diff mbox

serial: imx: half-duplex RS485 operation with RTS active low

Message ID 94f7fcf8873f7607087fc3a59a9422a8@agner.ch (mailing list archive)
State New, archived
Headers show

Commit Message

Stefan Agner April 16, 2018, 1:01 p.m. UTC 
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:

                        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

Comments

Uwe Kleine-König April 16, 2018, 1:42 p.m. UTC | #1 
On Mon, Apr 16, 2018 at 03:01:44PM +0200, Stefan Agner wrote:
> 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

So better use a gpio instead of the hardware-function?

Adding a respective check in the driver would be nice though.

Best regards
Uwe
Stefan Agner April 16, 2018, 4:12 p.m. UTC | #2 
On 16.04.2018 15:42, Uwe Kleine-König wrote:
> On Mon, Apr 16, 2018 at 03:01:44PM +0200, Stefan Agner wrote:
>> 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
> 
> So better use a gpio instead of the hardware-function?

I guess so, did not test that though.

Or use full duplex mode. That is what we used anyways in the past, even
when using a single RS485 bus (half duplex) on the physical layer
anyway.

> 
> Adding a respective check in the driver would be nice though.

Hm, yes agreed. Where would that check go? In probe? Then I guess it
only would warn when using device tree properties...?

--
Stefan
Uwe Kleine-König April 16, 2018, 6:02 p.m. UTC | #3 
Hallo Stefan,

On Mon, Apr 16, 2018 at 06:12:41PM +0200, Stefan Agner wrote:
> On 16.04.2018 15:42, Uwe Kleine-König wrote:
> > Adding a respective check in the driver would be nice though.
> 
> Hm, yes agreed. Where would that check go? In probe? Then I guess it
> only would warn when using device tree properties...?

look at the code between the calls to uart_get_rs485_mode() and
imx_uart_rs485_config(). Something like that would be needed and
imx_uart_rs485_config must adapt the flags accordingly.

Best regards
Uwe
diff mbox

Patch

--- 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)