linux-kernel - Re: [PATCH] serial: vt8500_serial: Fix a parameter of find_first_zero

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Date:   Mon, 22 Aug 2016 10:42:28 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     linux-arm-kernel@...ts.infradead.org
Cc:     Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        linux@...sktech.co.nz, gregkh@...uxfoundation.org, jslaby@...e.com,
        kernel-janitors@...r.kernel.org, linux-serial@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] serial: vt8500_serial: Fix a parameter of find_first_zero_bit.

On Sunday, August 21, 2016 11:20:25 PM CEST Christophe JAILLET wrote:
> The 2nd parameter of 'find_first_zero_bit' is the number of bits to search.
> In this case, we are passing 'sizeof(vt8500_ports_in_use)'.
> 'vt8500_ports_in_use' is an 'unsigned long'. So the sizeof is likely to
> return 4.
> 
> A few lines below, we check if it is below VT8500_MAX_PORTS, which is 6.
> 
> It is likely that the number of bits in a long was expected here, so use
> BITS_PER_LONG instead.
> 
> 
> It has been spotted by the following coccinelle script:
> @@
> expression ret, x;
> 
> @@
> *  ret = \(find_first_bit \| find_first_zero_bit\) (x, sizeof(...));
> 
> Signed-off-by: Christophe JAILLET <christophe.jaillet@...adoo.fr>
> ---
> Other options are possible:
>   - 'vt8500_ports_in_use' being a 'unsigned long', use ffz to reduce
>     code verbosity
>   - VT8500_MAX_PORTS, in order to be consistent with the test below

Sorry, but I'm not following the logic here.

> ---
>  drivers/tty/serial/vt8500_serial.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/serial/vt8500_serial.c b/drivers/tty/serial/vt8500_serial.c
> index 23cfc5e16b45..935076c50cb1 100644
> --- a/drivers/tty/serial/vt8500_serial.c
> +++ b/drivers/tty/serial/vt8500_serial.c
> @@ -664,7 +664,7 @@ static int vt8500_serial_probe(struct platform_device *pdev)
>  	if (port < 0) {
>  		/* calculate the port id */
>  		port = find_first_zero_bit(&vt8500_ports_in_use,
> -					sizeof(vt8500_ports_in_use));
> +					   BITS_PER_LONG);
>  	}

You argue that the two have the same meaning, which I see, but
why is it better than the existing code?

	Arnd