On 16.04.2018 12:29, Stefan Agner wrote: > On 16.04.2018 11:22, Uwe Kleine-König wrote: >> Hi Stefan, >> >> On Mon, Apr 16, 2018 at 11:14:32AM +0200, Stefan Agner wrote: >>> Using upstream I noticed that RS-485 does not work in the default >>> configuration for our platforms (Toradex Apalis/Colibri). Closer >> >> This is an i.MX6? >> > > Yes, sorry about that. > > But I think it is i.MX UART specific, I noticed the same behavior on > i.MX 7 too. > >>> debugging shows that it is related to "serial: imx: default to half >>> duplex rs485". >>> >>> We use the i.MX UART in DTE mode and control the RS-485 transceiver >>> using the RTS signal in low-active mode. >>> >>> uart-has-rtscts; >>> fsl,dte-mode; >>> linux,rs485-enabled-at-boot-time; >>> rs485-rts-active-low; >> >> That means you're not using a GPIO for RTS signaling, right? >> > > I use native RTS capability (which is in DTE mode the CTS signal...) > >>> Using this setting leads to the RTS signal not getting asserted (the >>> oscilloscope only shows a very short fluke before the start bit is >>> sent). > > Just FYI, the fluke looks like a proper assert, but it is really only > 50ns wide. > Also tried DCE mode, the same behavior. Two screenshots showing TX/RTS (CTS_B) in different settings: https://imgur.com/a/PbUex The little fluke disappears when disabling RX before setting CTSC: --- a/drivers/tty/serial/imx.c +++ b/drivers/tty/serial/imx.c @@ -657,6 +657,9 @@ static void imx_uart_start_tx(struct uart_port *port) if (port->rs485.flags & SER_RS485_ENABLED) { u32 ucr2; + if (!(port->rs485.flags & SER_RS485_RX_DURING_TX)) + imx_uart_stop_rx(port); + ucr2 = imx_uart_readl(sport, UCR2); if (port->rs485.flags & SER_RS485_RTS_ON_SEND) imx_uart_rts_active(sport, &ucr2); @@ -664,9 +667,6 @@ static void imx_uart_start_tx(struct uart_port *port) imx_uart_rts_inactive(sport, &ucr2); imx_uart_writel(sport, ucr2, UCR2); - if (!(port->rs485.flags & SER_RS485_RX_DURING_TX)) - imx_uart_stop_rx(port); - /* * Enable transmitter and shifter empty irq only if DMA is off. * In the DMA case this is done in the tx-callback. It seems that if the RX path is disabled, CTS_B is no longer controllable. It just stays high. That is not a problem in the high active RTS case... However, it breaks low active half-duplex... It seems that this thread is describing this situation: https://community.nxp.com/thread/385047 -- Stefan >>> >>> However, using >>> >>> uart-has-rtscts; >>> fsl,dte-mode; >>> linux,rs485-enabled-at-boot-time; >>> rs485-rts-active-low; >>> rs485-rx-during-tx; >>> >>> Asserts the RTS signal low active just fine... >>> >>> Is this a known problem? Any idea where that could come from? It looks >>> as if the receiver part is actually enabling RTS...? >> >> Which kernel version do you use? My latest rs485 related patches went >> into v4.17-rc1. With that I managed to make rs485 half duplex work on >> several customer boards. > > I used v4.17-rc1. > > I noticed your changes, but I it seems they make no difference, last > week I noticed the same issue in v4.16. > > Do those customer boards use DTE mode? > > -- > Stefan > >> >>> Also, isn't enabling RX even in half-duplex mode quite common in order >>> to detect collisions? >> >> I don't know. >> > > Probably also depends on the exact use case and the transceiver > configuration. > > -- > Stefan -- 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
