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

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Message-ID: <90fa8850f9424712074217dff03b229e@agner.ch>
Date:   Wed, 17 May 2017 10:35:43 -0700
From:   Stefan Agner <stefan@...er.ch>
To:     Dong Aisheng <dongas86@...il.com>
Cc:     Dong Aisheng <aisheng.dong@....com>, linux-serial@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        gregkh@...uxfoundation.org, jslaby@...e.com, fugang.duan@....com,
        Mingkai.Hu@....com, yangbo.lu@....com
Subject: Re: [PATCH V2 6/6] tty: serial: lpuart: add a more accurate baud rate
 calculation method

On 2017-05-16 20:47, Dong Aisheng wrote:
> 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@...uxfoundation.org>
>> > Cc: Jiri Slaby <jslaby@...e.com>
>> > Cc: Stefan Agner <stefan@...er.ch>
>> > Cc: Mingkai Hu <Mingkai.Hu@....com>
>> > Cc: Yangbo Lu <yangbo.lu@....com>
>> > Acked-by: Fugang Duan <fugang.duan@....com>
>> > Signed-off-by: Dong Aisheng <aisheng.dong@....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.

Ok, I see that is a good reason to not define a new feature property
now... 

But, if you are reasonable sure it should work, I am inclined to say
just enable it for LS1021a so it also really gets tested...

> 
>> 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.

Sounds good!

--
Stefan

> 
> 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 */