linux-kernel - Re: [RFC] fbdev/riva:change to use generice function to implement reverse

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Message-Id: <F9B74F97-EF3F-4051-B54B-B6CDD2946374@gmail.com>
Date:	Mon, 24 Aug 2015 16:31:13 +0800
From:	yalin wang <yalin.wang2010@...il.com>
To:	Afzal Mohammed <afzal.mohd.ma@...il.com>
Cc:	Tomi Valkeinen <tomi.valkeinen@...com>, adaplas@...il.com,
	plagnioj@...osoft.com, linux-fbdev@...r.kernel.org,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] fbdev/riva:change to use generice function to implement reverse_order()


> On Aug 22, 2015, at 15:53, Afzal Mohammed <afzal.mohd.ma@...il.com> wrote:
> 
> Hi,
> 
> On Fri, Aug 21, 2015 at 11:01:41AM +0300, Tomi Valkeinen wrote:
> 
>>>> Possibly the patches are still good for x86 also, but that needs to be
>>>> proven.
>>>> 
>>> not exactly, because x86_64 don’t have hardware instruction to do rbit OP,
>>> i compile by test :
>> 
>> For old drivers i386 may be more relevant than x86_64.
> 
> It seems asm bit reversal is supported in Kernel on arm & arm64 only,
> not sure whether any other arch even provide asm bit reversal
> instruction.

i only submit the bit reverse patch for arm / arm64 arch,
i am not sure if there are some other arch also have hard ware bit reverse 
instructions, need arch maintainers to submit if their arch also have these hard
ware instructions . :)

> 
>> These kind of optimizations should have some real world measurements,
> 
> Not for this case, but once measured on ARM, iirc, a 32-bit asm bit
> reversal as compared to doing it in C was taking 1 cycle as opposed to
> ~225 cycles!, of course writing optimized C could have made it fare
> better, but still would reach no-way near asm bit reversal.
> 


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