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Date: Sun, 28 May 2017 03:04:15 +0300 From: Andy Shevchenko <andy.shevchenko@...il.com> To: Dong Aisheng <aisheng.dong@....com> Cc: "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, linux-arm Mailing List <linux-arm-kernel@...ts.infradead.org>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Jiri Slaby <jslaby@...e.com>, fugang.duan@....com, Stefan Agner <stefan@...er.ch>, Mingkai.Hu@....com, yangbo.lu@....com Subject: Re: [PATCH 6/6] tty: serial: lpuart: add a more accurate baud rate calculation method On Tue, May 9, 2017 at 10:50 AM, Dong Aisheng <aisheng.dong@....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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