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Subject[PATCH 6/6] tty: serial: lpuart: add a more accurate baud rate calculation method
Date
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@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.com>
Cc: Fugang Duan <fugang.duan@nxp.com>
Cc: Stefan Agner <stefan@agner.ch>
Cc: Mingkai Hu <Mingkai.Hu@nxp.com>
Cc: Yangbo Lu <yangbo.lu@nxp.com>
Signed-off-by: Dong Aisheng <aisheng.dong@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 5b485e8..6b2abb7 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
@@ -1508,6 +1510,72 @@ 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)
{
@@ -1613,12 +1681,17 @@ 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")) {
+ 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 */
--
2.7.4
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