[Letux-kernel] OMAP5: Debugging kernel not starting... (and clock: dpll_abe_ck failed transition to 'locked')

[H. Nikolaus Schaller]

> Am 11.09.2018 um 18:26 schrieb H. Nikolaus Schaller <hns at goldelico.com>:
> 
> Hi,
> 
>> Am 11.09.2018 um 15:07 schrieb H. Nikolaus Schaller <hns at goldelico.com>:
>> 
>> Hi Tero,
>> 
>> After kernel has booted, there is a single difference
>> in CM_ABE_CLKSTCTRL.
>> 
>> The relevant bit is: CLKACTIVITY_DPLL_ABE_X2_CLK
>> 
>> After reboot, there are bigger differences not wiped
>> out by u-boot.
>> 
>> So the question is what disturbs this clock,
>> especially as CLKACTIVITY_DPLL_ABE_X2_ seems
>> to say that the clock is active (on Pyra) but
>> not on EVM (I had expected the reverse to be more
>> likely).
>> 
>> CM_ABE_CLKSTCTRL remains 0x803 on Pyra and 0x3 on EVM
> 
> Well, if we see the same setup by U-Boot after power-on
> it must be kernel + DTS. Or is this a false conclusion?
> 
> So I started to deconfigure kernel subsystems completely:
> 
> 1. all twl6040 / abe / aess
> 2. all OMAP3 & OMAP4
> 3. all wireless (wl1837 uses some external functional clocks)
> 4. all sound
> 
> And I have checked U-Boot and DTS sources but did not find
> Pyra specific overwrites for any system clock or abe or dpll
> related registers.
> 
> Then I did rm /lib/modules (the most critical ones are configured
> into the kernel so that I can still boot), and interestingly the
> CLKACTIVITY_DPLL_ABE_X2_CLK is now the same (0x3) on Pyra and
> OMAP5EVM. And "reboot" works.
> 
> So I can now simply modprobe blacklist them one after the other
> to find out which one is conflicting.

Bingo!

root at letux:~# more /etc/modprobe.d/blacklist.conf 
blacklist pwm_omap_dmtimer
root at letux:~#

makes the problem go away.

BTW: blacklisting pwm_bl makes CLKACTIVITY_DPLL_ABE_X2_CLK
show the right value but reboot still can't transition
dpll_abe_ck to 'locked'.


Why don't you see this on omap5uevm even if these modules
are loaded? I assume there is usually no panel connected
that makes use of pwm_bl and pwm_omap_dmtimer. And there is
no compatible = "ti,omap-dmtimer-pwm"; in omap5-board-common.dtsi
or omap5-uevm.dts. This is a hardware difference between Pyra
and OMAP5EVM.

So I think it is now up to you to help me to find out why
pwm_omap_dmtimer can influence the dpll_abe_ck...

BR and thanks,
Nikolaus