linux-kernel - Re: [PATCH] iommu/amd: Fix extended features logging

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Date:   Mon, 12 Apr 2021 12:49:19 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Alexander Monakov <amonakov@...ras.ru>,
        Paul Menzel <pmenzel@...gen.mpg.de>,
        Joerg Roedel <jroedel@...e.de>,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        iommu@...ts.linux-foundation.org,
        LKML <linux-kernel@...r.kernel.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joe Perches <joe@...ches.com>
Subject: Re: [PATCH] iommu/amd: Fix extended features logging

On Sun 2021-04-11 21:52:59, John Ogness wrote:
> On 2021-04-11, Alexander Monakov <amonakov@...ras.ru> wrote:
> >>> The second line is emitted via 'pr_cont', which causes it to have a
> >>> different ('warn') loglevel compared to the previous line ('info').
> >>> 
> >>> Commit 9a295ff0ffc9 attempted to rectify this by removing the newline
> >>> from the pci_info format string, but this doesn't work, as pci_info
> >>> calls implicitly append a newline anyway.
> >> 
> >> Hmm, did I really screw that up during my testing? I am sorry about that.
> >> 
> >> I tried to wrap my head around, where the newline is implicitly appended, and
> >> only found the definitions below.
> >> 
> > Yeah, it's not obvious: it happens in kernel/printk/printk.c:vprintk_store
> > where it does
> >
> > 	if (dev_info)
> > 		lflags |= LOG_NEWLINE;
> >
> > It doesn't seem to be documented; I think it prevents using pr_cont with
> > "rich" printk facilities that go via _dev_info.
> >
> > I suspect it quietly changed in commit c313af145b9bc ("printk() - isolate
> > KERN_CONT users from ordinary complete lines").
> 
> Yes, this behavior has been around for a while. I see no reason why it
> should be that way. These days printk does not care if there is dev_info
> included or not.

It seems that it has been this way since the dev_info was
introduced. It was called "dict" at that time. See

	if (buflen && (!cont || dict)) {
		/* no continuation; flush existing buffer */
		log_store(facility, buflevel, NULL, 0, buf, buflen);

in the commit 7ff9554bb578ba02166071d2d ("printk: convert byte-buffer
to variable-length record buffer").

I am not sure why. Well, dev_printk() previously printed the extra
information for every piece. See, the commit c4e00daaa96d3a0786f
("driver-core: extend dev_printk() to pass structured data").


> > Yeah, I saw that, but decided to go with the 'pr_info("")' solution, because
> > it is less magic, and already used in two other drivers.
> 
> Note that what I was suggesting was to fix a different issue: If the
> pr_cont() caller is interrupted by another printk user, then the
> following pr_cont() calls will use the default loglevel. By explicitly
> specifying the loglevel in pr_cont(), you can be sure that those pieces
> get the desired loglevel, even if those pieces get separated off because
> of an interrupting printk user.
> 
> So even if we fix dev_info to allow pr_cont continuation, it still may
> be desirable to specify the loglevel in the pr_cont pieces.
> 
> > pr_info("") will also prepend 'AMD-Vi:' to the feature list, which is
> > nice.
> 
> I'd rather fix dev_info callers to allow pr_cont and then fix any code
> that is using this workaround.
> 
> And if the print maintainers agree it is OK to encourage
> pr_cont(LOGLEVEL "...") usage, then people should really start using
> that if the loglevel on those pieces is important.

To be sure. We would need to repeat also the dev_info() information
in the continuous piece.

By other words. The clean solution would be:

   + remove the automatic newline in
     kernel/printk/printk.c:vprintk_store

   + create wrappers, for example, dev_cont_info()

Best Regards,
Petr