rtc: m48t86: verify that the RTC is actually present

The RTC is an optional feature at purchase time on some Technologic
Systems boards. Verify that it actually exists by checking if the
last two bytes of the NVRAM can be changed.
Signed-off-by: NH Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: NAlexandre Belloni <alexandre.belloni@free-electrons.com>

drivers/rtc/rtc-m48t86.c

@@ -201,6 +201,37 @@ static ssize_t m48t86_nvram_write(struct file *filp, struct kobject *kobj,
 static BIN_ATTR(nvram, 0644, m48t86_nvram_read, m48t86_nvram_write,
 		M48T86_NVRAM_LEN);
 
+/*
+ * The RTC is an optional feature at purchase time on some Technologic Systems
+ * boards. Verify that it actually exists by checking if the last two bytes
+ * of the NVRAM can be changed.
+ *
+ * This is based on the method used in their rtc7800.c example.
+ */
+static bool m48t86_verify_chip(struct platform_device *pdev)
+{
+	unsigned int offset0 = M48T86_NVRAM(M48T86_NVRAM_LEN - 2);
+	unsigned int offset1 = M48T86_NVRAM(M48T86_NVRAM_LEN - 1);
+	unsigned char tmp0, tmp1;
+
+	tmp0 = m48t86_readb(&pdev->dev, offset0);
+	tmp1 = m48t86_readb(&pdev->dev, offset1);
+
+	m48t86_writeb(&pdev->dev, 0x00, offset0);
+	m48t86_writeb(&pdev->dev, 0x55, offset1);
+	if (m48t86_readb(&pdev->dev, offset1) == 0x55) {
+		m48t86_writeb(&pdev->dev, 0xaa, offset1);
+		if (m48t86_readb(&pdev->dev, offset1) == 0xaa &&
+		    m48t86_readb(&pdev->dev, offset0) == 0x00) {
+			m48t86_writeb(&pdev->dev, tmp0, offset0);
+			m48t86_writeb(&pdev->dev, tmp1, offset1);
+
+			return true;
+		}
+	}
+	return false;
+}
+
 static int m48t86_rtc_probe(struct platform_device *pdev)
 {
 	struct m48t86_rtc_info *info;
@@ -231,6 +262,11 @@ static int m48t86_rtc_probe(struct platform_device *pdev)
 
 	dev_set_drvdata(&pdev->dev, info);
 
+	if (!m48t86_verify_chip(pdev)) {
+		dev_info(&pdev->dev, "RTC not present\n");
+		return -ENODEV;
+	}
+
 	info->rtc = devm_rtc_device_register(&pdev->dev, "m48t86",
 					     &m48t86_rtc_ops, THIS_MODULE);
 	if (IS_ERR(info->rtc))