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

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Date:   Tue, 12 Jan 2021 23:23:36 -0800
From:   Stephen Boyd <sboyd@...nel.org>
To:     Adam Ford <aford173@...il.com>, linux-clk@...r.kernel.org
Cc:     aford@...conembedded.com, Adam Ford <aford173@...il.com>,
        Luca Ceresoli <luca@...aceresoli.net>,
        Michael Turquette <mturquette@...libre.com>,
        Rob Herring <robh+dt@...nel.org>, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC 2/2] clk: vc5: Add support for optional load capacitance

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@...il.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)
>  {