Re: [PATCH 2/5] sound: soc: skylake: no need to check return value of debugfs_create functions

[Cezary Rojewski <cezary.rojewski@intel.com>]

On 2019-06-23 06:57, Greg Kroah-Hartman wrote:
> On Sat, Jun 22, 2019 at 09:57:07PM +0200, Cezary Rojewski wrote:
>>
>> On 2019-06-14 11:47, Greg Kroah-Hartman wrote:
>>> When calling debugfs functions, there is no need to ever check the
>>> return value.  The function can work or not, but the code logic should
>>> never do something different based on this.
>>>
>>
>> This change heavily impacts user space and development kits used by us
>> internally, and our clients. That is, if anything goes wrong during debugfs
>> initialization process.
> 
> As Takashi said, and as I said numerous times, how can anything go wrong
> during debugfs file creation that does not also cause the rest of your
> system to just crash. >
> userspace should NEVER care about a debugfs file being present or not.
> If it does, then you should not be using debugfs as it is never
> guaranteed to be present on a system (and is locked down and removed on
> many shipping systems for good reason.)
> 
> For development, it's wonderful, but it truely is just a debugging aid.
> 
>> Currently, apps may safely assume entire debugfs tree is up and running once
>> audio stack gets enumerated successfully. With your patch this is no longer
>> the case and user space is forced to verify status of all debugfs files and/
>> or directories manually.
> 
> What apps rely on debugfs for audio?  We need to fix those.
> 

Takashi,
Thanks for reply. I hope you don't mind me replying here and not 
explicitly on your post. My message would be exactly the same as the one 
you see below.


Greg,
Forgive me for not clarifying: by userspace of course I meant any 
development/ debug related app which we use exhaustively.

Look at this from different perspective: I'm "just" a user of debugfs 
interface. I call a function and given its declaration I receive either 
0 on success or != 0 on failure. Definition of said function may change 
in time and -ENOMEM might not be the only possible outcome, but that I 
leave to other developers and as long as behavior remains the same, 
changes are welcome.

Moreover, if we're compiling with CONFIG_DEBUGFS=1, driver may choose to 
collapse during probe if any of debugfs objects fail to initialize: in 
this case one can say CONFIG_DEBUGFS adds yet another condition for 
enumeration to be considered successful.

> Again, my goal with these changes is two things:
>    - no kernel operation should ever modify its behavior if debugfs is
>      enabled, or working, at all.
>    - no normal userspace code should ever care if debugfs is working
> 
> debugfs is for debugging things, that is all.  If you have system
> functionality relying on files in debugfs, they need to be moved to a
> system functionality that is always going to be present for your users
> (i.e. sysfs, configfs, tracefs, etc.)
> 
> thanks,
> 
> greg k-h
> 

With mindset "may or may not succeed" one might as well resign from 
debugfs entirely and move to sysfs and other fs you'd mentioned. And why 
would he otherwise, when the only way to verify debugfs object state is 
either log parsing (filtering errors) or file-exists check.

My app should not be guessing, instead, it should know upfront with what 
its working with.

Czarek