[PATCH] rtc: zynqmp: Enable crystal oscillator in initialization

[PATCH] rtc: zynqmp: Enable crystal oscillator in initialization

[Daniel Viaño]

Enable the RTC crystal oscillator bit in the control register (RTC_CTRL,
offset 0x40) during driver initialization.

Although the RTC_OSC_EN macro was defined when the driver was originally
introduced, it was never asserted in xlnx_init_rtc(). On cold boots, after
battery exhaustion, or on platforms where firmware does not pre-initialize
RTC_CTRL, the oscillator remains disabled and the counter never ticks.

Asserting RTC_OSC_EN alongside RTC_BATT_EN ensures the oscillator starts
without relying on bootloader initialization. Per the Zynq UltraScale+
TRM (UG1085), bit 24 (OSC_CNTRL) is a static level enable for the crystal
inverter rather than an edge-triggered reset; re-asserting it when the
oscillator is already running is a non-destructive no-op that does not
glitch or reset the ticking counters.

Tested on Zynq UltraScale+ hardware, confirming that the counter freezes
when RTC_OSC_EN is cleared, advances reliably once enabled, and is
unaffected when re-asserted while running.

Fixes: 11143c19eb57 ("rtc: add xilinx zynqmp rtc driver")
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Viaño <danividanivi@gmail.com>
---
 drivers/rtc/rtc-zynqmp.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/rtc/rtc-zynqmp.c b/drivers/rtc/rtc-zynqmp.c
index 5bcb7536e973..278705ec09f5 100644
--- a/drivers/rtc/rtc-zynqmp.c
+++ b/drivers/rtc/rtc-zynqmp.c
@@ -167,9 +167,12 @@ static void xlnx_init_rtc(struct xlnx_rtc_dev *xrtcdev)
 {
 	u32 rtc_ctrl;
 
-	/* Enable RTC switch to battery when VCC_PSAUX is not available */
+	/*
+	 * Enable crystal oscillator, and enable RTC switch to battery
+	 * when VCC_PSAUX is not available.
+	 */
 	rtc_ctrl = readl(xrtcdev->reg_base + RTC_CTRL);
-	rtc_ctrl |= RTC_BATT_EN;
+	rtc_ctrl |= RTC_BATT_EN | RTC_OSC_EN;
 	writel(rtc_ctrl, xrtcdev->reg_base + RTC_CTRL);
 }
 
-- 
2.53.0