git.openwrt.org Git - openwrt/staging/blocktrron.git/blob

   1 From 6952209ef220138189dd261d06441e1b2d50e994 Mon Sep 17 00:00:00 2001
   2 From: Weijie Gao <weijie.gao@mediatek.com>
   3 Date: Fri, 23 May 2025 17:26:02 +0800
   4 Subject: [PATCH] serial: mediatek: enable baudrate accuracy compensation
   5
   6 The high-speed UART from MediaTek supports baudrate accuracy
   7 compensation when using high-speed mode 3.
   8
   9 This is done by calculating the first digit of the fraction part of
  10 sample count value. The fraction value will be then used as the
  11 reference to insert 0 to 10 sample cycle(s) to one frame (assume
  12 that frame format is 8n1, i.e. 10 bits per frame).
  13
  14 The fracdiv_[l/m] registers are used to determine whether a bit in one frame
  15 should be inserted with one sample cycle.
  16
  17 With typical 40MHz source clock, the actual baudrates with/without
  18 accuracy compensation are:
  19
  20 Ideal    w/o compensation w/ compensation
  21 ======== ================ ===============
  22 9600     9603             9600
  23 115200   114942           115207
  24 921600   930232           921659
  25 3000000  3076923          3007519
  26
  27 Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>
  28 ---
  29  drivers/serial/serial_mtk.c | 24 +++++++++++++++++++++---
  30  1 file changed, 21 insertions(+), 3 deletions(-)
  31
  32 --- a/drivers/serial/serial_mtk.c
  33 +++ b/drivers/serial/serial_mtk.c
  34 @@ -99,10 +99,18 @@ struct mtk_serial_priv {
  35         bool upstream_highspeed_logic;
  36  };
  37
  38 +static const unsigned short fraction_l_mapping[] = {
  39 +       0, 1, 0x5, 0x15, 0x55, 0x57, 0x57, 0x77, 0x7F, 0xFF, 0xFF
  40 +};
  41 +
  42 +static const unsigned short fraction_m_mapping[] = {
  43 +       0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 3
  44 +};
  45 +
  46  static void _mtk_serial_setbrg(struct mtk_serial_priv *priv, int baud,
  47                                uint clk_rate)
  48  {
  49 -       u32 quot, realbaud, samplecount = 1;
  50 +       u32 quot, realbaud, samplecount = 1, fraction, frac_l = 0, frac_m = 0;
  51
  52         /* Special case for low baud clock */
  53         if (baud <= 115200 && clk_rate == 12000000) {
  54 @@ -147,7 +155,13 @@ use_hs3:
  55                 writel(3, &priv->regs->highspeed);
  56
  57                 quot = DIV_ROUND_UP(clk_rate, 256 * baud);
  58 -               samplecount = DIV_ROUND_CLOSEST(clk_rate, quot * baud);
  59 +               samplecount = clk_rate / (quot * baud);
  60 +
  61 +               fraction = ((clk_rate * 100) / quot / baud) % 100;
  62 +               fraction = DIV_ROUND_CLOSEST(fraction, 10);
  63 +
  64 +               frac_l = fraction_l_mapping[fraction];
  65 +               frac_m = fraction_m_mapping[fraction];
  66         }
  67
  68  set_baud:
  69 @@ -159,7 +173,11 @@ set_baud:
  70
  71         /* set highspeed mode sample count & point */
  72         writel(samplecount - 1, &priv->regs->sample_count);
  73 -       writel((samplecount - 2) >> 1, &priv->regs->sample_point);
  74 +       writel((samplecount >> 1) - 1, &priv->regs->sample_point);
  75 +
  76 +       /* set baudrate fraction compensation */
  77 +       writel(frac_l, &priv->regs->fracdiv_l);
  78 +       writel(frac_m, &priv->regs->fracdiv_m);
  79  }
  80
  81  static int _mtk_serial_putc(struct mtk_serial_priv *priv, const char ch)