clk: mxs: Invalid register access on HW_CLKCTRL_FRAC0?

Stefan Wahren stefan.wahren at
Sat Dec 13 04:23:29 PST 2014

Hi Fabio,

yesterday i stumble on this note in the i.MX28 reference manual (p. 931):

    10.8.24 Fractional Clock Control Register 0 (HW_CLKCTRL_FRAC0)

    NOTE: This register can only be addressed by byte instructions. Addressing
word or half-word are not allowed.

The same applies to HW_CLKCTRL_FRAC1.

But clk_misc_init() doesn't care about that in clk_imx28.c:

    val = readl_relaxed(FRAC0);
    val &= ~((0x3f << BP_FRAC0_IO0FRAC) | (0x3f << BP_FRAC0_IO1FRAC));
    val |= (30 << BP_FRAC0_IO0FRAC) | (30 << BP_FRAC0_IO1FRAC);
    writel_relaxed(val, FRAC0);

The function clk_ref_set_rate() in clk_ref.c write also the complete register at
once, but change only a byte.

Which of them are invalid?

Would you prefer to use writeb() to fix this?


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