linux-kernel - Re: First kernel patch (optimization)

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Date:	Tue, 15 Sep 2015 14:57:05 -0700
From:	Alexander Duyck <alexander.duyck@...il.com>
To:	Eric Curtin <ericcurtin17@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: First kernel patch (optimization)

On 09/15/2015 12:52 PM, Eric Curtin wrote:
> My first kernel patch, hope I did everything correctly! Instead of calling strlen on every iteration of the for loop, just call it once instead and store in a variable.
>
> Signed-off-by: Eric Curtin <ericcurtin17@...il.com>
>
> diff --git a/tools/usb/usbip/src/usbip_detach.c b/tools/usb/usbip/src/usbip_detach.c
> index 05c6d15..9db9d21 100644
> --- a/tools/usb/usbip/src/usbip_detach.c
> +++ b/tools/usb/usbip/src/usbip_detach.c
> @@ -47,7 +47,9 @@ static int detach_port(char *port)
>   	uint8_t portnum;
>   	char path[PATH_MAX+1];
>
> -	for (unsigned int i = 0; i < strlen(port); i++)
> +	unsigned int port_len = strlen(port);
> +
> +	for (unsigned int i = 0; i < port_len; i++)
>   		if (!isdigit(port[i])) {
>   			err("invalid port %s", port);
>   			return -1;
>

You should probably run this through scripts/checkpatch.pl as I don't 
think declaring i inside the loop is consistent with the kernel coding 
standard.  Also you don't want the space between port_len and the 
declaration of path.

Also you might want to consider just running the loop from port_len down 
to 0 doing something like:
	while (port_len) {
		if (!isdigit(port[--port_len])) {
			err("invalid port %s", port);		
			return -1;

The advantage to running the loop backwards is that you have to carry 
one less variable which means one less register to mess with in the 
final compiled code.

- Alex

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