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rtc: armada38x: add support for trimming the RTC

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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>
This commit is contained in:
Russell King 2017-09-29 11:23:31 +01:00 committed by Alexandre Belloni
parent 8a25c8f66c
commit f94ffbc2c2
1 changed files with 101 additions and 0 deletions
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101
drivers/rtc/rtc-armada38x.c
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@ -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_irq(int irq, void *data)
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 = {