linux-kernel - Re: [PATCH 2/2] tty: serial: fsl_lpuart: writing a 1 and then a 0 to trigger a break character

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Date:   Fri, 15 Jul 2022 08:47:00 +0200
From:   Michael Walle <michael@...le.cc>
To:     Sherry Sun <sherry.sun@....com>
Cc:     gregkh@...uxfoundation.org, jirislaby@...nel.org,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-imx@....com
Subject: Re: [PATCH 2/2] tty: serial: fsl_lpuart: writing a 1 and then a 0 to
 trigger a break character

Hi,

Am 2022-07-15 04:59, schrieb Sherry Sun:
> According to the lpuart reference manual, need to writing a 1 and then 
> a
> 0 to the UARTCTRL_SBK field queues a break character in the transmit
> data stream. Only writing a 1 cannot trigger the break character, so 
> fix
> it.

I don't think this is correct. The tty core will already call this:
   .break_ctl(port, 1)
   usleep()
   .break_ctl(port, 0)

So you'll have your 1->0 transition.

My RM from the LS1028A says the following:

| Writing a 1 and then a 0 to SBK queues a break character in the
| transmit data stream. Additional break characters of 10 to 13,
| or 13 to 16 if LPUART_STATBRK13] is set, bit times of logic 0
| are queued as long as SBK is set. Depending on the timing of
| the set and clear of SBK relative to the information currently
| being transmitted, a second break character may be queued
| before software clears SBK.

To me it seems that setting the SBK bit just pulls the TX line
low and releasing it will return to normal transmitter mode.

-michael