linux-kernel - Re: [PATCH] of: Custom printk format specifier for device node

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Message-Id: <20150331170229.58DCEC40856@trevor.secretlab.ca>
Date:	Tue, 31 Mar 2015 10:02:29 -0700
From:	Grant Likely <grant.likely@...retlab.ca>
To:	Pantelis Antoniou <pantelis.antoniou@...sulko.com>
Cc:	Joe Perches <joe@...ches.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rob Herring <robherring2@...il.com>,
	Randy Dunlap <rdunlap@...radead.org>,
	linux-doc@...r.kernel.org, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] of: Custom printk format specifier for device node

Hi Pantelis,

Thanks for the quick reply. Comments below...

On Tue, 31 Mar 2015 13:03:05 +0300
, Pantelis Antoniou <pantelis.antoniou@...sulko.com>
 wrote:
> Hi Grant,
> 
> > On Mar 30, 2015, at 22:04 , Grant Likely <grant.likely@...retlab.ca> wrote:
> > 
> > On Thu, 22 Jan 2015 22:31:46 +0200
> > , Pantelis Antoniou <panto@...oniou-consulting.com>
> > wrote:
> >> Hi Joe,
> >> 
> >>> On Jan 21, 2015, at 19:37 , Joe Perches <joe@...ches.com> wrote:
> >>> 
> >>> On Wed, 2015-01-21 at 19:06 +0200, Pantelis Antoniou wrote:
> >>>> 90% of the usage of device node's full_name is printing it out
> >>>> in a kernel message. Preparing for the eventual delayed allocation
> >>>> introduce a custom printk format specifier that is both more
> >>>> compact and more pleasant to the eye.
> >>>> 
> >>>> For instance typical use is:
> >>>> 	pr_info("Frobbing node %s\n", node->full_name);
> >>>> 
> >>>> Which can be written now as:
> >>>> 	pr_info("Frobbing node %pO\n", node);
> >> 
> >>> Still disliking use of %p0.
> >>> 
> >> 
> >> pO - Open Firmware
> >> 
> >> pT for tree is bad, cause we plan to use a tree type in the future in OF.
> > 
> > So, here's a radical thought. How about we reserve '%pO' for objects, as
> > in kobjects.  We'll use extra flags to narrow down specifically to
> > device tree nodes, but we could teach vsprintf() to treat a plain '%pO'
> > as plain kobject pointer, and if it is able to recognize the kobj_type,
> > then run a specific decoder to format it.
> > 
> > This also gives us a namespace for various kinds of firmware data
> > output. %Od could be a struct device, %On for device tree node, %Oa for
> > an ACPI node, etc.
> > 
> 
> I’m fine with this. I also have a patch to turn an overlay to a kobj
> so this fits naturally.
> 
> OTOH if we do this, I would expect to rework the custom printk infrastructure
> to be more generic.
> 
> IMHO having the format specifier and the format print methods in lib/vsprintf.c
> is not very nice.
> 
> How about having a way to register object printk handlers with something like that?
> We could put that in a special linker section and have the printk method pass control
> there.
> 
> PRINTK_OBJFMT(’n’, printk_objfmt_device_node);
> 
> We might have to make a few printk methods public however.

Honestly, I think trying to add registration is an overengineered
solution at this point. We're not hitting a wall on the complexity of
vsprintf.c, and having them all in one place helps to ensure we don't
have conflicts.

> 
> > I've dropped the refcount decoder. I know it is useful for debugging the
> > core DT code, but it isn't something that will be used generally. Plus
> > the returned value cannot be relied upon to be stable because there may
> > be other code currently iterating over the tree.
> > 
> 
> Yeah, I know it’s not something to rely on. If we do %pOk to be kobj
> debug I can add it back in.

Yes, that would be a good place to have refcount output.

> > +Device tree nodes:
> > +
> > +	%pOn[fnpPcCFr]
> > +
> > +	For printing device tree nodes. The optional arguments are:
> > +	    f device node full_name
> > +	    n device node name
> > +	    p device node phandle
> > +	    P device node path spec (name + @unit)
> > +	    F device node flags
> > +	    c major compatible string
> > +	    C full compatible string
> > +	Without any arguments prints full_name (same as %pOnf)
> > +	The separator when using multiple arguments is ‘:’
> ^ separator is ‘.'

? I'm confused? The separator that I'm using is a colon. ':'  Where do
you see ','? I don't think ',' would be a good separator because it
appears in node names and compatible strings. Originally, I think you
were using pipe '|', but my personal opinion is that ':' is better
because there is already precidence as a separator.

g.
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