[LEDE-DEV] Difference between AutoLoad vs. AutoProbe for kernel modules?

[John Crispin]

On 10/01/2017 10:53, Zefir Kurtisi wrote:
> On 01/10/2017 09:20 AM, John Crispin wrote:
>>
>>
>> On 10/01/2017 09:17, Baptiste Jonglez wrote:
>>> On Tue, Jan 10, 2017 at 07:56:36AM +0100, John Crispin wrote:
>>>>>> While investigating an issue with module loading order¹, I discovered
>>>>>> that
>>>>>> some kernel packages use AutoProbe, like this:
>>>>>>
>>>>>>      AUTOLOAD:=$(call AutoProbe,xt_hashlimit)
>>>>>>
>>>>>> while some kernel packages use the AutoLoad helper I was used to, with a
>>>>>> priority:
>>>>>>
>>>>>>      AUTOLOAD:=$(call AutoLoad,28,raid0)
>>>>>>
>>>>>> Judging from this commit² and `include/kernel.mk`, it seems the only
>>>>>> difference is that AutoProbe does not include a priority.
>>>>>>
>>>>>> Is the loading order determined automatically for AutoProbe?  If so,
>>>>>> where
>>>>>> is the magic, and why is AutoLoad still needed in some cases?
>>>>>
>>>>> I opened the issue, so using autoload the modules will get a priority
>>>>> specified by the number, for wireguard above 90 would issue only one
>>>>> warning and using autoprobe the module would be loaded by the order of
>>>>> the name ? so setting the xt_hashlimit with a lower number (autoload)
>>>>> will start wireguard without complaining
>>>>>
>>>>
>>>> autoload is like insmod while autoproe is more liek modprobe. kmodloader
>>>> will first load all numbered modules in the given order and then probe
>>>> the remaining ones.
>>>
>>> So, with AutoProbe, there is dependency resolution, similarly to modprobe?
>>> But without using depmod?
>>>
>>
>> correct. kmodloader will scan the ELF header and work out which other
>> modules are required to be loaded. depmod caches this dependency info
>> while kmodloader generates it when needed.
>>
>> 	John
>>
> Worth to note that only explicit dependencies are resolved, i.e. if one ko uses
> another ko (like ath9k -> ath9k_common). It obviously does not (and can't) work if
> one subsystem makes use of a component in another subsystem.
> 
> Let me share a recent experience for illustration and amusement. I had to debug a
> system where a gianfar ETH driver connects to an at803x PHY driver. Approaching
> the problem the usual printk based technique, I ended up in a situation where
> printing a string of size n in gianfar made the system work, while reducing the
> length of the printed string to (n-1) did not. Realizing that prink-ing random
> chars might not be the correct fix for the issue, I was wondering wtf!?
> 
> Turned out problem was caused by the kernel module autoloader. While the PHY and
> ETH driver were loaded and initialized in the background, the additional printk
> caused enough delay to allow the PHY to finish registration before the ETH
> attaches to it. If otherwise the ETH attaches to an enumerated PHY on the bus
> whose driver did not yet finalize its registration, it gets attach to genphy.
> Since everything works more or less, you won't notice unless you look for printk
> output - which itself changes measures :\
> 
> 
> Lesson learned: autoprobe is convenient, but if you need to ensure lower layer
> drivers are loaded before upper layer ones, use autoload.

correct, which is why we tend to add subsystem and lib stuff using
AutoLoad and the rest using AutoProbe

	John

> 
> NOTE: Since module initialization / registration still happens in the background,
> this might not prevent from the effect described above from happening (e.g. in
> case you delay PHY registration long enough with printk), but at least it gives
> you some control for spreading the loading of dependent modules.
> 
> 
> Cheers,
> Zefir
>