Re: [MIPS]clocks_calc_mult_shift() may gen a too big mult value

[zhangfx]

Dear Sirs,
> > Thanks for the suggestion. And sorry for I didn't notice the upstream
> > code has already hooked to clocksource_register_hz() in csrc-r4k.c
> > (We're using r4000 clock source)
> >
> > I'm afraid this still doesn't fix my case. Through
> > clocksource_register_hz()->__clocksource_register_scale()->__clocksource_updatefreq_scale,
> > I got a calculated maxsec = (0xffffffff - (0xffffffff>>5))/250000500 =
> > 16        # assume mips_hpt_frequency=250000500
> >
> > With this maxsec, I got a mult of 0xffffde72, still too big.
> Hrmm. Yong Zang is right to suggest clocksource_register_hz(), as the
> intention of that code is to try to avoid these sorts of issues.
>
> What is the corresponding shift value you're getting for the value
> above?
>
> Could you annotate clocks_calc_mult_shift() a little bit to see where
> things might be going wrong?
Let me give some real world data:
in one machine with 500MHz freq,
the calculated freq = 500084016, and clocks_calc_mult_shift() give
  mult = 4294245725
  shift = 30

but in the 1785th call to update_wall_time, due to error correction 
algorithm, the mult become 4293964632,
in next update_wall_time, the ntp_error is 0x301c93b7927c, which lead to 
an adj of 20, then mult is overflow:
   mult = 4293964632 + (1<<20) = 45912
with this mult, if anyone call timekeeping_get_ns or others using mult, 
the time concept will be extremely wrong, so some sleep will 
(almost)never return => virtually hang

We are not abosulately sure that the error source is normal, but anyway 
it is a possible for the code to overflow, and it will cause hang.

For this case, the timekeeping_bigadjust should be able to control adj 
to a maximum of around 20 with the lookahead for any error. So if the 
mult is chosen at shift = 29, then mult becomes 4294245725/2, it will 
not be possible to be overflowed.

In short, choosing a mult close to 2^32 is dangerous. But I don't know 
what's the best way to avoid it for general cases, because I don't know 
how big error can be and the adj can be for different systems.

Regards

Yours
Fuxin Zhang

> thanks
> -john
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