Re: Regression on colibri-imx7 due to commit 9c1b13b3fd27 ("cyclic: reduce get_timer_us() calls inside hlist_for_each_entry_safe()")

[Emanuele Ghidoli <ghidoliemanuele@gmail.com>]

On 7/24/26 02:00, Marek Vasut wrote:
> On 7/23/26 10:41 PM, Emanuele Ghidoli wrote:
>>
>>
>> On 7/23/26 20:35, Marek Vasut wrote:
>>> On 7/23/26 4:53 PM, Emanuele Ghidoli wrote:
>>>
>>> Hello everyone,
>>>
>>>> I found that Colibri iMX7 no longer boots when commit 9c1b13b3fd27
>>>> ("cyclic: reduce get_timer_us() calls inside hlist_for_each_entry_safe()")
>>>> is applied. Nothing is printed on the serial.
>>>>
>>>> I bisected the issue: booting the parent commit works fine, and
>>>> reverting this patch on top of current main also restores
>>>> boot. It's not yet obvious to me what in this change causes the
>>>> regression.
>>>>
>>>> Would it make sense to revert it until the root cause is found?
>>>
>>> Give Patrice a bit of time to analyze the problem.
>>>
>>
>> Hello,
>>
>> after the regression, get_timer_us() is called on every cyclic_run() call,
>> even when the cyclic list is empty. On imx7 this happens earlier than before,
>> when the timer has not yet been initialized: get_timer_us() calls get_tbclk(),
>> whose value is zero, and uses it as a divisor in tick_to_time_us(), leading to
>> a division by zero.
>>
>> I would fix this by ensuring get_tbclk() returns at least 1 on imx7, but we
>> could hide bugs in this way. However, this bug could also be present on other
>> architectures, so I would just guard against entering the cyclic loop by
>> checking whether the list is empty before doing anything else, so
>> get_timer_us() is called "at the same point in time" as before.
>> Something like:
>> if (hlist_empty(cyclic_get_list())) {
>>     gd->flags &= ~GD_FLG_CYCLIC_RUNNING;
>>     return;
>> }
> It looks like there are two topics -- optimize the cyclic run and fix the MX7
> timer.
> 
> I agree the cyclic run should exit if the list is empty, but please wait for
> input from Rasmus on that.
> 
> Regarding the timer, how come the timer is initialized so late on MX7 ?
timer_init() called in initcall_run_f().>
> The Cortex-A7 should have its own ARM timer, that should be available right
> from the beginning. Is that ARM timer in use on your system, or does your
> system use GPT timer ?
iMX7 uses arch/arm/mach-imx/syscounter.c.

udelay() is called in initcall_run_f(), before timer_init().
arch_cpu_init() (arch/arm/mach-imx/mx7/soc.c) -> imx_gpcv2_init -> udelay
(which runs schedule/cyclic).

And this is interesting, udelay() before timer_init() leads to a 0 us delay,
so there is another bug.

I have verified that removing this udelay(65) the board boots.
> 
> Which board is this ?
board/toradex/colibri_imx7/
> 
> +CC Peng

Kind regards,
Emanuele