Re: [PATCH 2/2] docs/misra: add Rule 5.1

[Jan Beulich <jbeulich@xxxxxxxx>]

On 26.05.2022 03:18, Stefano Stabellini wrote:
> On Wed, 25 May 2022, Jan Beulich wrote:
>> On 25.05.2022 02:35, Stefano Stabellini wrote:
>>> From: Stefano Stabellini <stefano.stabellini@xxxxxxxxxx>
>>>
>>> Add Rule 5.1, with the additional note that the character limit for Xen
>>> is 63 characters.
>>>
>>> The max length identifiers found by ECLAIR are:
>>>
>>> __mitigate_spectre_bhb_clear_insn_start
>>> domain_pause_by_systemcontroller_nosync
>>>
>>> Both of them are 40 characters long. A limit of 63 characters work for
>>> the existing code.
>>
>> I have to admit that it hasn't become clear to me why we want to
>> permit (if not to say encourage) the use of such long identifiers.
>> If 40 is the longest we've got, why not limit it to 40 for now
>> with a goal of further reducing? A 40-char symbol plus some
>> indentation will already pose problems with 80-char line length.
>  
> We can go lower than 63 if we want to. I chose the closest power-of-two
> length -1 for the NUL terminator. But it doesn't have to be a
> power-of-two. So we could go with "41" if we want to.
> 
> Anyone in favor of that? I am happy with any number between 41 and 63.

Why 41, not 40? 41 seems yet more arbitrary to me than e.g. 40.

>> Otoh, as said on the call, I think the public headers want
>> mentioning explicitly here in some way. Part of them (most or all
>> of what's under io/) aren't used when building Xen, so won't be
>> seen by Eclair (aiui). Yet they are a formal part of the code
>> base, and e.g. ring.h has some pretty long names (albeit still
>> below 40 chars as it looks). So once we're able to go down to e.g.
>> 32 for the bulk of the code base, public headers should imo still
>> be explicitly allowed to use longer identifiers.
> 
> Actually I thought about writing something for the public header but I
> wasn't sure what to write. What about:
> 
> - Note: the Xen characters limit for identifiers is 41. Public headers
>   (xen/include/public/) are allowed to retain longer identifiers for
>   backward compatibility.

Fine with me, except I wonder in how far going forward we actually
need to play by that limit there. Proper name-spacing is particularly
important in the public headers, so may warrant a higher limit for
certain (unusual?) circumstances.

Jan