Re: [PATCH 0/2] Add rs485 support to uartps driver

[m . brock]

Guntupalli, Manikanta schreef op 2023-05-10 18:26:
> Hi Maarten,
>
> > -----Original Message-----
> > From: m.brock@xxxxxxxxxxxxx <m.brock@xxxxxxxxxxxxx>
> > Sent: Thursday, May 4, 2023 5:52 PM
> > To: Guntupalli, Manikanta <manikanta.guntupalli@xxxxxxx>
> > Cc: gregkh@xxxxxxxxxxxxxxxxxxx; robh+dt@xxxxxxxxxx;
> > krzysztof.kozlowski+dt@xxxxxxxxxx; michal.simek@xxxxxxxxxx; linux-
> > serial@xxxxxxxxxxxxxxx; devicetree@xxxxxxxxxxxxxxx; linux-
> > kernel@xxxxxxxxxxxxxxx; jirislaby@xxxxxxxxxx; linux-arm-
> > kernel@xxxxxxxxxxxxxxxxxxx; Simek, Michal <michal.simek@xxxxxxx>; git
> > (AMD-Xilinx) <git@xxxxxxx>; Pandey, Radhey Shyam
> > <radhey.shyam.pandey@xxxxxxx>; Datta, Shubhrajyoti
> > <shubhrajyoti.datta@xxxxxxx>; Goud, Srinivas <srinivas.goud@xxxxxxx>;
> > manion05gk@xxxxxxxxx
> > Subject: Re: [PATCH 0/2] Add rs485 support to uartps driver
> >
> > Manikanta Guntupalli wrote 2023-04-26 14:29:
> > > Add optional gpio property to uartps node to support rs485 Add rs485
> > > support to uartps driver
> > >
> > > Manikanta Guntupalli (2):
> > >   dt-bindings: Add optional gpio property to uartps node to support
> > >     rs485
> > >   tty: serial: uartps: Add rs485 support to uartps driver
> > >
> > >  .../devicetree/bindings/serial/cdns,uart.yaml |  5 +
> > >  drivers/tty/serial/xilinx_uartps.c            | 96 ++++++++++++++++++-
> > >  2 files changed, 100 insertions(+), 1 deletion(-)
> >
> > Why would you want to use a GPIO and not RTS for choosing the 
> > direction as
> > is more common in this case?
> In ZynqMp platform Cadence UART Controller RTS signal routed to
> external through the PL(Programmable Logic) design not through
> Multiplexed IO.

Then why not route RXD & TXD to the PL as well and connect the module to 
a
PMOD connector connected to the PL? But I admit that a GPIO always works 
as
well.

> > And have you thought about configuring the polarity?
> GPIO polarity configured through device tree property.
>
> &uart0 {
>         ...
>         txrx-gpios = <&gpio 72 GPIO_ACTIVE_LOW>;
>         linux,rs485-enabled-at-boot-time;
> };

Useable, but not honoring 
SER_RS485_RTS_ON_SEND/SER_RS485_RTS_AFTER_SEND.

> > How long will the signal be active before the real transmission begins 
> > so the
> > driver can settle?
> Default is RE(GPIO LOW) and while sending we drive the pin to HIGH. We
> wait for transmission completion, for that we check Transmitter state
> machine active status to ZERO and TX FIFO EMPTY.

How does that take delay_rts_before_send/delay_rts_after_send into 
account?
Not every driver switches direction as fast as you would like.

> Thanks,
> Manikanta.

Greetings,
Maarten