Vinod Koul <vinod.koul@xxxxxxxxx> writes: > On Thu, Dec 08, 2016 at 12:20:30PM +0000, Måns Rullgård wrote: >> > >> > I'm far from claiming that drivers/tty/serial/sh-sci.c is perfect, but >> > it does request DMA channels at open time, not at probe time. >> >> In the part quoted above, I said most drivers request dma channels in >> their probe or open functions. For the purposes of this discussion, >> that distinction is irrelevant. In either case, the channel is held >> indefinitely. > > And the answer was it is wrong and not _all_ do that!! Show me one that doesn't. >> If this wasn't the correct way to use the dmaengine, >> there would be no need for the virt-dma helpers which are specifically >> designed for cases the one currently at hand. > > That is incorrect. > > virt-dma helps to have multiple request from various clients. For many > controllers which implement a SW mux, they can transfer data for one client > and then next transfer can be for some other one. > > This allows better utilization of dma channels and helps in case where we > have fewer dma channels than users. Which is *exactly* the situation we have here. I have no idea what you're arguing for or against any more. Perhaps you're just arguing. >> 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. > > That is pretty common hardware optimization but usually hardware shows up > with flush commands to let in flight transactions be completed. Well, this hardware seems to lack that particular feature. Pretending otherwise isn't helping. -- 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
