Date:   Mon, 30 Mar 2020 23:29:24 +0100
From:   Peter Lister <peter@...eshed.quignogs.org.uk>
To:     Jonathan Corbet <corbet@....net>,
        Jani Nikula <jani.nikula@...ux.intel.com>
Cc:     Matthew Wilcox <willy@...radead.org>, linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>, linux-doc@...r.kernel.org
Subject: Re: [PATCH v3 0/1] Compactly make code examples into literal blocks

On 27/03/2020 16:41, Jonathan Corbet wrote:

> On Fri, 27 Mar 2020 13:28:54 +0200
> Jani Nikula <jani.nikula@...ux.intel.com> wrote:
>
>> IMHO the real problem is kernel-doc doing too much preprocessing on the
>> input, preventing us from doing what would be the sensible thing in
>> rst. The more we try to fix the problem by adding more kernel-doc
>> processing, the further we dig ourselves into this hole.
>>
>> If kernel-doc didn't have its own notion of section headers, such as
>> "example:", we wouldn't have this problem to begin with. We could just
>> use the usual rst construct; "example::" followed by an indented block.
>>
>> I'm not going to stand in the way of the patch, but I'm telling you,
>> this is going to get harder, not easier, on this path.
> I agree with you in principle.  The problem, of course, is that this is a
> legacy gift from before the RST days and it will be hard to change.
>
> A quick grep shows that the pattern:
>
> 	* Example:
>
> appears nearly 100 times in current kernels.  It is not inconceivable to
> make a push to get rid of all of those, turning them into ordinary RST
> syntax - especially since not all of those are actually kerneldoc
> comments.
>
> The same quick grep says that "returns?:" appears about 10,000 times.
> *That* will be painful to change, and I can only imagine that some
> resistance would have to be overcome at some point.
>
> So what do folks think we should do? :)
>
> I want to ponder on this for a bit.  Peter, that may mean that I hold this
> patch past the 5.7 merge window, which perhaps makes sense at this point
> anyway, sorry.  But I really would like to push things into a direction
> that moves us away from gnarly perl hacks and toward something more
> maintainable in the long term.

I would have been surprised if it had been accepted as is.

Matthew and Greg, thanks for reviewing - I have a feeling you might need 
to do this a few times more.

Over the past few days, I too have been pondering, certain thatthis 
patch, a mini tweak of the existing kernel-doc, is not theright 
answer.Equally, I'm unconvinced that the "right" answer is a wholesale 
move to ReST, so where's the happy medium?

<alert: long email, tldr: "Finding the happy medium on kerneldoc layout 
in C source comments">

A week or two back, I tried to fix doc build "indentation" warnings due 
to return value listsin sfb-bus.c. Russell King didn't like my patch 
saying "I think it's more important that the documentation interferes to 
a minimal degree with the code in the file".[ Mauro's patch to sfb-bus.c 
is now in linux-next. He fixed the problem with a bullet list - 
thriftier with the line breaks andthe official fix in the kerneldoc 
notes - but I'll argue that a simple list or definition list might be 
more appropriate. ]

Russell and Matthew argue that the primary purpose of source annotation 
is to aid developers and that any significant detraction (verbosity, 
whitespace) is not excused by prettier docs. FWIW, my background is 
sysadmin (much perl) and system/kernel programming (mostly C) and I 
agree with them.

Jani, if you see risks of complexity and maintenance problems, then so 
do I. But the point of kernel-doc is surely to be a specific semantic 
markup which works for developers and maintainers and allows doc authors 
access to their annotation.  The format clearly needs thought,perl is 
less fashionable than it was and kernel-doc - er - needs work, but I 
don't see the *idea* as inherently broken. I suggest that there would be 
good developer buy-in to searchable pretty docs if a few compact idioms 
reliably did The Right Thing and didn't spit out doc build warnings.

The build warnings I currently observe do not tell me "the author got it 
wrong", but rather "kernel-doc didn't understand the author's intention".

Consider this excerpt from the kerneldoc comment for bitmap_cut(), 
recently added to lib/bitmap.c. (NB - this is an example in the 
description section - the parameters, including src, have already been 
described).

  * In pictures, example for a big-endian 32-bit architecture:
  *
  * @src:
  * 31                                   63
  * |                                    |
  * 10000000 11000001 11110010 00010101  10000000 11000001 01110010 00010101
  *                 | |              |                                    |
  *                16 14             0                                   32
  *

This fails because 1) the diagram isn't made literal (and so generates a 
indentation build warning) and 2) "@src:" is interpreted as an extra 
definition of src which scrunges the first one. I find it hard to assert 
that the author's intentions were wrong; it's the kind of good 
annotation we should hope for in a newly added function. If "* @src:" 
gained an double colon and we guaranteed that, after a description 
section header, references to a parameter didn't overwrite the original 
definition, this would work fine.

My instinct is to fix doc build issues with minimal changes: not actual 
ReST, but clear idioms reliably generating good ReST. This should be 
accompanied by tests for developers and reviewers so that we can have a 
fair stab at getting it right first time and (of course) documentation.

Could I ask anyone who disagrees to suggest their preferred way to lay 
out the comment for bitmap_cut()?

One head-on approachis to literalise *all* kerneldoc comments for 
functions and structures. The kerneldoc keywords then serve only to 
generate links; the ReST output is minimal but guaranteed validand 
warning free. Would any readers of API docs be inconvenienced? The 
target readership are presumably programmers, and the searchability of 
the sphinx RTD is more useful to me than the formatting.

All the best,
Peter

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