Re: [PATCH v2] vsprintf: introduce %dE for error constants

[Jani Nikula]

On Wed, 28 Aug 2019, Petr Mladek <pmladek@xxxxxxxx> wrote:
> On Tue 2019-08-27 23:12:44, Uwe Kleine-KÃnig  wrote:
>> Petr Mladek had some concerns:
>> > The array is long, created by cpu&paste, the index of each code
>> > is not obvious.
>> 
>> Yeah right, the array is long. This cannot really be changed because we
>> have that many error codes. I don't understand your concern about the
>> index not being obvious. The array was just a list of (number, string)
>> pairs where the position in the array didn't have any semantic.
>
> I missed that the number was stored in the array as well. I somehow
> expected that it was array of strings.
>
>
>> > There are ideas to make the code even more tricky to reduce
>> > the size, keep it fast.
>> 
>> I think Enrico Weigelt's suggestion to use a case is the best
>> performance-wise so that's what I picked up. Also I hope that
>> performance isn't that important because the need to print an error
>> should not be so common that it really hurts in production.
>
> I personally do not like switch/case. It is a lot of code.
> I wonder if it even saved some space.
>
> If you want to safe space, I would use u16 to store the numbers.
> Or I would use array of strings. There will be only few holes.
>
> You might also consider handling only the most commonly
> used codes from errno.h and errno-base.h (1..133). There will
> be no holes and the codes are stable.
>
>
>> > Both, %dE modifier and the output format (ECODE) is non-standard.
>> 
>> Yeah, obviously right. The problem is that the new modifier does
>> something that wasn't implemented before, so it cannot match any
>> standard. %pI is only known on Linux either, so I think being
>> non-standard is a weak argument.
>
> I am not completely sure that %p modifiers were a good idea.
> They came before I started maintaining printk(). They add more
> complex algorithms into paths where we could not report problems
> easily (printk recursion). Also they are causing problems with
> unit testing that might be done in userspace. These non-standard
> formats cause that printk() can't be simply substituted by printf().
>
> I am not keen to spread these problems over more formats.
> Also %d format is more complicated. It is often used with
> already existing modifiers.
>
>
>> > Upper letters gain a lot of attention. But the error code is
>> > only helper information. Also many error codes are misleading because
>> > they are used either wrongly or there was no better available.
>> 
>> This isn't really an argument against the patch I think. Sure, if a
>> function returned (say) EIO while ETIMEOUT would be better, my patch
>> doesn't improve that detail. Still
>>
>>         mydev: Failed to initialize blablub: EIO
>>
>> is more expressive than
>> 
>>         mydev: Failed to initialize blablub: -5
>
> OK, upper letters probably are not a problem.
>
> But what about EWOULDBLOCK and EDEADLOCK? They have the same
> error codes as EAGAIN and EDEADLK. It might cause a lot of confusion.
> People might spend a lot of time searching for EAGAIN before they
> notice that EWOULDBLOCK was used in the code instead.
>
> Also you still did not answer the question where the idea came from.
> Did it just look nice? Anyone asked for it? Who? Why?
>
>
>> > There is no proof that this approach would be widely acceptable for
>> > subsystem maintainers. Some might not like mass and "blind" code
>> > changes. Some might not like the output at all.
>> 
>> I don't intend to mass convert existing code. I would restrict myself to
>> updating the documentation and then maybe send a patch per subsystem as an
>> example to let maintainers know and judge for themselves if they like it or
>> not. And if it doesn't get picked up, we can just remove the feature again next
>> year (or so).
>
> It looks like a lot of potentially useless work.
>
>
>> I dropped the example conversion, I think the idea should be clear now
>> even without an explicit example.
>
> Please, do the opposite. Add conversion of few subsystems into the
> patchset and add more people into CC. We will see immediately whether
> it makes sense to spend time on this.
>
> I personally think that this feature is not worth the code, data,
> and bikeshedding.

The obvious alternative, I think already mentioned, is to just add
strerror() or similar as a function. I doubt there'd be much opposition
to that. Folks could use %s and strerr(ret). And a follow-up could add
the special format specifier if needed.

BR,
Jani.



-- 
Jani Nikula, Intel Open Source Graphics Center