The Si5341 datasheet warns that before accessing any other registers,
including the PAGE register, we need to wait for the DEVICE_READY register
to indicate the device is ready, or the process of the device loading its
state from NVM can be corrupted. Wait for DEVICE_READY on startup before
continuing initialization. This is done using a raw I2C register read
prior to setting up regmap to avoid any potential unwanted automatic PAGE
register accesses from regmap at this stage.

Fixes: 3044a860fd ("clk: Add Si5341/Si5340 driver")
Signed-off-by: Robert Hancock <redacted>
---
  drivers/clk/clk-si5341.c | 31 +++++++++++++++++++++++++++++++
  1 file changed, 31 insertions(+)

diff --git a/drivers/clk/clk-si5341.c b/drivers/clk/clk-si5341.c
index e0446e66fa64..f210860fb96e 100644
--- a/drivers/clk/clk-si5341.c
+++ b/drivers/clk/clk-si5341.c

@@ -94,6 +94,7 @@ struct clk_si5341_output_config {
  #define SI5341_STATUS		0x000C
  #define SI5341_SOFT_RST		0x001C
  #define SI5341_IN_SEL		0x0021
+#define SI5341_DEVICE_READY	0x00FE
  #define SI5341_XAXB_CFG		0x090E
  #define SI5341_IN_EN		0x0949
  #define SI5341_INX_TO_PFD_EN	0x094A

@@ -1189,6 +1190,31 @@ static const struct regmap_range_cfg si5341_regmap_ranges[] = {
  	},
  };
  
+static int si5341_wait_device_ready(struct i2c_client *client)
+{
+	int count;
+
+	/* Datasheet warns: Any attempt to read or write any register other
+	 * than DEVICE_READY before DEVICE_READY reads as 0x0F may corrupt the
+	 * NVM programming and may corrupt the register contents, as they are
+	 * read from NVM. Note that this includes accesses to the PAGE register.
+	 * Also: DEVICE_READY is available on every register page, so no page
+	 * change is needed to read it.
+	 * Do this outside regmap to avoid automatic PAGE register access.
+	 */
+	for (count = 0; count < 10; ++count) {
+		s32 result = i2c_smbus_read_byte_data(client,
+						      SI5341_DEVICE_READY);
+		if (result < 0)
+			return result;
+		if (result == 0x0F)
+			return 0;
+		usleep_range(1000, 20000);
+	}
+	dev_err(&client->dev, "timeout waiting for DEVICE_READY\n");

+	return -EIO;
+}
+
  static const struct regmap_config si5341_regmap_config = {
  	.reg_bits = 8,
  	.val_bits = 8,

@@ -1385,6 +1411,11 @@ static int si5341_probe(struct i2c_client *client,
  
  	data->i2c_client = client;
  
+	/* Must be done before otherwise touching hardware */
+	err = si5341_wait_device_ready(client);
+	if (err)
+		return err;
+
  	for (i = 0; i < SI5341_NUM_INPUTS; ++i) {
  		input = devm_clk_get(&client->dev, si5341_input_clock_names[i]);
  		if (IS_ERR(input)) {