Messages in this thread | | | From | Andy Shevchenko <> | Date | Sun, 28 May 2017 03:04:15 +0300 | Subject | Re: [PATCH 6/6] tty: serial: lpuart: add a more accurate baud rate calculation method |
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On Tue, May 9, 2017 at 10:50 AM, Dong Aisheng <aisheng.dong@nxp.com> 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.
> +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++) {
I _think_ you may simplify this and avoid for-loop if you reconsider approach.
> + /* 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;
(32 - 4 + 1) times division is called...
> + > + /* 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");
Shouldn't you fall back to previous setting?
> + > + tmp = lpuart32_read(sport->port.membase + UARTBAUD); > +
> + if ((osr > 3) && (osr < 8))
Isn't it
if (osr ^ BIT(2) < BIT(2))
?
> + tmp |= UARTBAUD_BOTHEDGE;
> +}
> + if (of_device_is_compatible(port->dev->of_node, "fsl,imx7ulp-lpuart")) { > + 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); > + }
Perhaps it makes sense to split this to a helper function as well (in a separate patch).
-- With Best Regards, Andy Shevchenko
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