rtc: sun6i: Add NVMEM provider

The sun6i RTC provides 32 bytes of general-purpose data registers.
They can be used to save data in the always-on RTC power domain.
The registers are writable via 32-bit MMIO accesses only.

Expose them with a NVMEM provider so they can be used by other drivers.
Signed-off-by: Samuel Holland <samuel@sholland.org>
Acked-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20220413231731.56709-1-samuel@sholland.org

drivers/rtc/rtc-sun6i.c

@@ -71,6 +71,10 @@
 #define SUN6I_LOSC_OUT_GATING			0x0060
 #define SUN6I_LOSC_OUT_GATING_EN_OFFSET		0
 
+/* General-purpose data */
+#define SUN6I_GP_DATA				0x0100
+#define SUN6I_GP_DATA_SIZE			0x20
+
 /*
  * Get date values
  */
@@ -662,6 +666,39 @@ static const struct rtc_class_ops sun6i_rtc_ops = {
 	.alarm_irq_enable	= sun6i_rtc_alarm_irq_enable
 };
 
+static int sun6i_rtc_nvmem_read(void *priv, unsigned int offset, void *_val, size_t bytes)
+{
+	struct sun6i_rtc_dev *chip = priv;
+	u32 *val = _val;
+	int i;
+
+	for (i = 0; i < bytes / 4; ++i)
+		val[i] = readl(chip->base + SUN6I_GP_DATA + offset + 4 * i);
+
+	return 0;
+}
+
+static int sun6i_rtc_nvmem_write(void *priv, unsigned int offset, void *_val, size_t bytes)
+{
+	struct sun6i_rtc_dev *chip = priv;
+	u32 *val = _val;
+	int i;
+
+	for (i = 0; i < bytes / 4; ++i)
+		writel(val[i], chip->base + SUN6I_GP_DATA + offset + 4 * i);
+
+	return 0;
+}
+
+static struct nvmem_config sun6i_rtc_nvmem_cfg = {
+	.type		= NVMEM_TYPE_BATTERY_BACKED,
+	.reg_read	= sun6i_rtc_nvmem_read,
+	.reg_write	= sun6i_rtc_nvmem_write,
+	.size		= SUN6I_GP_DATA_SIZE,
+	.word_size	= 4,
+	.stride		= 4,
+};
+
 #ifdef CONFIG_PM_SLEEP
 /* Enable IRQ wake on suspend, to wake up from RTC. */
 static int sun6i_rtc_suspend(struct device *dev)
@@ -795,6 +832,11 @@ static int sun6i_rtc_probe(struct platform_device *pdev)
 	if (ret)
 		return ret;
 
+	sun6i_rtc_nvmem_cfg.priv = chip;
+	ret = devm_rtc_nvmem_register(chip->rtc, &sun6i_rtc_nvmem_cfg);
+	if (ret)
+		return ret;
+
 	dev_info(&pdev->dev, "RTC enabled\n");
 
 	return 0;