linux-kernel - Re: [PATCH 1/2] kfifo: round up the fifo size power of 2

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Message-ID: <1351243827.8719.5.camel@wall-e>
Date:	Fri, 26 Oct 2012 11:30:27 +0200
From:	Stefani Seibold <stefani@...bold.net>
To:	Yuanhan Liu <yuanhan.liu@...ux.intel.com>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 1/2] kfifo: round up the fifo size power of 2

Am Freitag, den 26.10.2012, 15:56 +0800 schrieb Yuanhan Liu:
> Say, if we want to allocate a filo with size of 6 bytes, it would be safer
> to allocate 8 bytes instead of 4 bytes.
> ----
> I know it works with rounddown_pow_of_two as well, since size is maintained
> in the kfifo internal part. But, I'm quite curious why Stefani chose
> rounddown_pow_of_two. To reduce memory?
> 

Yes, exactly, if a user do the wrong thing, than the user will get also
a wrong result, and did not waste memory.

But anyway, if the majority like this patch it is okay for me.

> Thanks,
> Yuanhan Liu
> -----
> 
> Cc: Stefani Seibold <stefani@...bold.net>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Signed-off-by: Yuanhan Liu <yuanhan.liu@...ux.intel.com>
> ---
>  kernel/kfifo.c |    6 +++---
>  1 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/kfifo.c b/kernel/kfifo.c
> index 59dcf5b..0f78378 100644
> --- a/kernel/kfifo.c
> +++ b/kernel/kfifo.c
> @@ -39,11 +39,11 @@ int __kfifo_alloc(struct __kfifo *fifo, unsigned int size,
>  		size_t esize, gfp_t gfp_mask)
>  {
>  	/*
> -	 * round down to the next power of 2, since our 'let the indices
> +	 * round up to the next power of 2, since our 'let the indices
>  	 * wrap' technique works only in this case.
>  	 */
>  	if (!is_power_of_2(size))
> -		size = rounddown_pow_of_two(size);
> +		size = roundup_pow_of_two(size);
>  
>  	fifo->in = 0;
>  	fifo->out = 0;
> @@ -84,7 +84,7 @@ int __kfifo_init(struct __kfifo *fifo, void *buffer,
>  	size /= esize;
>  
>  	if (!is_power_of_2(size))
> -		size = rounddown_pow_of_two(size);
> +		size = roundup_pow_of_two(size);
>  
>  	fifo->in = 0;
>  	fifo->out = 0;


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