On Fri, Jun 17, 2016 at 1:40 AM, Hoan Tran <hotran@xxxxxxx> wrote:
> This patch adds fractional scale clock support.
> Fractional scale clock is implemented for a single register field.
> Output rate = parent_rate * scale / denominator
> For example, for 1 / 8 fractional scale, denominator will be 8 and scale
> will be computed and programmed accordingly.
>
> Signed-off-by: Hoan Tran <hotran@xxxxxxx>
> Signed-off-by: Loc Ho <lho@xxxxxxx>
> --- /dev/null
> +++ b/drivers/clk/clk-fractional-scale.c
> +static long clk_fs_round_rate(struct clk_hw *hw, unsigned long rate,
> + unsigned long *parent_rate)
> +{
> + struct clk_fractional_scale *fd = to_clk_sf(hw);
> + u64 ret, scale;
> +
> + if (!rate || rate >= *parent_rate)
> + return *parent_rate;
> +
> + /* freq = parent_rate * scaler / denom */
> + ret = rate * fd->denom;
Can this overflow? No, because fd->denom is u64.
But if fd->denom is changed to u32, it needs a cast to u64 here.
> + scale = ret / *parent_rate;
As detected by the kbuild test robot, this should use do_div().
> +
> + ret = (u64) *parent_rate * scale;
> + do_div(ret, fd->denom);
> +
> + return ret;
> +}
> +
> +static int clk_fs_set_rate(struct clk_hw *hw, unsigned long rate,
> + unsigned long parent_rate)
> +{
> + struct clk_fractional_scale *fd = to_clk_sf(hw);
> + unsigned long flags = 0;
> + u64 scale, ret;
> + u32 val;
> +
> + /*
> + * Compute the scaler:
> + *
> + * freq = parent_rate * scaler / denom, or
> + * scaler = freq * denom / parent_rate
> + */
> + ret = rate * fd->denom;
Can this overflow? No, because fd->denom is u64.
But if fd->denom is changed to u32, it needs a cast to u64 here.
> + scale = ret / (u64)parent_rate;
Don't cast the divider to u64, as this will turn a 64-by-32 division into a
64-by-64 division.
As detected by the kbuild test robot, this should use do_div().
> --- a/include/linux/clk-provider.h
> +++ b/include/linux/clk-provider.h
> +struct clk_fractional_scale {
> + struct clk_hw hw;
> + void __iomem *reg;
> + u8 shift;
> + u32 mask;
> + u64 denom;
u64 sounds overkill to me, but it looks like that was done to avoid overflow in
the multiplications?
> + u32 flags;
> + spinlock_t *lock;
> +};
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
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