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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID:
 <AS8PR04MB917638F081F9D6C1F046694C89D62@AS8PR04MB9176.eurprd04.prod.outlook.com>
Date: Mon, 10 Mar 2025 15:43:51 +0000
From: Shenwei Wang <shenwei.wang@....com>
To: Sherry Sun <sherry.sun@....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: Sunday, March 9, 2025 8:52 PM
> To: Shenwei Wang <shenwei.wang@....com>; gregkh@...uxfoundation.org;
> jirislaby@...nel.org
> Cc: linux-serial@...r.kernel.org; linux-kernel@...r.kernel.org;
> imx@...ts.linux.dev
> Subject: RE: [PATCH] tty: serial: fsl_lpuart: disable transmitter before changing
> RS485 related registers
> 
> 
> 
> > -----Original Message-----
> > From: Shenwei Wang <shenwei.wang@....com>
> > Sent: Saturday, March 8, 2025 3:20 AM
> > To: Sherry Sun <sherry.sun@....com>; gregkh@...uxfoundation.org;
> > jirislaby@...nel.org
> > Cc: linux-serial@...r.kernel.org; linux-kernel@...r.kernel.org;
> > 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.

Thanks,
Shenwei

> 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