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Subject: Re: [PATCH] nand_base: optimize checking of erased buffers
From: Mason <slash.tmp@free.fr>
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
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Date: Wed, 17 May 2017 13:39:16 +0200
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On 17/05/2017 13:27, Mason wrote:

> 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

Doh! As Boris points out, the prologue/epilogue handle
all alignment & size issues.

Regards.