Re: [PATCH] nand_base: optimize checking of erased buffers

From	Mason <slash.tmp@free.fr>
Newsgroups	linux.kernel
Subject	Re: [PATCH] nand_base: optimize checking of erased buffers
Date	2017-05-17 13:30 +0200
Message-ID	<tI5js-2pQ-3@gated-at.bofh.it> (permalink)
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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.

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Re: [PATCH] nand_base: optimize checking of erased buffers Mason <slash.tmp@free.fr> - 2017-05-17 13:30 +0200
  Re: [PATCH] nand_base: optimize checking of erased buffers Mason <slash.tmp@free.fr> - 2017-05-17 13:50 +0200
  Re: [PATCH] nand_base: optimize checking of erased buffers Pavel Machek <pavel@ucw.cz> - 2017-05-17 14:00 +0200

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