Mason <slash.tmp@xxxxxxx> writes: > On 08/12/2016 13:44, Måns Rullgård wrote: > >> Mason <slash.tmp@xxxxxxx> writes: >> >>> On 08/12/2016 13:20, Måns Rullgård wrote: >>> >>>> The only problem we have is that nobody envisioned hardware where the >>>> dma engine indicates completion slightly too soon. I suspect there's a >>>> fifo or such somewhere, and the interrupt is triggered when the last >>>> byte has been placed in the fifo rather than when it has been removed >>>> which would have been more correct. >>> >>> As I (tried to) explain here: >>> https://marc.info/?l=dmaengine&m=148007808418242&w=2 >>> >>> A *read* MBUS agent raises its IRQ when it is safe for the memory >>> to be overwritten (i.e. every byte has been pushed into the pipe). >>> >>> A *write* MBUS agent raises its IRQ when it is safe for another >>> agent to read any one of the transferred bytes. >>> >>> The issue comes from the fact that, for a memory-to-device transfer, >>> the system will receive the read agent's IRQ, but most devices >>> (NFC, SATA) don't have an IRQ line to signal that their part of the >>> operation is complete. >> >> SATA does, actually. Nevertheless, it's an unusual design. > > Thanks, I was mistaken about the SATA controller. > > On tango3 (and also tango4, I assume) > > IRQ 41 = Serial ATA #0 > IRQ 42 = Serial ATA DMA #0 > IRQ 54 = Serial ATA #1 > IRQ 55 = Serial ATA DMA #1 > > But in the end, whether there is a device interrupt (SATA) > or not (NFC), for a memory-to-device transfer, the DMA > driver will get the read agent notification (which should > be ignored) and the client driver should either spin until > idle (NFC) or wait for its completion IRQ (SATA). > > Correct? Yes, and when the client device is finished, the driver needs to signal this to the dma driver so it can reuse the channel. It's this last piece that's missing. -- Måns Rullgård -- To unsubscribe from this list: send the line "unsubscribe dmaengine" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
