linux-kernel - Re: [PATCH v2 01/28] lib/printbuf: New data structure for printing strings

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Date:   Fri, 27 May 2022 12:29:20 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Kent Overstreet <kent.overstreet@...il.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...r.kernel.org,
        rostedt@...dmis.org, senozhatsky@...omium.org,
        andriy.shevchenko@...ux.intel.com, willy@...radead.org
Subject: Re: [PATCH v2 01/28] lib/printbuf: New data structure for printing
 strings

On Thu 2022-05-26 11:21:27, Kent Overstreet wrote:
> On Thu, May 26, 2022 at 05:06:15PM +0200, Petr Mladek wrote:
> > On Thu 2022-05-19 13:23:54, Kent Overstreet wrote:
> > > This adds printbufs: a printbuf points to a char * buffer and knows the
> > > size of the output buffer as well as the current output position.
> > > 
> > > Future patches will be adding more features to printbuf, but initially
> > > printbufs are targeted at refactoring and improving our existing code in
> > > lib/vsprintf.c - so this initial printbuf patch has the features
> > > required for that.
> > 
> > > diff --git a/include/linux/printbuf.h b/include/linux/printbuf.h
> > > new file mode 100644
> > > index 0000000000..40dc07040d
> > > --- /dev/null
> > > +++ b/include/linux/printbuf.h
> > > +static inline void pr_chars(struct printbuf *out, char c, unsigned n)
> > > +{
> > > +	memset(out->buf + out->pos,
> > > +	       c,
> > > +	       min(n, printbuf_remaining(out)));
> > > +	out->pos += n;
> > > +	printbuf_nul_terminate(out);
> > > +}
> > 
> > This function is not later used. Please, do not add API
> > that will not have users in the same patchset.
> > 
> > There are several other cases. I am not going to comment
> > all of them.
> 
> It is used in this patchset, in lib/vsnprintf.c. You sure about the other cases?

Ah, I used outdated cscope. This was bad example.

> > > +static inline void __pr_char(struct printbuf *out, char c)
> > > +{
> > > +	if (printbuf_remaining(out))
> > > +		out->buf[out->pos] = c;
> > > +	out->pos++;
> > > +}
> > > +
> > > +static inline void pr_char(struct printbuf *out, char c)
> > > +{
> > > +	__pr_char(out, c);
> > > +	printbuf_nul_terminate(out);
> > > +}
> > 
> > The "pr_" prefix is a nightmare for me because the same prefix
> > is used also for printk() API ;-)
> > 
> > Could we please use "pb_" instead?
> 
> I'm not entirely against that, but I see printbufs as already in this patchset
> tightly coupled to vsprintf.c and thus quite related to printk, as well - and
> there aren't that many different pr_ things. So I think the shared prefix makes
> some sense, I'd like to hear what others think before making that change.

I would really like to keep the three APIs separated and easy to
distinguish. They are principally different:

1. pr_*() API:

       + wrapper to printk(). They makes the messages available on
	 console and for user-space log daemons while printf()

      + the various pr_*() variants are used to define kernel
	specific features and behavior, for example:
	loglevel, ratelimit, only once. deferred console handling.

       + uses implicit (system) buffer

       + The message format is defined by the 1st parameter. It
	 is the same way as printf() in user-space.

       + It is inspired by printf() from user-space that prints
	 the messages to the standard output.


2. *s*printf() APIs:

       + basically duplicate the same user-space API. It supports
	 some extra %p modifiers. There might be few more
	 incompatibilities.

       + use simple "char *" buffer provided as the 1st parameter

       + the messages format is defined the same way as in
	 the user-space counterparts.


3. printbuf API:

       + append messages into the given printbuf by small pieces

       + format defined by the suffix, for example, _char(),
	 bytes(), units_64(), _tab(), indent()

       + allows to do special operations on the buffer,
	 for example, _reset(), make_room(), atomic_inc()

       + it will be used as low-level API for vscnprinf()
	 implementation, pretty printing API, or
	 stand alone uses.

       + I wonder if there will be variant that will allow
	 to pass the format in the printf() way, e.g.
	 int pb_printf(printbuf *buf, const char *fmt, ...);

       + is there any user space counter part?


Now, it is clear that printfbuf API must be distinguished by another
prefix:

       + it must be clear that it stores the output into printbuf.
	 It is similar to dprintf(), fprintf(), sprintf().

       + It can't be done by the suffix because it is already used
	 to define format of the appended string or extra operation.

       + It must be clear what is low-level API used to implement
	 vsprintf() and high-level API that uses vsprintf().
	 I mean pb_char() vs. pb_printf().


Best Regards,
Petr

PS: I probably won't find time to write more comments on this patchset
    today. I'll continue the following week. It seems that it will
    be a long journey.