linux-kernel - Re: [PATCH V3 2/2] tty: serial: uartps: Add rs485 support to uartps driver

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Date:   Sat, 18 Nov 2023 20:35:06 +0100
From:   Lino Sanfilippo <LinoSanfilippo@....de>
To:     "Guntupalli, Manikanta" <manikanta.guntupalli@....com>,
        "git (AMD-Xilinx)" <git@....com>,
        "Simek, Michal" <michal.simek@....com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "krzysztof.kozlowski+dt@...aro.org" 
        <krzysztof.kozlowski+dt@...aro.org>,
        "conor+dt@...nel.org" <conor+dt@...nel.org>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jirislaby@...nel.org" <jirislaby@...nel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Cc:     "Pandey, Radhey Shyam" <radhey.shyam.pandey@....com>,
        "Goud, Srinivas" <srinivas.goud@....com>,
        "Datta, Shubhrajyoti" <shubhrajyoti.datta@....com>,
        "manion05gk@...il.com" <manion05gk@...il.com>
Subject: Re: [PATCH V3 2/2] tty: serial: uartps: Add rs485 support to uartps
 driver

Hi,

On 15.11.23 07:50, Guntupalli, Manikanta wrote:

>> From: Lino Sanfilippo <LinoSanfilippo@....de>
>>
>> So, this will be executed each time (including the rts_before_send delay) the
>> core wants to send data? This is not how it is supposed to work: The tx setup
>> (and the delay before send) has to be done once when transmission starts.
>> Note that when sending a bulk of data the core may call cdns_uart_start_tx()
>> several times before it eventually calls cdns_uart_stop_tx() to stop the
>> transmission.
> We have tested bulk transfer (5000 bytes) and observed that cdns_uart_start_tx() is getting called only once. Can you please suggest how to reproduce the case where the core may call cdns_uart_start_tx() several times before it eventually calls cdns_uart_stop_tx().

Thats strange. Normally the uart_ports start_tx() function is called whenever there is new
data in the tty circular buffer (see uart_write()) and the writing process is suspended after
that (see n_tty_write() in case of N_TTY line discipline). The writing process is woken up via
uart_write_wakeup() called from the uart driver and then it writes new data into the circular
buffer which results in another call to the uart_ports start_tx(). There is no stop_tx() until
all data from the passed userspace buffer is written. But there is a start_tx for every new bulk
of data that is available in the circular buffer.
This is at least what I can observe in my test setup (using a PL011 UART with the amba driver). If
I write a test buffer of 9212 bytes to the tty device using one single write() I can see 10
consecutive calls of tx_start() before tx_stop() eventually is called. What does your test setup look like?

Regards,
Lino