On Fri, Aug 15, 2014 at 09:27:59PM +0200, Sebastian Andrzej Siewior wrote: > If you want to change this to reduce the gap, then you have first > change 8250 core code. Currently it waits until the shift register is > empty. Oh the 8250 normally works this way? I didn't know that. > On the other hand if you use DMA then it can handle transfers > 64bytes > in one go and you can start transfers while the FIFO is not completely > empty. You can dma more than the fifo size? > If you use DMA. You program one transfer says 100 bytes. You get an > dma-transfer complete once the 100 bytes are transfered which means the > FIFO has 63 bytes. From this point on you could enqueue the next > transfer with say another 100 bytes. In that scenario you don't see the > gap. > > You get only to the gap if you use the non-DMA mode (and not UARTs > support DMA). In that case, yes waiting till there only 16 bytes before > starting the refill would make sense if you want to utilize the port by > 100%. But as I said in 0/15, you need to teach the core this first. > Otherwise it will return doing nothing until the shift register is > empty (i.e. until the FIFO is completely empty). Well if DMA takes care of it, and the normal 8250 is already like this, then I suppose it is already better than the typical case. > There is patch in Greg's tty tree already where you are able to > configure the RX trigger level. We could wire this up once we agree > which levels we want support. The OMAP supports all levels from 1…63. All? or just every 4 (that's what I just read in the DRA7xx docs). > Yes, true. However this is only an issue without HW control. With DMA > the buffer is slightly larger. The DMA engine starts the transfer on > its own once there 48 bytes in the FIFO (except in the few cases where > it does not). That's nice of it. I will have to give this a try. -- Len Sorensen -- To unsubscribe from this list: send the line "unsubscribe linux-omap" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
