* Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx> wrote:
> Hi Ingo,
>
> On 01/22/2017 05:04 PM, Ingo Molnar wrote:
> > * Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx> wrote:
> >
> >>>> +static void xdbc_runtime_delay(unsigned long count)
> >>>> +{
> >>>> + udelay(count);
> >>>> +}
> >>>> +static void (*xdbc_delay)(unsigned long) = xdbc_early_delay;
> >>> Is this udelay() complication really necessary? udelay() should work fine even in
> >>> early code. It might not be precisely calibrated, but should be good enough.
> >> I tried udelay() in the early code. It's not precise enough for the
> >> hardware handshaking.
> > Possibly because on x86 early udelay() did not work at all - i.e. there's no delay
> > whatsoever.
>
> Yes.
>
> >
> > Could you try it on top of this commit in tip:timers/core:
> >
> > 4c45c5167c95 x86/timer: Make delay() work during early bootup
> >
> > ?
>
> I tried tip:timers/core. It's not precise enough for my context either.
>
> __const_udelay().
>
> 157 inline void __const_udelay(unsigned long xloops)
> 158 {
> 159 unsigned long lpj = this_cpu_read(cpu_info.loops_per_jiffy) ? : loops_per_jiffy;
> 160 int d0;
> 161
> 162 xloops *= 4;
> 163 asm("mull %%edx"
> 164 :"=d" (xloops), "=&a" (d0)
> 165 :"1" (xloops), "0" (lpj * (HZ / 4)));
> 166
> 167 __delay(++xloops);
> 168 }
>
>
> In my early code, loops_per_jiffy is not initialized yet. Hence "lpj" for the asm line
> is 4096 (default value).
>
> The cpu_info.loops_per_jiffy actually reads 8832000 after initialization. They are
> about 2000 times different.
>
> I did a hacky test in kernel to check the difference between these two different
> "lpj" values. (The hacky patch is attached.) Below is the output for 100ms delay.
>
> [ 2.494751] udelay_test uninitialized ---->start
> [ 2.494820] udelay_test uninitialized ---->end
> [ 2.494828] udelay_test initialized ---->start
> [ 2.595234] udelay_test initialized ---->end
>
> For 100ms delay, udelay() with uninitialized loops_per_jiffy only gives a delay of
> only 69us.
Ok, then could we add some simple calibration to make udelay work much better - or
perhaps move the udelay calibration up earlier?
Hiding essentially an early udelay() implementation in an early-printk driver is
ugly and counterproductive.
Thanks,
Ingo
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