serial: imx: Prevent TX buffer PIO write when a DMA has been started
Function imx_transmit_buffer starts a TX DMA if DMA is enabled, since commit 91a1a909 ("serial: imx: Support sw flow control in DMA mode"). It also carries on and attempts to write the same TX buffer using PIO. This results in TX data corruption and double-incrementing xmit->tail with the knock-on effect of tail passing head and a page of garbage being sent out. This seems to be triggered mostly when using RS485 half duplex on SMP systems, but is probably not limited to just those. Tested locally on an i.MX6Q with an RS485 half duplex transceiver on UART3, and also by Clemens Gruber. Tested-by: Clemens Gruber <clemens.gruber@pqgruber.com> Signed-off-by: Ian Jamison <ian.dev@arkver.com> Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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