[PATCH V2 6/6] tty: serial: lpuart: add a more accurate baud rate calculation method

[Dong Aisheng]

On Mon, May 15, 2017 at 10:06:41AM -0700, Stefan Agner wrote:
> On 2017-05-15 00:48, Dong Aisheng wrote:
> > On new LPUART versions, the oversampling ratio for the receiver can be
> > changed from 4x (00011) to 32x (11111) which could help us get a more
> > accurate baud rate divider.
> > 
> > The idea is to use the best OSR (over-sampling rate) possible.
> > Note, OSR is typically hard-set to 16 in other LPUART instantiations.
> > Loop to find the best OSR value possible, one that generates minimum
> > baud diff iterate through the rest of the supported values of OSR.
> > 
> > Currently only i.MX7ULP is using it.
> > 
> > Cc: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
> > Cc: Jiri Slaby <jslaby at suse.com>
> > Cc: Stefan Agner <stefan at agner.ch>
> > Cc: Mingkai Hu <Mingkai.Hu at nxp.com>
> > Cc: Yangbo Lu <yangbo.lu at nxp.com>
> > Acked-by: Fugang Duan <fugang.duan at nxp.com>
> > Signed-off-by: Dong Aisheng <aisheng.dong at nxp.com>
> > ---
> >  drivers/tty/serial/fsl_lpuart.c | 85 ++++++++++++++++++++++++++++++++++++++---
> >  1 file changed, 79 insertions(+), 6 deletions(-)
> > 
> > diff --git a/drivers/tty/serial/fsl_lpuart.c b/drivers/tty/serial/fsl_lpuart.c
> > index 107d0911..bda4b0c 100644
> > --- a/drivers/tty/serial/fsl_lpuart.c
> > +++ b/drivers/tty/serial/fsl_lpuart.c
> > @@ -140,6 +140,8 @@
> >  #define UARTBAUD_SBNS		0x00002000
> >  #define UARTBAUD_SBR		0x00000000
> >  #define UARTBAUD_SBR_MASK	0x1fff
> > +#define UARTBAUD_OSR_MASK       0x1f
> > +#define UARTBAUD_OSR_SHIFT      24
> >  
> >  #define UARTSTAT_LBKDIF		0x80000000
> >  #define UARTSTAT_RXEDGIF	0x40000000
> > @@ -1506,6 +1508,72 @@ lpuart_set_termios(struct uart_port *port,
> > struct ktermios *termios,
> >  }
> >  
> >  static void
> > +lpuart32_serial_setbrg(struct lpuart_port *sport, unsigned int baudrate)
> > +{
> > +	u32 sbr, osr, baud_diff, tmp_osr, tmp_sbr, tmp_diff, tmp;
> > +	u32 clk = sport->port.uartclk;
> > +
> > +	/*
> > +	 * The idea is to use the best OSR (over-sampling rate) possible.
> > +	 * Note, OSR is typically hard-set to 16 in other LPUART instantiations.
> > +	 * Loop to find the best OSR value possible, one that generates minimum
> > +	 * baud_diff iterate through the rest of the supported values of OSR.
> > +	 *
> > +	 * Calculation Formula:
> > +	 *  Baud Rate = baud clock / ((OSR+1) × SBR)
> > +	 */
> > +	baud_diff = baudrate;
> > +	osr = 0;
> > +	sbr = 0;
> > +
> > +	for (tmp_osr = 4; tmp_osr <= 32; tmp_osr++) {
> > +		/* calculate the temporary sbr value  */
> > +		tmp_sbr = (clk / (baudrate * tmp_osr));
> > +		if (tmp_sbr == 0)
> > +			tmp_sbr = 1;
> > +
> > +		/*
> > +		 * calculate the baud rate difference based on the temporary
> > +		 * osr and sbr values
> > +		 */
> > +		tmp_diff = clk / (tmp_osr * tmp_sbr) - baudrate;
