On Thu, Feb 2, 2017 at 8:22 AM, Clemens Gruber <clemens.gruber@xxxxxxxxxxxx> wrote: > On Thu, Feb 02, 2017 at 07:40:53AM -0800, Tim Harvey wrote: >> Something I noticed this time around while testing however is that in >> some cases I'm not getting the dma_tx_callback telling us the tx dma >> completed causing the stop_tx to return and the TXEN to not be >> de-asserted. I hadn't noticed this before. Have you encountered it in >> your use/testing? > > Hi Tim, > > a few weeks ago, Fabio and I discussed a DMA-related problem I have with > RS485 on an i.MX6Q board. > > Are you observing this behavior you described on an i.MX6Q? The board I have and am seeing this on is an IMX6Q, but the customer board that I'm looking into this for is a DL so I think the issue is not just IMX6Q > > As a quick workaround, I changed the imx driver to not enable DMA. > (I commented out the call to imx_uart_dma_init in imx_startup) > Yes, this is how I worked around the issue currently. > For the full discussion, see: https://lkml.org/lkml/2017/1/4/579 > > TL/DR: Sending a character leads to the whole circular buffer being sent > and TXEN is asserted during that transmission, which takes about 4 > seconds for 9600 baud. Here is how it looks on my logic analyzer: > https://pqgruber.com/rs485_results.png Yes - exactly what I'm seeing as well. > > It looks like it can only be reproduced on SMP systems, Fabio had only a > i.MX6S available and could not reproduce it. > > What's interesting: If the transmit-complete interrupt is disabled > (UCR4_TCEN), the data on TXD is fine. > I'll have to check this > Somehow, the actions occuring after the transmit-complete interrupt, in > the transmit_buffer function, lead to the the circular buffer struct's > xmit->tail to jump over xmit->head. > So instead of 2 bytes I wanted to send, additional 4094 bytes are sent > out, during which TXEN stays asserted. > > Could this be the same issue you are observing now? Yes - same issue Tim -- To unsubscribe from this list: send the line "unsubscribe linux-serial" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
