clk: vc5: Add support for optional load capacitance

There are two registers which can set the load capacitance for
XTAL1 and XTAL2. These are optional registers when using an
external crystal.  Parse the device tree and set the
corresponding registers accordingly.

Signed-off-by: Adam Ford <aford173@gmail.com>
Link: https://lore.kernel.org/r/20210207185140.3653350-2-aford173@gmail.com
Reviewed-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>

drivers/clk/clk-versaclock5.c

@@ -759,6 +759,63 @@ static int vc5_update_power(struct device_node *np_output,
 	return 0;
 }
 
+static int vc5_map_cap_value(u32 femtofarads)
+{
+	int mapped_value;
+
+	/*
+	 * The datasheet explicitly states 9000 - 25000 with 0.5pF
+	 * steps, but the Programmer's guide shows the steps are 0.430pF.
+	 * After getting feedback from Renesas, the .5pF steps were the
+	 * goal, but 430nF was the actual values.
+	 * Because of this, the actual range goes to 22760 instead of 25000
+	 */
+	if (femtofarads < 9000 || femtofarads > 22760)
+		return -EINVAL;
+
+	/*
+	 * The Programmer's guide shows XTAL[5:0] but in reality,
+	 * XTAL[0] and XTAL[1] are both LSB which makes the math
+	 * strange.  With clarfication from Renesas, setting the
+	 * values should be simpler by ignoring XTAL[0]
+	 */
+	mapped_value = DIV_ROUND_CLOSEST(femtofarads - 9000, 430);
+
+	/*
+	 * Since the calculation ignores XTAL[0], there is one
+	 * special case where mapped_value = 32.  In reality, this means
+	 * the real mapped value should be 111111b.  In other cases,
+	 * the mapped_value needs to be shifted 1 to the left.
+	 */
+	if (mapped_value > 31)
+		mapped_value = 0x3f;
+	else
+		mapped_value <<= 1;
+
+	return mapped_value;
+}
+static int vc5_update_cap_load(struct device_node *node, struct vc5_driver_data *vc5)
+{
+	u32 value;
+	int mapped_value;
+
+	if (!of_property_read_u32(node, "idt,xtal-load-femtofarads", &value)) {
+		mapped_value = vc5_map_cap_value(value);
+		if (mapped_value < 0)
+			return mapped_value;
+
+		/*
+		 * The mapped_value is really the high 6 bits of
+		 * VC5_XTAL_X1_LOAD_CAP and VC5_XTAL_X2_LOAD_CAP, so
+		 * shift the value 2 places.
+		 */
+		regmap_update_bits(vc5->regmap, VC5_XTAL_X1_LOAD_CAP, ~0x03, mapped_value << 2);
+		regmap_update_bits(vc5->regmap, VC5_XTAL_X2_LOAD_CAP, ~0x03, mapped_value << 2);
+	}
+
+	return 0;
+}
+
 static int vc5_update_slew(struct device_node *np_output,
 			   struct vc5_out_data *clk_out)
 {
@@ -884,6 +941,13 @@ static int vc5_probe(struct i2c_client *client, const struct i2c_device_id *id)
 		return -EINVAL;
 	}
 
+	/* Configure Optional Loading Capacitance for external XTAL */
+	if (!(vc5->chip_info->flags & VC5_HAS_INTERNAL_XTAL)) {
+		ret = vc5_update_cap_load(client->dev.of_node, vc5);
+		if (ret)
+			goto err_clk_register;
+	}
+
 	init.name = kasprintf(GFP_KERNEL, "%pOFn.mux", client->dev.of_node);
 	init.ops = &vc5_mux_ops;
 	init.flags = 0;