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

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Date:	Wed, 14 Nov 2012 08:03:49 +0100
From:	Stefani Seibold <stefani@...bold.net>
To:	Yuanhan Liu <yuanhan.liu@...ux.intel.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] kfifo: round up the fifo size power of 2

Am Freitag, den 09.11.2012, 10:32 +0800 schrieb Yuanhan Liu:
> On Thu, Nov 08, 2012 at 01:37:15PM +0100, Stefani Seibold wrote:
> > Am Donnerstag, den 08.11.2012, 20:24 +0800 schrieb Yuanhan Liu:

> Yes, it is. I will try log API then.
> 
> Stefani, I found an issue while rework to current API. Say the current
> code of __kfifo_init:
>     int __kfifo_init(struct __kfifo *fifo, void *buffer,
>                     unsigned int size, size_t esize)
>     {
>         size /= esize;
> 
>         if (!is_power_of_2(size))
>                 size = rounddown_pow_of_two(size);
>         ....
>     }
> 
> Even thought I changed the API to something like:
>     int __kfifo_init(struct __kfifo *fifo, void *buffer,
>                     int size_order, size_t esize)
>     {
> 	unsigned int size = 1 << size_order;
> 
> 	size /= esize;
> 	...
>     }
> 
> See? There is still a divide and we can't make it sure that it will be
> power of 2 after that.
> 
> So, I came up 2 proposal to fix this.
> 
> 1. refactor the meaning of 'size' argument first.
> 
>    'size' means the size of pre-allocated buffer. We can refactor it to
>    meaning of 'the number of fifo elements' just like __kfifo_alloc, so
>    that we don't need do the size /= esize stuff.
> 
> 2. remove kfifo_init
> 
>    As we can't make sure that kfifo will do exactly what users asked(in
>    the way of fifo size). It would be safe and good to maintain buffer
>    and buffer size inside kfifo. So, I propose to remove it and use
>    kfifo_alloc instead.
> 
>    git grep 'kfifo_init\>' shows that we currently have 2 users only.
> 
> 
> The first way is hacky, and it doesn't make much sense to me. Since
> buffer is pre-allocated by user but not kfifo. User has to calculate
> element size and the number of elements, which is not friendly.
> 
> The second way does make more sense to me.

kfifo_init() was requested by some kernel developers, i never liked it.
If you have a better and cleaner solution than do it, otherwise kick it
away if you like.

- Stefani



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