mvebu: Backport RTC trimming support.

This enables a higher precision mode for the RTC.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

diff --git a/target/linux/mvebu/patches-4.14/420-rtc-trimming-support.patch b/target/linux/mvebu/patches-4.14/420-rtc-trimming-support.patch
new file mode 100644 (file)
index 0000000..2974a69
--- /dev/null
+++ b/target/linux/mvebu/patches-4.14/420-rtc-trimming-support.patch
@@ -0,0 +1,143 @@
+commit f94ffbc2c2a4128c4412bb483d0807722dfb682b
+Author: Russell King <rmk+kernel@armlinux.org.uk>
+Date:   Fri Sep 29 11:23:31 2017 +0100
+
+    rtc: armada38x: add support for trimming the RTC
+    
+    Add support for trimming the RTC using the offset mechanism.  This RTC
+    supports two modes: low update mode and high update mode.  Low update
+    mode has finer precision than high update mode, so we use the low mode
+    where possible.
+    
+    Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
+    Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
+
+--- a/drivers/rtc/rtc-armada38x.c
++++ b/drivers/rtc/rtc-armada38x.c
+@@ -28,6 +28,8 @@
+ #define RTC_IRQ_AL_EN             BIT(0)
+ #define RTC_IRQ_FREQ_EN                   BIT(1)
+ #define RTC_IRQ_FREQ_1HZ          BIT(2)
++#define RTC_CCR                   0x18
++#define RTC_CCR_MODE              BIT(15)
+ 
+ #define RTC_TIME          0xC
+ #define RTC_ALARM1        0x10
+@@ -343,18 +345,117 @@ static irqreturn_t armada38x_rtc_alarm_i
+       return IRQ_HANDLED;
+ }
+ 
++/*
++ * The information given in the Armada 388 functional spec is complex.
++ * They give two different formulas for calculating the offset value,
++ * but when considering "Offset" as an 8-bit signed integer, they both
++ * reduce down to (we shall rename "Offset" as "val" here):
++ *
++ *   val = (f_ideal / f_measured - 1) / resolution   where f_ideal = 32768
++ *
++ * Converting to time, f = 1/t:
++ *   val = (t_measured / t_ideal - 1) / resolution   where t_ideal = 1/32768
++ *
++ *   =>  t_measured / t_ideal = val * resolution + 1
++ *
++ * "offset" in the RTC interface is defined as:
++ *   t = t0 * (1 + offset * 1e-9)
++ * where t is the desired period, t0 is the measured period with a zero
++ * offset, which is t_measured above. With t0 = t_measured and t = t_ideal,
++ *   offset = (t_ideal / t_measured - 1) / 1e-9
++ *
++ *   => t_ideal / t_measured = offset * 1e-9 + 1
++ *
++ * so:
++ *
++ *   offset * 1e-9 + 1 = 1 / (val * resolution + 1)
++ *
++ * We want "resolution" to be an integer, so resolution = R * 1e-9, giving
++ *   offset = 1e18 / (val * R + 1e9) - 1e9
++ *   val = (1e18 / (offset + 1e9) - 1e9) / R
++ * with a common transformation:
++ *   f(x) = 1e18 / (x + 1e9) - 1e9
++ *   offset = f(val * R)
++ *   val = f(offset) / R
++ *
++ * Armada 38x supports two modes, fine mode (954ppb) and coarse mode (3815ppb).
++ */
++static long armada38x_ppb_convert(long ppb)
++{
++      long div = ppb + 1000000000L;
++
++      return div_s64(1000000000000000000LL + div / 2, div) - 1000000000L;
++}
++
++static int armada38x_rtc_read_offset(struct device *dev, long *offset)
++{
++      struct armada38x_rtc *rtc = dev_get_drvdata(dev);
++      unsigned long ccr, flags;
++      long ppb_cor;
++
++      spin_lock_irqsave(&rtc->lock, flags);
++      ccr = rtc->data->read_rtc_reg(rtc, RTC_CCR);
++      spin_unlock_irqrestore(&rtc->lock, flags);
++
++      ppb_cor = (ccr & RTC_CCR_MODE ? 3815 : 954) * (s8)ccr;
++      /* ppb_cor + 1000000000L can never be zero */
++      *offset = armada38x_ppb_convert(ppb_cor);
++
++      return 0;
++}
++
++static int armada38x_rtc_set_offset(struct device *dev, long offset)
++{
++      struct armada38x_rtc *rtc = dev_get_drvdata(dev);
++      unsigned long ccr = 0;
++      long ppb_cor, off;
++
++      /*
++       * The maximum ppb_cor is -128 * 3815 .. 127 * 3815, but we
++       * need to clamp the input.  This equates to -484270 .. 488558.
++       * Not only is this to stop out of range "off" but also to
++       * avoid the division by zero in armada38x_ppb_convert().
++       */
++      offset = clamp(offset, -484270L, 488558L);
++
++      ppb_cor = armada38x_ppb_convert(offset);
++
++      /*
++       * Use low update mode where possible, which gives a better
++       * resolution of correction.
++       */
++      off = DIV_ROUND_CLOSEST(ppb_cor, 954);
++      if (off > 127 || off < -128) {
++              ccr = RTC_CCR_MODE;
++              off = DIV_ROUND_CLOSEST(ppb_cor, 3815);
++      }
++
++      /*
++       * Armada 388 requires a bit pattern in bits 14..8 depending on
++       * the sign bit: { 0, ~S, S, S, S, S, S }
++       */
++      ccr |= (off & 0x3fff) ^ 0x2000;
++      rtc_delayed_write(ccr, rtc, RTC_CCR);
++
++      return 0;
++}
++
+ static const struct rtc_class_ops armada38x_rtc_ops = {
+       .read_time = armada38x_rtc_read_time,
+       .set_time = armada38x_rtc_set_time,
+       .read_alarm = armada38x_rtc_read_alarm,
+       .set_alarm = armada38x_rtc_set_alarm,
+       .alarm_irq_enable = armada38x_rtc_alarm_irq_enable,
++      .read_offset = armada38x_rtc_read_offset,
++      .set_offset = armada38x_rtc_set_offset,
+ };
+ 
+ static const struct rtc_class_ops armada38x_rtc_ops_noirq = {
+       .read_time = armada38x_rtc_read_time,
+       .set_time = armada38x_rtc_set_time,
+       .read_alarm = armada38x_rtc_read_alarm,
++      .read_offset = armada38x_rtc_read_offset,
++      .set_offset = armada38x_rtc_set_offset,
+ };
+ 
+ static const struct armada38x_rtc_data armada38x_data = {