linux-kernel - Re: [PATCH 3/4] of: Custom printk format specifier for device node

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Date:   Wed, 14 Jun 2017 13:56:48 -0700
From:   Joe Perches <joe@...ches.com>
To:     Rob Herring <robh@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Mark Rutland <mark.rutland@....com>
Cc:     Pantelis Antoniou <pantelis.antoniou@...sulko.com>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] of: Custom printk format specifier for device node

On Wed, 2017-06-14 at 15:30 -0500, Rob Herring wrote:
> From: Pantelis Antoniou <pantelis.antoniou@...sulko.com>

I think the commit subject is wrong.
It adds an "of" specific bit to vsprintf.c.
The subject should be
'vsprintf:  Add %p extension "%pO" for device tree'

> 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 %pOF\n", node);

Somehow I think this example is poor as node->full_name
is a pretty obvious to read use.  %pOF requires you to
look up or know what the output is going to be.

> More fine-grained control of formatting includes printing the name,
> flag, path-spec name, reference count and others, explained in the
> documentation entry.
> 
> Originally written by Pantelis, but pretty much rewrote the core
> function using existing string/number functions. The 2 passes were
> unnecessary and have been removed. Also, updated the checkpatch.pl
> check.

Some comments about the code.

> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> []
> @@ -1470,6 +1471,123 @@ char *flags_string(char *buf, char *end, void *flags_ptr, const char *fmt)
>  	return format_flags(buf, end, flags, names);
>  }
>  
> +static noinline_for_stack
> +char *device_node_gen_full_name(const struct device_node *np, char *buf, char *end)
> +{
> +	int len, ret;
> +
> +	if (!np || !np->parent)
> +		return buf;
> +
> +	buf = device_node_gen_full_name(np->parent, buf, end);

This is recursive.  How many levels of parents could there be?
Perhaps there should be a recursion limit.

> +
> +	if (buf < end)
> +		len = end - buf;
> +	else
> +		len = 0;
> +	ret = snprintf(buf, len, "/%s", kbasename(np->full_name));
> +	if (ret <= 0)
> +		return buf;
> +	else if (len == 0 || ret < len)
> +		return buf + ret;
> +	return buf + len;
> +}

Does this work with %p<len>OF for a right justified or padded
length string?  Perhaps widen_string should be added.

> +static noinline_for_stack
> +char *device_node_string(char *buf, char *end, struct device_node *dn,
> +			 struct printf_spec spec, const char *fmt)
> +{
> +	char tbuf[sizeof("xxxxxxxxxx") + 1];
> +	const char *fmtp, *p;
> +	int ret;
> +	char *buf_start = buf;
> +	struct property *prop;
> +	bool has_mult, pass;
> +	const struct printf_spec num_spec = {
> +		.flags = SMALL,
> +		.field_width = -1,
> +		.precision = -1,
> +		.base = 10,
> +	};
> +
> +	struct printf_spec str_spec = spec;
> +	str_spec.field_width = -1;
> +
> +	if (!IS_ENABLED(CONFIG_OF))
> +		return string(buf, end, "(!OF)", spec);
> +
> +	if ((unsigned long)dn < PAGE_SIZE)
> +		return string(buf, end, "(null)", spec);
> +
> +	/* simple case without anything any more format specifiers */
> +	if (fmt[1] == '\0' || strcspn(fmt + 1,"fnpPFcCr") > 0)
> +		fmt = "Ff";
> +
> +	for (fmtp = fmt + 1, pass = false; strspn(fmtp,"fnpPFcCr"); fmtp++, pass = true) {

why not
	while (isalpha(*++fmt))
like ip6 or isalnum like FORMAT_TYPE_PTR uses?

> +		if (pass && (*fmtp != 'f')) {
> +			if (buf < end)
> +				*buf = '|';
> +			buf++;
> +		}
> +
> +		switch (*fmtp) {
> +		case 'f':	/* full_name */
> +			if (pass) {
> +				if (buf < end)
> +					*buf = ':';
> +				buf++;
> +			}
> +			buf = device_node_gen_full_name(dn, buf, end);
> +			break;
> +		case 'n':	/* name */
> +			buf = string(buf, end, dn->name, str_spec);
> +			break;
> +		case 'p':	/* phandle */
> +			buf = number(buf, end, (unsigned int)dn->phandle, num_spec);
> +			break;
> +		case 'P':	/* path-spec */
> +			buf = string(buf, end, kbasename(of_node_full_name(dn)), str_spec);
> +			break;
> +		case 'F':	/* flags */
> +			snprintf(tbuf, sizeof(tbuf), "%c%c%c%c",
> +				of_node_check_flag(dn, OF_DYNAMIC) ?
> +					'D' : '-',
> +				of_node_check_flag(dn, OF_DETACHED) ?
> +					'd' : '-',
> +				of_node_check_flag(dn, OF_POPULATED) ?
> +					'P' : '-',
> +				of_node_check_flag(dn,
> +					OF_POPULATED_BUS) ?  'B' : '-');

I'd try to avoid all uses of snprintf as it's effectively
another fairly
large stack frame.

It's probably better to avoid more recursion stack depth use
and just use *buf++ as appropriate.