[PATCH v4] i2c: busses: i2c-bcm2835: limits cdiv to allowed values

[Silvan Wicki]

Adds: make sure bits 16-31 of DIV register are always 0
Adds: assume minimal divider of 2 if divider resulted in 0
      (bcm2835 sets divider to 32768 if cdiv is set to 0)

See page 33/34 of BCM2835-ARM-Peripherals.pdf for the DIV register.
https://www.raspberrypi.org/wp-content/uploads/2012/02/BCM2835-ARM-Peripherals.pdf

Signed-off-by: Silvan Wicki <linux_wi at tinag.ch>
---
 drivers/i2c/busses/i2c-bcm2835.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/drivers/i2c/busses/i2c-bcm2835.c b/drivers/i2c/busses/i2c-bcm2835.c
index c9336a3..7a5b96d 100644
--- a/drivers/i2c/busses/i2c-bcm2835.c
+++ b/drivers/i2c/busses/i2c-bcm2835.c
@@ -50,6 +50,9 @@
 #define BCM2835_I2C_S_CLKT	BIT(9)
 #define BCM2835_I2C_S_LEN	BIT(10) /* Fake bit for SW error reporting */
 
+#define BCM2835_I2C_CDIV_MIN   0x0002
+#define BCM2835_I2C_CDIV_MAX   0xFFFE
+
 #define BCM2835_I2C_TIMEOUT (msecs_to_jiffies(1000))
 
 struct bcm2835_i2c_dev {
@@ -252,12 +255,28 @@ static int bcm2835_i2c_probe(struct platform_device *pdev)
 
 	divider = DIV_ROUND_UP(clk_get_rate(i2c_dev->clk), bus_clk_rate);
 	/*
+	 * A divider of 0 results in extremely high bus_clk_rate values
+	 * such as bus_clk_rate >= 4044967297 with core_clock = 250MHz.
+	 * In such a case assume the minimal possible divider, since
+	 * bcm2835 chip sets divisor internally to 32768 if cdiv is 0.
+	 */
+	if (divider < BCM2835_I2C_CDIV_MIN) {
+		dev_warn(&pdev->dev,
+			 "Clock too fast, falling back to max clock speed\n");
+		divider = BCM2835_I2C_CDIV_MIN;
+	}
+	/*
 	 * Per the datasheet, the register is always interpreted as an even
 	 * number, by rounding down. In other words, the LSB is ignored. So,
 	 * if the LSB is set, increment the divider to avoid any issue.
 	 */
 	if (divider & 1)
 		divider++;
+	if (divider > BCM2835_I2C_CDIV_MAX) {
+		dev_warn(&pdev->dev,
+			 "Clock too slow, falling back to min clock speed\n");
+		divider = BCM2835_I2C_CDIV_MAX;
+	}
 	bcm2835_i2c_writel(i2c_dev, BCM2835_I2C_DIV, divider);
 
 	irq = platform_get_resource(pdev, IORESOURCE_IRQ, 0);
-- 
2.1.4