On Fri, Jun 17, 2016 at 1:40 AM, Hoan Tran [off-list ref] 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 <redacted>
Signed-off-by: Loc Ho <redacted>

+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().

+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@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