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

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Message-ID: <45cd5b50-9854-fce7-5f08-f7660abb8691@suse.com>
Date:   Thu, 29 Aug 2019 10:27:04 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Petr Mladek <pmladek@...e.com>,
        Uwe Kleine-König <uwe@...ine-koenig.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Jonathan Corbet <corbet@....net>,
        metux IT consult Enrico Weigelt <lkml@...ux.net>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] vsprintf: introduce %dE for error constants

On 29.08.19 10:12, Petr Mladek wrote:
> On Wed 2019-08-28 21:18:37, Uwe Kleine-König  wrote:
>> Hello Petr,
>>
>> On 8/28/19 1:32 PM, Petr Mladek wrote:
>>> On Tue 2019-08-27 23:12:44, Uwe Kleine-König  wrote:
>>>> Petr Mladek had some concerns:
>>>>> 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.
> 
> This is contadicting. The "best" performance-wise solution was
> choosen in favor of space. The next sentence says that performance
> is not important.
> 
>>> I personally do not like switch/case. It is a lot of code.
>>> I wonder if it even saved some space.
>>
>> I guess we have to die either way. Either it is quick or it is space
>> efficient.
> 
> I am more concerned about the size. Well, array of strings will
> be both fast and size efficient.
> 
>> With the big case I trust the compiler to pick something
>> sensible expecting that it adapts for example to -Osize.
> 
> I am not sure what are the expectations here. I can't imagine
> another translation than:
> 
>     if (val == 1)
>       str = "EPERM";
>     else if (val == 2)
>       str = "ENOENT"
>     else if (val == 3)
>       str = "ESRCH"
>     ...
> 
> It means that all constans will be hardcoded in the code instead
> of in data section. Plus there will be instructions for each
> if/else part.
> 
>>> 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.
>>
>> I'd like to postpone the discussion about "how" until we agreed about
>> the "if at all".
> 
> It seems that all people like this feature.

Hmm, what about already existing format strings conatining "%dE"?

Yes, I could find only one (drivers/staging/speakup/speakup_bns.c), but
nevertheless...

> 
> BTW: I though more about generating or cut&pasting the arrary.
> I can't find any reasonable way how to generate it.

Generate the array and errno.h/errno-base.h from the same source?


Juergen