> > +
> > +		/* select best values between sbr and sbr+1 */
> > +		tmp = clk / (tmp_osr * (tmp_sbr + 1));
> > +		if (tmp_diff > (baudrate - tmp)) {
> > +			tmp_diff = baudrate - tmp;
> > +			tmp_sbr++;
> > +		}
> > +
> > +		if (tmp_diff <= baud_diff) {
> > +			baud_diff = tmp_diff;
> > +			osr = tmp_osr;
> > +			sbr = tmp_sbr;
> > +		}
> > +	}
> > +
> > +	/* handle buadrate outside acceptable rate */
> > +	if (baud_diff > ((baudrate / 100) * 3))
> > +		dev_warn(sport->port.dev,
> > +			 "unacceptable baud rate difference of more than 3%%\n");
> > +
> > +	tmp = lpuart32_read(sport->port.membase + UARTBAUD);
> > +
> > +	if ((osr > 3) && (osr < 8))
> > +		tmp |= UARTBAUD_BOTHEDGE;
> > +
> > +	tmp &= ~(UARTBAUD_OSR_MASK << UARTBAUD_OSR_SHIFT);
> > +	tmp |= (((osr-1) & UARTBAUD_OSR_MASK) << UARTBAUD_OSR_SHIFT);
> > +
> > +	tmp &= ~UARTBAUD_SBR_MASK;
> > +	tmp |= sbr & UARTBAUD_SBR_MASK;
> > +
> > +	tmp &= ~(UARTBAUD_TDMAE | UARTBAUD_RDMAE);
> > +
> > +	lpuart32_write(tmp, sport->port.membase + UARTBAUD);
> > +}
> > +
> > +static void
> >  lpuart32_set_termios(struct uart_port *port, struct ktermios *termios,
> >  		   struct ktermios *old)
> >  {
> > @@ -1611,12 +1679,17 @@ lpuart32_set_termios(struct uart_port *port,
> > struct ktermios *termios,
> >  	lpuart32_write(old_ctrl & ~(UARTCTRL_TE | UARTCTRL_RE),
> >  			sport->port.membase + UARTCTRL);
> >  
> > -	sbr = sport->port.uartclk / (16 * baud);
> > -	bd &= ~UARTBAUD_SBR_MASK;
> > -	bd |= sbr & UARTBAUD_SBR_MASK;
> > -	bd |= UARTBAUD_BOTHEDGE;
> > -	bd &= ~(UARTBAUD_TDMAE | UARTBAUD_RDMAE);
> > -	lpuart32_write(bd, sport->port.membase + UARTBAUD);
> > +	if (of_device_is_compatible(port->dev->of_node, "fsl,imx7ulp-lpuart")) {
> 
> Shouldn't we be consequent here and also use a flag in the soc data
> instead of of_device_is_compatible...?
> 

The original purpose is that this is a temporary code and supposed will
be deleted later once LS platforms confirmed the new baud setting API
works for them as well.

That's why i did not make it a property, as i stated in the cover letter.

> Btw, instead of using 3 bools, I would prefer using a single flags like
> your patchset is proposing for the GPIO driver, what do you think?
> 

Yes, good suggestion.
Probably we could convert the below two.
.is_32 = true,
.is_be = true,

But reg_off seems better to be kept.

Regards
Dong Aisheng

> --
> Stefan
> 
> 
> > +		lpuart32_serial_setbrg(sport, baud);
> > +	} else {
> > +		sbr = sport->port.uartclk / (16 * baud);
> > +		bd &= ~UARTBAUD_SBR_MASK;
> > +		bd |= sbr & UARTBAUD_SBR_MASK;
> > +		bd |= UARTBAUD_BOTHEDGE;
> > +		bd &= ~(UARTBAUD_TDMAE | UARTBAUD_RDMAE);
> > +		lpuart32_write(bd, sport->port.membase + UARTBAUD);
> > +	}
> > +
> >  	lpuart32_write(modem, sport->port.membase + UARTMODIR);
> >  	lpuart32_write(ctrl, sport->port.membase + UARTCTRL);
> >  	/* restore control register */