linux-kernel - RE: [PATCH] tty: serial: fsl_lpuart: disable transmitter before changing RS485 related registers

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Message-ID:
 <DB9PR04MB84290573E5B49798799E028C92D12@DB9PR04MB8429.eurprd04.prod.outlook.com>
Date: Tue, 11 Mar 2025 01:58:52 +0000
From: Sherry Sun <sherry.sun@....com>
To: Shenwei Wang <shenwei.wang@....com>, "gregkh@...uxfoundation.org"
	<gregkh@...uxfoundation.org>, "jirislaby@...nel.org" <jirislaby@...nel.org>
CC: "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"imx@...ts.linux.dev" <imx@...ts.linux.dev>
Subject: RE: [PATCH] tty: serial: fsl_lpuart: disable transmitter before
 changing RS485 related registers



> > >
> > > > -----Original Message-----
> > > > From: Sherry Sun <sherry.sun@....com>
> > > > Sent: Thursday, March 6, 2025 8:20 PM
> > > > To: gregkh@...uxfoundation.org; jirislaby@...nel.org
> > > > Cc: linux-serial@...r.kernel.org; linux-kernel@...r.kernel.org;
> > > > imx@...ts.linux.dev; Shenwei Wang <shenwei.wang@....com>
> > > > Subject: [PATCH] tty: serial: fsl_lpuart: disable transmitter
> > > > before changing RS485 related registers
> > > >
> > > > According to the LPUART reference manual, TXRTSE and TXRTSPOL of
> > > MODIR
> > > > register only can be changed when the transmitter is disabled.
> > > > So disable the transmitter before changing RS485 related registers
> > > > and re-enable it after the change is done.
> > > >
> > > > Fixes: 67b01837861c ("tty: serial: lpuart: Add RS485 support for
> > > > 32-bit uart
> > > > flavour")
> > > > Signed-off-by: Sherry Sun <sherry.sun@....com>
> > > > ---
> > > >  drivers/tty/serial/fsl_lpuart.c | 14 ++++++++++++++
> > > >  1 file changed, 14 insertions(+)
> > > >
> > > > diff --git a/drivers/tty/serial/fsl_lpuart.c
> > > > b/drivers/tty/serial/fsl_lpuart.c index
> > > > 91d02c55c470..4dc2f3e2b8e0 100644
> > > > --- a/drivers/tty/serial/fsl_lpuart.c
> > > > +++ b/drivers/tty/serial/fsl_lpuart.c
> > > > @@ -1484,6 +1484,16 @@ static int lpuart32_config_rs485(struct
> > > > uart_port *port, struct ktermios *termio
> > > >
> > > >  	unsigned long modem = lpuart32_read(&sport->port, UARTMODIR)
> > > >  				& ~(UARTMODIR_TXRTSPOL |
> > > > UARTMODIR_TXRTSE);
> > > > +	u32 ctrl;
> > > > +
> > > > +	/* TXRTSE and TXRTSPOL only can be changed when transmitter is
> > > > disabled. */
> > > > +	ctrl = lpuart32_read(&sport->port, UARTCTRL);
> > > > +	if (ctrl & UARTCTRL_TE) {
> > > > +		/* wait transmit engin complete */
> > > > +		lpuart32_wait_bit_set(&sport->port, UARTSTAT,
> > > UARTSTAT_TC);
> > > > +		lpuart32_write(&sport->port, ctrl & ~UARTCTRL_TE,
> > > > UARTCTRL);
> > >
> > > Since there may be a delay between writing to the register and the
> > > UARTCTRL_TE bit actually changing to 0, we should poll the UARTCTRL
> > > register and verify that UARTCTRL_TE has really become 0 before
> proceeding.
> > > Otherwise, subsequent operations would still execute while the
> > > UARTCTRL_TE bit remains in the status of 1, which is not the
> > > intention of this
> > patch.
> > >
> >
> > Hi Shenwei,
> >
> > Description of TE bit in LPUART RM: "After this field becomes 0, the
> > field reads 1 until the transmitter has completed the current
> > character and the TXD pin is tristated".
> > I added the UARTSTAT_TC status check to make sure the transmitter has
> > completed, not sure if it is reasonable to add the TE bit poll read,
> > since usually we poll read the status register bits instead of the control bits.
> >
> 
> Sorry, had a typo in the last mail. Fixed below.
> 
> I am not sure if the condition "TE==0" is equivalent to "TC==1" in this context.
> If you confirm it, I am fine with the patch.


Hi Shenwei,
It seems that "TE==0" is not completely equivalent to "TC==1", I will add the TE polling read check in V2, thanks for the comment.

Best Regards
Sherry

> >
> > > Thanks,
> > > Shenwei
> > >
> > > > +	}
> > > > +
> > > >  	lpuart32_write(&sport->port, modem, UARTMODIR);
> > > >
> > > >  	if (rs485->flags & SER_RS485_ENABLED) { @@ -1503,6 +1513,10 @@
> > > > static int lpuart32_config_rs485(struct uart_port *port, struct
> > > > ktermios
> > > *termio
> > > >  	}
> > > >
> > > >  	lpuart32_write(&sport->port, modem, UARTMODIR);
> > > > +
> > > > +	if (ctrl & UARTCTRL_TE)
> > > > +		lpuart32_write(&sport->port, ctrl, UARTCTRL);
> > > > +
> > > >  	return 0;
> > > >  }
> > > >
> > > > --
> > > > 2.34.1