AM3352BZCZ 断电后时钟保存问题

我的问题是， RTC硬件时钟，断电后重启时间没有保存，如何解决。

热启动时钟是保存的，冷启动没保存。

初始化程序， 是内核源码，没改动，如下：

static void am335x_rtc_init(int evm_id, int profile)
{
 void __iomem *base;
 struct clk *clk;
 struct omap_hwmod *oh;
 struct platform_device *pdev;
 char *dev_name = "am33xx-rtc";

 clk = clk_get(NULL, "rtc_fck");
 if (IS_ERR(clk)) {
  pr_err("rtc : Failed to get RTC clock\n");
  return;
 }

 if (clk_enable(clk)) {
  pr_err("rtc: Clock Enable Failed\n");
  return;
 }

 base = ioremap(AM33XX_RTC_BASE, SZ_4K);

 if (WARN_ON(!base))
  return;

 /* Unlock the rtc's registers */
 writel(0x83e70b13, base + 0x6c);
 writel(0x95a4f1e0, base + 0x70);

 /*
  * Enable the 32K OSc
  * TODO: Need a better way to handle this
  * Since we want the clock to be running before mmc init
  * we need to do it before the rtc probe happens
  */
 writel(0x48, base + 0x54);

 iounmap(base);

 switch (evm_id) {
 case BEAGLE_BONE_A3:
 case BEAGLE_BONE_OLD:
  am335x_rtc_info.pm_off = true;
  break;
 default:
  break;
 }

 clk_disable(clk);
 clk_put(clk);

 if (omap_rev() == AM335X_REV_ES2_0)
  am335x_rtc_info.wakeup_capable = 1;

 oh = omap_hwmod_lookup("rtc");
 if (!oh) {
  pr_err("could not look up %s\n", "rtc");
  return;
 }

 pdev = omap_device_build(dev_name, -1, oh, &am335x_rtc_info,
   sizeof(struct omap_rtc_pdata), NULL, 0, 0);
 WARN(IS_ERR(pdev), "Can't build omap_device for %s:%s.\n",
   dev_name, oh->name);
}