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

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Message-ID: <309c98bbe25b8493ac35c8da97f9bff1@vanmierlo.com>
Date:   Sun, 14 May 2023 13:01:10 +0200
From:   m.brock@...mierlo.com
To:     "Guntupalli, Manikanta" <manikanta.guntupalli@....com>
Cc:     gregkh@...uxfoundation.org, robh+dt@...nel.org,
        krzysztof.kozlowski+dt@...aro.org, michal.simek@...inx.com,
        linux-serial@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, jirislaby@...nel.org,
        linux-arm-kernel@...ts.infradead.org,
        "Simek, Michal" <michal.simek@....com>,
        "git (AMD-Xilinx)" <git@....com>,
        "Pandey, Radhey Shyam" <radhey.shyam.pandey@....com>,
        "Datta, Shubhrajyoti" <shubhrajyoti.datta@....com>,
        "Goud, Srinivas" <srinivas.goud@....com>, manion05gk@...il.com
Subject: Re: [PATCH 0/2] Add rs485 support to uartps driver

Guntupalli, Manikanta schreef op 2023-05-10 18:26:
> Hi Maarten,
> 
>> -----Original Message-----
>> From: m.brock@...mierlo.com <m.brock@...mierlo.com>
>> Sent: Thursday, May 4, 2023 5:52 PM
>> To: Guntupalli, Manikanta <manikanta.guntupalli@....com>
>> Cc: gregkh@...uxfoundation.org; robh+dt@...nel.org;
>> krzysztof.kozlowski+dt@...aro.org; michal.simek@...inx.com; linux-
>> serial@...r.kernel.org; devicetree@...r.kernel.org; linux-
>> kernel@...r.kernel.org; jirislaby@...nel.org; linux-arm-
>> kernel@...ts.infradead.org; Simek, Michal <michal.simek@....com>; git
>> (AMD-Xilinx) <git@....com>; Pandey, Radhey Shyam
>> <radhey.shyam.pandey@....com>; Datta, Shubhrajyoti
>> <shubhrajyoti.datta@....com>; Goud, Srinivas <srinivas.goud@....com>;
>> manion05gk@...il.com
>> 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