From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1752006AbdH3C6K (ORCPT <rfc822;w@1wt.eu>);
        Tue, 29 Aug 2017 22:58:10 -0400
Received: from smtprelay0018.hostedemail.com ([216.40.44.18]:36747 "EHLO
        smtprelay.hostedemail.com" rhost-flags-OK-OK-OK-FAIL)
        by vger.kernel.org with ESMTP id S1751318AbdH3C6G (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Tue, 29 Aug 2017 22:58:06 -0400
X-Session-Marker: 6A6F6540706572636865732E636F6D
X-Spam-Summary: 2,0,0,,d41d8cd98f00b204,joe@perches.com,:::::::::::::::::::::::,RULES_HIT:41:355:379:541:599:960:973:988:989:1260:1277:1311:1313:1314:1345:1359:1373:1437:1515:1516:1518:1534:1542:1593:1594:1711:1730:1747:1777:1792:2393:2559:2562:2692:2693:2828:3138:3139:3140:3141:3142:3353:3622:3865:3866:3867:3868:3870:3871:3872:3873:3874:4250:4321:4605:5007:6742:10004:10400:10848:11026:11232:11473:11658:11914:12043:12296:12438:12740:12760:12895:13161:13229:13439:14659:14721:21080:21088:21433:21627:30054:30060:30075:30090:30091,0,RBL:none,CacheIP:none,Bayesian:0.5,0.5,0.5,Netcheck:none,DomainCache:0,MSF:not bulk,SPF:,MSBL:0,DNSBL:none,Custom_rules:0:0:0,LFtime:1,LUA_SUMMARY:none
X-HE-Tag: pies85_807a559f7d120
X-Filterd-Recvd-Size: 4260
Message-ID: <1504061881.2786.11.camel@perches.com>
Subject: Re: printk: what is going on with additional newlines?
From: Joe Perches <joe@perches.com>
To: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Pavel Machek <pavel@ucw.cz>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Petr Mladek <pmladek@suse.com>, Jan Kara <jack@suse.cz>,
        Andrew Morton <akpm@linux-foundation.org>,
        Jiri Slaby <jslaby@suse.com>, Andreas Mohr <andi@lisas.de>,
        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Date: Tue, 29 Aug 2017 19:58:01 -0700
In-Reply-To: <20170830024703.GA17175@jagdpanzerIV.localdomain>
References: <20170828124634.GD492@amd>
         <20170829134048.GA437@jagdpanzerIV.localdomain>
         <CA+55aFwmwdY_mMqdEyFPpRhCKRyeqj=+aCqe5nN108v8ELFvPw@mail.gmail.com>
         <CA+55aFyzR4LKhJKLFgvvd9OTsos2_g4-9fova782BX4kyA3bLA@mail.gmail.com>
         <20170829195013.5048dc42@gandalf.local.home>
         <20170830010348.GB654@jagdpanzerIV.localdomain>
         <20170829211046.74644c8a@gandalf.local.home>
         <1504057959.2786.4.camel@perches.com>
         <20170830022528.GA17968@jagdpanzerIV.localdomain>
         <1504060296.2786.8.camel@perches.com>
         <20170830024703.GA17175@jagdpanzerIV.localdomain>
Content-Type: text/plain; charset="ISO-8859-1"
X-Mailer: Evolution 3.22.6-1ubuntu1 
Mime-Version: 1.0
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Wed, 2017-08-30 at 11:47 +0900, Sergey Senozhatsky wrote:
> On (08/29/17 19:31), Joe Perches wrote:
> [..]
> > > the idea is not to do printk() on that seq buffer at all, but to
> > > log_store(), atomically, seq buffer messages
> > > 
> > > 	spin_lock(&logbuf_lock)
> > > 	while (offset < seq_buffer->len) {
> > > 		...
> > > 		log_store(seq->buffer + offset);
> > > 		...
> > > 	}
> > > 	spin_unlock(&logbuf_unlock)
> > 
> > Why?
> > 
> > What's wrong with a simple printk?
> > It'd still do a log_store.
> 
> sure, it will. but in separate logbuf entries, and between two
> consequent printk calls on the same CPU a lot of stuff can happen:

I think you don't quite understand how this would work.
The idea is that the entire concatenated bit would be emitted
in one go.

One use case already in place with seq_buf_init is in
drivers/clk/tegra/clk-bpmp.c

Basically, it's

    static void tegra_bpmp_clk_info_dump(struct tegra_bpmp *bpmp,
    	    	    	    	         const char *level,
    	    	    	    	         const struct tegra_bpmp_clk_info *info)
    {
    	    const char *prefix = "";
    	    struct seq_buf buf;
    	    unsigned int i;
    	    char flags[64];

    	    seq_buf_init(&buf, flags, sizeof(flags));

    	    if (info->flags)
    	    	    seq_buf_printf(&buf, "(");

    	    if (info->flags & TEGRA_BPMP_CLK_HAS_MUX) {
    	    	    seq_buf_printf(&buf, "%smux", prefix);
    	    	    prefix = ", ";
    	    }

    	    if ((info->flags & TEGRA_BPMP_CLK_HAS_SET_RATE) == 0) {
    	    	    seq_buf_printf(&buf, "%sfixed", prefix);
    	    	    prefix = ", ";
    	    }

    	    if (info->flags & TEGRA_BPMP_CLK_IS_ROOT) {
    	    	    seq_buf_printf(&buf, "%sroot", prefix);
    	    	    prefix = ", ";
    	    }

    	    if (info->flags)
    	    	    seq_buf_printf(&buf, ")");

    	    [...]
    	    dev_printk(level, bpmp->dev, "  flags: %lx %s\n", info->flags, flags);

so that the dev_printk is simply emitting a buffer from
concatenated strings via the seq_buf_printf uses.

The other use case would be an entire printk buffer
all at once.

ala:

	seq_buf_init(seq, buf, sizeof(buf));
	seq_buf_printf(&buf, "KERN_<LEVEL> fmt...", args...)
	for (i = 0; i < bar; i++)
		seq_buf_printf(&buf, fmt, ...)
	seq_buf_printk(&buf);

or:

	seq_buf_init(seq, buf, sizeof(buf));
	seq_buf_printf(&buf,
fmt, args...)
	for (i = 0; i < bar; i++)
		seq_buf_printf(&bu
f, fmt, args...)
	seq_buf_printk(&buf, KERN_<LEVEL>);

I can't think of another use case.

Can you?