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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 = {
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