linux-kernel - Re: [PATCH v9 4/4] serial: pl011: add console matching function

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Date:	Mon, 15 Aug 2016 13:39:34 +0100
From:	Russell King - ARM Linux <linux@...linux.org.uk>
To:	Aleksey Makarov <aleksey.makarov@...aro.org>
Cc:	"Rafael J . Wysocki" <rjw@...ysocki.net>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-serial@...r.kernel.org, linux-acpi@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	Len Brown <lenb@...nel.org>,
	Leif Lindholm <leif.lindholm@...aro.org>,
	Graeme Gregory <graeme.gregory@...aro.org>,
	Al Stone <ahs3@...hat.com>,
	Christopher Covington <cov@...eaurora.org>,
	Yury Norov <ynorov@...iumnetworks.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Andy Shevchenko <andy.shevchenko@...il.com>,
	"Zheng, Lv" <lv.zheng@...el.com>, Mark Salter <msalter@...hat.com>,
	Kefeng Wang <wangkefeng.wang@...wei.com>,
	Jiri Slaby <jslaby@...e.com>
Subject: Re: [PATCH v9 4/4] serial: pl011: add console matching function

On Thu, Aug 11, 2016 at 06:31:41PM +0300, Aleksey Makarov wrote:
> +/**
> + *	pl011_console_match - non-standard console matching
> + *	@co:	  registering console
> + *	@name:	  name from console command line
> + *	@idx:	  index from console command line
> + *	@options: ptr to option string from console command line
> + *
> + *	Only attempts to match console command lines of the form:
> + *	    console=pl011,mmio|mmio32,<addr>[,<options>]
> + *	    console=pl011,0x<addr>[,<options>]
> + *	This form is used to register an initial earlycon boot console and
> + *	replace it with the amba_console at pl011 driver init.
> + *
> + *	Performs console setup for a match (as required by interface)
> + *	If no <options> are specified, then assume the h/w is already setup.
> + *
> + *	Returns 0 if console matches; otherwise non-zero to use default matching
> + */
> +static int __init pl011_console_match(struct console *co, char *name, int idx,
> +				      char *options)
> +{
> +	char match[] = "pl011";	/* pl011-specific earlycon name */

This is inefficient - the compiler will probably store "pl011" in the
rodata section, then allocate an array on the stack, and them memcpy()
it onto the stack.  This is really a false optimisation.

> +	unsigned char iotype;
> +	unsigned long addr;
> +	int i;
> +
> +	if (strncmp(name, match, 5) != 0)

Just do:

	if (strncmp(name, "pl011", 5) != 0)

here, and let the compiler work it out - it'll probably place "pl011"
in the rodata section, and use a pointer to it rather than messing around
with the stack.

What if "console=pl011x,..." is passed?  Should this be matched too?
Maybe this should compare with "pl011," to ensure that the name is
correctly terminated?

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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