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

[Lino Sanfilippo <LinoSanfilippo@gmx.de>]

Hi,

On 15.11.23 07:50, Guntupalli, Manikanta wrote:

>> From: Lino Sanfilippo <LinoSanfilippo@gmx.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