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Subject: Re: [PATCH] nand_base: optimize checking of erased buffers
To: Pavel Machek <pavel@ucw.cz>,
        Boris Brezillon <boris.brezillon@free-electrons.com>
Cc: Richard Weinberger <richard@nod.at>,
        David Woodhouse <dwmw2@infradead.org>,
        Brian Norris <computersforpeace@gmail.com>,
        Marek Vasut <marek.vasut@gmail.com>,
        Cyrille Pitchen <cyrille.pitchen@wedev4u.fr>,
        linux-mtd <linux-mtd@lists.infradead.org>,
        LKML <linux-kernel@vger.kernel.org>, mark.marshall@omicronenergy.com,
        b44839@freescale.com, prabhakar@freescale.com
References: <20170419121332.GA26979@amd> <20170419231804.5a04ed69@bbrezillon>
 <20170419221507.GA24914@amd> <20170420002748.5c76c9b9@bbrezillon>
 <20170420114057.GA4705@amd> <20170421105107.GA7259@amd>
From: Mason <slash.tmp@free.fr>
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Date: Wed, 17 May 2017 13:27:43 +0200
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On 21/04/2017 12:51, Pavel Machek wrote:

> If we see ~0UL in flash, there's no need for hweight, and no need to
> check number of bitflips. So this should be net win.
> 
> Signed-off-by: Pavel Machek <pavel@denx.de>
> 
> diff --git a/drivers/mtd/nand/nand_base.c b/drivers/mtd/nand/nand_base.c
> index b0524f8..96c27ec 100644
> --- a/drivers/mtd/nand/nand_base.c
> +++ b/drivers/mtd/nand/nand_base.c
> @@ -1357,7 +1357,10 @@ static int nand_check_erased_buf(void *buf, int len, int bitflips_threshold)
>  
>  	for (; len >= sizeof(long);
>  	     len -= sizeof(long), bitmap += sizeof(long)) {
> -		weight = hweight_long(*((unsigned long *)bitmap));

I hadn't noticed this earlier. There is, obviously, an implicit
requirement that 'buf' must be 4-byte aligned on 32-bit platforms,
and 8-byte aligned on 64-bit platforms.

This is not true for my platform, as the ecc pointer is
chip->oob_poi + 10

I suppose it's not a problem if the platform can handle
unaligned loads, otherwise it spells trouble.


> +		unsigned long d = *((unsigned long *)bitmap);
> +		if (d == ~0UL)
> +			continue;
> +		weight = hweight_long(d);
>  		bitflips += BITS_PER_LONG - weight;
>  		if (unlikely(bitflips > bitflips_threshold))
>  			return -EBADMSG;
> 

The optimization makes sense in itself, but given that it's on an
error path (?) I'm not sure it will bring any tangible benefits?

Regards.