I need to switch to polling or something like this. Any information is helpful. Thanks!
