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

From: Jonas Gorski <jogo@openwrt.org>
To: Simon Arlott <simon@fire.lp0.eu>
Cc: Florian Fainelli <f.fainelli@gmail.com>,
	Kamal Dasu <kdasu.kdev@gmail.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	MTD Maling List <linux-mtd@lists.infradead.org>,
	bcm-kernel-feedback-list <bcm-kernel-feedback-list@broadcom.com>,
	Brian Norris <computersforpeace@gmail.com>,
	David Woodhouse <dwmw2@infradead.org>
Subject: Re: [PATCH] mtd: brcmnand: Workaround false ECC uncorrectable errors
Date: Wed, 2 Dec 2015 21:44:04 +0100	[thread overview]
Message-ID: <CAOiHx=mLmEW=gK_HE6oVqaFZjMUAWoP1PxZchc_mC_A5PJn-Cw@mail.gmail.com> (raw)
In-Reply-To: <565F51D2.4010204@simon.arlott.org.uk>

On Wed, Dec 2, 2015 at 9:17 PM, Simon Arlott <simon@fire.lp0.eu> wrote:
> On 01/12/15 10:41, Jonas Gorski wrote:
>> On Sat, Nov 28, 2015 at 8:23 PM, Simon Arlott <simon@fire.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 threshold would be the amount of bitflips the code can correct, so
basically ecc.strength (at least that is my understanding).

> 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.

According to brcmnand.c hamming can fix up to fifteen bitflips, but in
the current code you would fail a hamming protected all-0xff-page for
even a single bitflip in the data or in the ecc bytes, which means
that all-0xff-pages wouldn't be protected at all.

Jonas