Hi Geert,
On Thu, Jun 16, 2016 at 11:43 PM, Geert Uytterhoeven
<ge...@linux-m68k.org> wrote:
> On Fri, Jun 17, 2016 at 1:40 AM, Hoan Tran <hot...@apm.com> 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 <hot...@apm.com>
>> Signed-off-by: Loc Ho <l...@apm.com>
>
>> --- /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().
Yes, my mistake. I'll fix it.
>
>> +
>> + 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().
Will fix it.
Thanks
Hoan
>
>> --- 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 --
> ge...@linux-m68k.org
>
> 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
