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Subject: Re: [PATCH 5/5] kfifo: log based kfifo API
From: Andy Walls <awalls@md.metrocast.net>
Date: Tue, 08 Jan 2013 16:10:07 -0500
To: Dmitry Torokhov <dmitry.torokhov@gmail.com>,
	Yuanhan Liu <yuanhan.liu@linux.intel.com>
CC: linux-kernel@vger.kernel.org,
	Stefani Seibold <stefani@seibold.net>,
	Andrew Morton <akpm@linux-foundation.org>,
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Dmitry Torokhov <dmitry.torokhov@gmail.com> wrote:

>Hi Yuanhan,
>
>On Tue, Jan 08, 2013 at 10:57:53PM +0800, Yuanhan Liu wrote:
>> The current kfifo API take the kfifo size as input, while it rounds
>>  _down_ the size to power of 2 at __kfifo_alloc. This may introduce
>> potential issue.
>> 
>> Take the code at drivers/hid/hid-logitech-dj.c as example:
>> 
>> 	if (kfifo_alloc(&djrcv_dev->notif_fifo,
>>                        DJ_MAX_NUMBER_NOTIFICATIONS * sizeof(struct
>dj_report),
>>                        GFP_KERNEL)) {
>> 
>> Where, DJ_MAX_NUMBER_NOTIFICATIONS is 8, and sizeo of(struct
>dj_report)
>> is 15.
>> 
>> Which means it wants to allocate a kfifo buffer which can store 8
>> dj_report entries at once. The expected kfifo buffer size would be
>> 8 * 15 = 120 then. While, in the end, __kfifo_alloc will turn the
>> size to rounddown_power_of_2(120) =  64, and then allocate a buf
>> with 64 bytes, which I don't think this is the original author want.
>> 
>> With the new log API, we can do like following:
>> 
>> 	int kfifo_size_order = order_base_2(DJ_MAX_NUMBER_NOTIFICATIONS *
>> 					    sizeof(struct dj_report));
>> 
>> 	if (kfifo_alloc(&djrcv_dev->notif_fifo, kfifo_size_order,
>GFP_KERNEL)) {
>> 
>> This make sure we will allocate enough kfifo buffer for holding
>> DJ_MAX_NUMBER_NOTIFICATIONS dj_report entries.
>
>Why don't you simply change __kfifo_alloc to round the allocation up
>instead of down?
>
>Thanks.
>
>-- 
>Dmitry
>--
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Hi Dmitry,

I agree.   I don't see the benefit in pushing up the change to a kfifo internal decision/problem to many different places in the kernel.

Regards,
Andy