w1: mxc_w1: Add warning for base frequency calculation

Base frequency should be as close as possible to 1 MHz. This patch
adds warnings when clock is unreliable, according to i.MX datasheet.
Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Acked-by: Evgeniy Polyakov <zbr@ioremap.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/w1/masters/mxc_w1.c

@@ -105,6 +105,7 @@ static u8 mxc_w1_ds2_touch_bit(void *data, u8 bit)
 static int mxc_w1_probe(struct platform_device *pdev)
 {
 	struct mxc_w1_device *mdev;
+	unsigned long clkrate;
 	struct resource *res;
 	unsigned int clkdiv;
 	int err = 0;
@@ -118,7 +119,16 @@ static int mxc_w1_probe(struct platform_device *pdev)
 	if (IS_ERR(mdev->clk))
 		return PTR_ERR(mdev->clk);
 
-	clkdiv = (clk_get_rate(mdev->clk) / 1000000) - 1;
+	clkrate = clk_get_rate(mdev->clk);
+	if (clkrate < 10000000)
+		dev_warn(&pdev->dev,
+			 "Low clock frequency causes improper function\n");
+
+	clkdiv = DIV_ROUND_CLOSEST(clkrate, 1000000);
+	clkrate /= clkdiv;
+	if ((clkrate < 980000) || (clkrate > 1020000))
+		dev_warn(&pdev->dev,
+			 "Incorrect time base frequency %lu Hz\n", clkrate);
 
 	res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
 	mdev->regs = devm_ioremap_resource(&pdev->dev, res);
@@ -126,7 +136,7 @@ static int mxc_w1_probe(struct platform_device *pdev)
 		return PTR_ERR(mdev->regs);
 
 	clk_prepare_enable(mdev->clk);
-	__raw_writeb(clkdiv, mdev->regs + MXC_W1_TIME_DIVIDER);
+	__raw_writeb(clkdiv - 1, mdev->regs + MXC_W1_TIME_DIVIDER);
 
 	mdev->bus_master.data = mdev;
 	mdev->bus_master.reset_bus = mxc_w1_ds2_reset_bus;