linux-kernel - Re: [PATCH] mtd: brcmnand: Workaround false ECC uncorrectable errors

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Message-ID: <565F51D2.4010204@simon.arlott.org.uk>
Date:	Wed, 2 Dec 2015 20:17:22 +0000
From:	Simon Arlott <simon@...e.lp0.eu>
To:	Jonas Gorski <jogo@...nwrt.org>
Cc:	Brian Norris <computersforpeace@...il.com>,
	Kamal Dasu <kdasu.kdev@...il.com>,
	David Woodhouse <dwmw2@...radead.org>,
	MTD Maling List <linux-mtd@...ts.infradead.org>,
	bcm-kernel-feedback-list <bcm-kernel-feedback-list@...adcom.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Florian Fainelli <f.fainelli@...il.com>
Subject: Re: [PATCH] mtd: brcmnand: Workaround false ECC uncorrectable errors

On 01/12/15 10:41, Jonas Gorski wrote:
> On Sat, Nov 28, 2015 at 8:23 PM, Simon Arlott <simon@...e.lp0.eu> wrote:
>> +
>> +       /* Go to start of buffer */
>> +       buf -= FC_WORDS;
>> +
>> +       /* Erased if all data bytes are 0xFF */
>> +       buf_erased = memchr_inv(buf, 0xFF, FC_WORDS) == NULL;
>> +
>> +       if (!buf_erased)
>> +               goto out_free;
> 
> We now have a function exactly for that use case in 4.4,
> nand_check_erased_buf [1], consider using that. This also has the
> benefit of treating bit flips as correctable as long as the ECC scheme
> is strong enough.

I have no idea whether or not it's appropriate to specify
bitflips_threshold > 0 so it'd just be a more complex way to do
a memchr_inv() search for 0xFF.

The code also has to check for the hamming code bytes being all 0x00,
because according to the comments [2], the controller also has
difficulty with the non-erased all-0xFFs scenario too.

[2] https://github.com/lp0/bcm963xx_4.12L.06B_consumer/blob/dd8fcb13046f738c311507dc2fcfd3e5d57a88e0/kernel/linux/drivers/mtd/brcmnand/brcmnand_base.c#L2459

> 
> Jonas
> 
> [1] https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/drivers/mtd/nand/nand_base.c#n1110
> 

-- 
Simon Arlott
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