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    SubjectRe: [PATCH v6 06/36] nds32: Kernel booting and initialization
    2018-01-20 0:41 GMT+08:00 Arnd Bergmann <arnd@arndb.de>:
    > On Fri, Jan 19, 2018 at 5:34 PM, Greentime Hu <green.hu@gmail.com> wrote:
    >> Hi, Arnd:
    >>
    >> 2018-01-18 18:11 GMT+08:00 Arnd Bergmann <arnd@arndb.de>:
    >>> On Mon, Jan 15, 2018 at 6:53 AM, Greentime Hu <green.hu@gmail.com> wrote:
    >>>
    >>> I had not looked at this patch in enough detail earlier, sorry about
    >>> that. It should be
    >>> easy enough to fix though.
    >>>
    >>>> +#ifdef CONFIG_VGA_CONSOLE
    >>>> +struct screen_info screen_info;
    >>>> +#endif
    >>>
    >>> I would assume that you can't ever have a VGA console. Just drop all
    >>> the references
    >>> here and instead send a patch to the fbdev maintainer to add the dependency
    >>> at CONFIG_VGA_CONSOLE to prevent selecting it with nds32.
    >>
    >> I found it can be built pass for now because we disable it in defconfig.
    >> Should I send the patch in v7 series?
    >
    > yes, I think that would be best.
    >
    >>>> +
    >>>> +extern void __init early_init_devtree(void *params);
    >>>> +extern void __init early_trap_init(void);
    >>>
    >>> similarly, these are declared in include/linux/of_fdt.h
    >>>
    >>
    >> early_trap_init is a nds32 function. I will move it to nds32.h
    >
    > Right, makes sense.
    >
    >>>> +void calibrate_delay(void)
    >>>> +{
    >>>> + const int *val;
    >>>> + struct device_node *cpu = NULL;
    >>>> + cpu = of_find_compatible_node(NULL, NULL, "andestech,nds32v3");
    >>>> + val = of_get_property(cpu, "clock-frequency", NULL);
    >>>> + if (!val || !*val)
    >>>> + panic("no cpu 'clock-frequency' parameter in device tree");
    >>>> + loops_per_jiffy = be32_to_cpup(val) / HZ;
    >>>> + pr_cont("%lu.%02lu BogoMIPS (lpj=%lu)\n",
    >>>> + loops_per_jiffy / (500000 / HZ),
    >>>> + (loops_per_jiffy / (5000 / HZ)) % 100, loops_per_jiffy);
    >>>> +}
    >>>
    >>> This seems very odd to me: The 'clock-frequency' property in the
    >>> cpu node should refer to the actual frequency it is running at, but that
    >>> tends to be different from the bogomips as needed by the ndelay()
    >>> function. Can you explain what is going on here?
    >>>
    >>
    >> This implementation is referenced from openrisc.
    >> https://lkml.org/lkml/2017/11/17/228
    >
    > It's correct on openrisc, because that has a reliable cycle counter,
    > and that gets used in its delay function:
    >
    > void __delay(unsigned long cycles)
    > {
    > cycles_t start = get_cycles();
    > while ((get_cycles() - start) < cycles)
    > cpu_relax();
    > }
    >
    > In my review comment that you cited, I assumed that nds32 had similar
    > hardware.
    >
    > However, as you explained earlier, the nds32 architecture does not provide
    > a cycle counter and the clocksource resolution is not high enough to
    > be a good replacement, so you have to use the traditional delay
    > calibration.
    >

    Hi, Arnd:

    Thank you for your explanation.
    Will it be ok if I code it like this?

    config GENERIC_CALIBRATE_DELAY
    def_bool y

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