iio: adc: aspeed: Fix the calculate error of clock.

The ADC clock formula is
ast2400/2500:
ADC clock period = PCLK * 2 * (ADC0C[31:17] + 1) * (ADC0C[9:0] + 1)
ast2600:
ADC clock period = PCLK * 2 * (ADC0C[15:0] + 1)
They all have one fixed divided 2 and the legacy driver didn't handle it.
This patch register the fixed factory clock device as the parent of ADC
clock scaler to fix this issue.

Signed-off-by: Billy Tsai <billy_tsai@aspeedtech.com>
Link: https://lore.kernel.org/r/20210922081520.30580-8-billy_tsai@aspeedtech.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/aspeed_adc.c

@@ -4,6 +4,12 @@
  *
  * Copyright (C) 2017 Google, Inc.
  * Copyright (C) 2021 Aspeed Technology Inc.
+ *
+ * ADC clock formula:
+ * Ast2400/Ast2500:
+ * clock period = period of PCLK * 2 * (ADC0C[31:17] + 1) * (ADC0C[9:0] + 1)
+ * Ast2600:
+ * clock period = period of PCLK * 2 * (ADC0C[15:0] + 1)
  */
 
 #include <linux/clk.h>
@@ -85,6 +91,7 @@ struct aspeed_adc_data {
 	struct regulator	*regulator;
 	void __iomem		*base;
 	spinlock_t		clk_lock;
+	struct clk_hw		*fixed_div_clk;
 	struct clk_hw		*clk_prescaler;
 	struct clk_hw		*clk_scaler;
 	struct reset_control	*rst;
@@ -197,6 +204,13 @@ static const struct iio_info aspeed_adc_iio_info = {
 	.debugfs_reg_access = aspeed_adc_reg_access,
 };
 
+static void aspeed_adc_unregister_fixed_divider(void *data)
+{
+	struct clk_hw *clk = data;
+
+	clk_hw_unregister_fixed_factor(clk);
+}
+
 static void aspeed_adc_reset_assert(void *data)
 {
 	struct reset_control *rst = data;
@@ -320,6 +334,19 @@ static int aspeed_adc_probe(struct platform_device *pdev)
 	spin_lock_init(&data->clk_lock);
 	snprintf(clk_parent_name, ARRAY_SIZE(clk_parent_name), "%s",
 		 of_clk_get_parent_name(pdev->dev.of_node, 0));
+	snprintf(clk_name, ARRAY_SIZE(clk_name), "%s-fixed-div",
+		 data->model_data->model_name);
+	data->fixed_div_clk = clk_hw_register_fixed_factor(
+		&pdev->dev, clk_name, clk_parent_name, 0, 1, 2);
+	if (IS_ERR(data->fixed_div_clk))
+		return PTR_ERR(data->fixed_div_clk);
+
+	ret = devm_add_action_or_reset(data->dev,
+				       aspeed_adc_unregister_fixed_divider,
+				       data->fixed_div_clk);
+	if (ret)
+		return ret;
+	snprintf(clk_parent_name, ARRAY_SIZE(clk_parent_name), clk_name);
 
 	if (data->model_data->need_prescaler) {
 		snprintf(clk_name, ARRAY_SIZE(clk_name), "%s-prescaler",