RFC: Fix kdump failed with 'notsc'

["Wei, Jiangang" <weijg.fnst@cn.fujitsu.com>]

Hi,

When I trigger kernel crash and specify 'notsc' for capture-kernel,
The process of kdump will be blocked at calibrate_delay_converge().

/* wait for "start of" clock tick */
ticks = jiffies;
while (ticks == jiffies)
        ; /* nothing */

The reason is that the jiffies remains the same, no changed.

serial console log as following,
............
[    0.000000] Linux version 4.7.0-rc2+ (root@localhost.localdomain)
(gcc version 4.8.2 20140120 (Red Hat 4.8.2-16) (GCC) ) #2 SMP Wed Jun
156
[    0.000000] Kernel command line: BOOT_IMAGE=/vmlinuz-4.7.0-rc2+ 
root=/dev/mapper/centos-root ro rd.lvm.lv=centos/swap
vconsole.font=latarcyrheb-sun16 rd.lvm.lv=centos/root crashkernel=256M
vconsole.keymap=us console=tty0 console=ttyS0,115200n8 LANG=en_US.UTF-8
irqpoll nr_cpus=1 reset_devices cgroup_disable=memory mce=off numa=off
panic=10 rootflags=nofail acpi_no_memhotplug notsc 
............
[    0.000000] tsc: Kernel compiled with CONFIG_X86_TSC, cannot disable
TSC completely
............
[    0.000000] clocksource: hpet: mask: 0xffffffff max_cycles:
0xffffffff, max_idle_ns: 133484882848 ns
[    0.000000] tsc: Fast TSC calibration using PIT
[    0.000000] tsc: Detected 3192.714 MHz processor
[    0.000000] Calibrating delay loop... 

# The last log is raised by calibrate_delay(), which calls
calibrate_delay_converge() to compute the lpj value. 

# So far, I don't know why the jiffies stays the same.
# But I found two methods can avoid this problem。

1）specify the 'lpj=<n>'  with 'notsc' together.

2)  revert the 70de9a9.

    commit 70de9a97049e0ba79dc040868564408d5ce697f9
    Author: Alok Kataria <akataria@vmware.com>
    Date:   Mon Nov 3 11:18:47 2008 -0800

    x86: don't use tsc_khz to calculate lpj if notsc is passed
    
    Impact: fix udelay when "notsc" boot parameter is passed
    
    With notsc passed on commandline, tsc may not be used for
    udelays, make sure that we do not use tsc_khz to calculate
    the lpj value in such cases.

IMO,
The flow of getting tsc_khz as following,
tsc_init()->x86_platform.calibrate_tsc()->native_calibrate_tsc()->quick_pit_calibrate().
No codes use or call 'rdtsc'.

Even if  ‘notsc’  is passed, the tsc_khz is credible.
and we can get lpj by it.

So I want to push a patch to revert the 70de9a9.
Any comments or suggestions is appreciated.

Thanks,
wei