linux-kernel - Re: [PATCH] mtd/nand: use string library

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Message-ID: <CAN8TOE_e8FkTTABRTyrXr5EW3wsJeXaODE4Wpud4ype=tcRFFQ@mail.gmail.com>
Date:	Tue, 31 Jan 2012 09:57:26 -0800
From:	Brian Norris <computersforpeace@...il.com>
To:	Akinobu Mita <akinobu.mita@...il.com>
Cc:	Joe Perches <joe@...ches.com>,
	Scott Branden <sbranden@...adcom.com>,
	linux-kernel@...r.kernel.org, linux-mtd@...ts.infradead.org,
	Jiandong Zheng <jdzheng@...adcom.com>,
	akpm@...ux-foundation.org, David Woodhouse <dwmw2@...radead.org>,
	eric.dumazet@...il.com
Subject: Re: [PATCH] mtd/nand: use string library

On Fri, Jan 27, 2012 at 3:52 PM, Akinobu Mita <akinobu.mita@...il.com> wrote:
> 2012/1/28 Brian Norris <computersforpeace@...il.com>:
>> On Fri, Jan 27, 2012 at 9:16 AM, Joe Perches <joe@...ches.com> wrote:
>>> On Fri, 2012-01-27 at 23:24 +0900, Akinobu Mita wrote:
>>>> - Use memchr_inv to check if the data contains all 0xFF bytes.
>>>>   It is faster than looping for each byte.
>>>
>>> Stupid question:
>>>
>>> Are there any mtd devices modified that are slower
>>> at 64 bit accesses than repeated 8 bit accesses?
>>
>> I believe this patch deals with kernel buffers, not any kind of direct
>> access to the MTD, so the question (which is not stupid IMO) should be
>> regarding CPU architectures. And my educated guess is that 64-bit
>> access should not be any slower. I do know that 8-bit access *is*
>> slower for some relevant architectures.
>
> It could be slower when the number of bytes scanned is very small
> (found a unmatched character immediately, or the size of the area
> is very small), because memchr_inv() needs to generate a 64bit pattern
> to compare before starting the loop.  I recalled that Eric Dumazet
> pointed out it could generate the 64bit pattern more efficiently.
> (https://lkml.org/lkml/2011/8/8/480)
>
> Even if that small scanning is slower, this change can be assumed cleanup
> patch that simplifies the code.

Well, I agree that it qualifies as cleanup as well, but we should at
least make an attempt not to cause performance regression...

So by my understanding, the use of memchr_inv() is on buffers of
minimum length of 10 in this patch, so we're likely to have decent
results. And memcmp() usage looks fine to me.

So unless other concerns arise:

Acked-by: Brian Norris <computersforpeace@...il.com>
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