Re: [RFC 2/2] clk: vc5: Add support for optional load capacitance

Stephen Boyd
2021-01-13T07:24:19Z

Quoting Adam Ford (2021-01-06 09:39:00)
> 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>
> ---
> drivers/clk/clk-versaclock5.c | 64 +++++++++++++++++++++++++++++++++++
> 1 file changed, 64 insertions(+)
>
> diff --git a/drivers/clk/clk-versaclock5.c b/drivers/clk/clk-versaclock5.c
> index 43db67337bc0..445abc3731fb 100644
> --- a/drivers/clk/clk-versaclock5.c
> +++ b/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 */
> + if ((femtofarads < 9000) || (femtofarads > 25000))

Please remove useless parenthesis.

> + return -EINVAL;
> +
> + /* The lowest target we can hit is 9430, so exit if it's less */
> + if (femtofarads < 9430)
> + return 0;
> +
> + /*
> + * According to VersaClock 6E Programming Guide, there are 6
> + * bits which translate to 64 entries in XTAL registers 12 and
> + * 13. Because bits 0 and 1 increase the capacitance the
> + * same, some of the values can be repeated. Plugging this
> + * into a spreadsheet and generating a trendline, the output
> + * equation becomes x = (y-9098.29) / 216.44, where 'y' is
> + * the desired capacitance in femtofarads, and x is the value
> + * of XTAL[5:0].
> + * To help with rounding, do fixed point math
> + */
> + femtofarads *= 100;
> + mapped_value = (femtofarads - 909829) / 21644;
> +
> + /*
> + * The datasheet states, the maximum capacitance is 25000,
> + * but the programmer guide shows a max value is 22832,
> + * so values higher values could overflow, so cap it.
> + */
> + mapped_value = max(mapped_value/100, 0x3f);
> +
> + return mapped_value;
> +}
> +static int vc5_update_cap_load(struct device_node *node, struct vc5_driver_data *vc5)
> +{
> + u32 value, mapped_value;
> +
> + if (!of_property_read_u32(node, "idt,xtal1-load-femtofarads", &value)) {
> + mapped_value = vc5_map_cap_value(value);
> + if (mapped_value < 0)

How can it be less than 0? It's unsigned.

> + return mapped_value;
> +
> + regmap_write(vc5->regmap, VC5_XTAL_X1_LOAD_CAP, (mapped_value << 2));
> + }
> +
> + if (!of_property_read_u32(node, "idt,xtal2-load-femtofarads", &value)) {
> + mapped_value = vc5_map_cap_value(value);
> + if (mapped_value < 0)

Same!

> + return mapped_value;
> + regmap_write(vc5->regmap, VC5_XTAL_X2_LOAD_CAP, (mapped_value << 2));
> + }
> +
> + return 0;
> +}
> +
> static int vc5_update_slew(struct device_node *np_output,
> struct vc5_out_data *clk_out)
> {