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 07:20:16 +0000
From:   Sherry Sun <sherry.sun@....com>
To:     Michael Walle <michael@...le.cc>
CC:     "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "jirislaby@...nel.org" <jirislaby@...nel.org>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        dl-linux-imx <linux-imx@....com>
Subject: RE: [PATCH 2/2] tty: serial: fsl_lpuart: writing a 1 and then a 0 to
 trigger a break character

> 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.
>

Hi Michael,

Actually set break_ctl(tty, -1) then break_ctl(tty, 0) is only done in the send_break() function.
If we call TIOCSBRK from user space, it will only set break_ctl(tty, -1) without break_ctl(tty, 0).

And from the definition of .break_ctl(port,ctl), the callback is used to Control the transmission of a break signal(Documentation/driver-api/serial/driver.rst), if ctl is nonzero, it should queues a break character. I don't think it is reasonable to call break_ctl() twice in order to send one break signal.

Also I have tried other uart IP, such as drivers/tty/serial/imx.c, it also queues a break character if we call break_ctl() once. So I believe the break_ctl() in lpuart driver should be fixed.

Best regards
Sherry