From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from callisto.digivation.com.au ([198.35.45.124]) by bombadil.infradead.org with esmtps (Exim 4.90_1 #2 (Red Hat Linux)) id 1frYGl-0007BG-Eh for linux-mtd@lists.infradead.org; Mon, 20 Aug 2018 00:42:09 +0000 Subject: Re: [RFC PATCH] UBI fixable bit-flip issue To: Boris Brezillon Cc: linux-mtd@lists.infradead.org, Richard Weinberger References: <1bcbe82a-85e5-52f8-dc54-9d22e5b390fa@digivation.com.au> <20180817102559.7ab14fd9@bbrezillon> <227ad167-4e57-638d-a4cb-1e82613de2bc@digivation.com.au> <20180817165322.61958720@bbrezillon> <20180817172246.45fa784c@bbrezillon> From: Mark Spieth Message-ID: <32f5f211-5ae3-6088-ba38-b7ebb7e24f8e@digivation.com.au> Date: Mon, 20 Aug 2018 10:40:14 +1000 MIME-Version: 1.0 In-Reply-To: <20180817172246.45fa784c@bbrezillon> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US List-Id: Linux MTD discussion mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On 18/08/18 01:22, Boris Brezillon wrote: > On Fri, 17 Aug 2018 16:53:22 +0200 > Boris Brezillon wrote: > >> On Sat, 18 Aug 2018 00:33:25 +1000 >> Mark Spieth wrote: >> >>>>> I hope this description is clear enough. >>>> Well, I think selecting the bitflip threshold properly is really >>>> important, simply because some NANDs (including SLC NANDs) are showing >>>> bitflips even on blocks that have a low EC. Check the NAND ECC >>>> requirements, and if it's something like 8bit/512bytes, I guess that's >>>> more or less expected (it all depends on how many bitflips you have in >>>> the faulty block). It's less likely on NANDs requiring 1bit/512bytes >>>> ECC, and if that happens on such NANDs, you may have a problem in the >>>> controller driver. >>> 4 bits ECC per 512 bytes, from memory 28 bytes in OOB, using software >>> ECC in the MTD driver. >>> As I said, I believe the better threshold is hiding the root cause. It >>> is only a band-aid. >> What you describe will anyway happen sooner or later: if you're using >> almost al LEBs, and the remaining free ones are all impacted by the >> correctable bit-flip issue you'll have to use them anyway. So, yes, >> this is a band-aid, just like your solution is just improving things >> but not really solving the issue. This being said, if the blocks >> really show too many bitflips, they should be marked bad at some point, >> because during the scrubbing process we do write a pattern and check >> that we can read it back. I'll have to double check, but I think we're >> also checking for EUCLEAN and mark the block bad when that happens. > Hm, actually we're not torturing the source PEB when moving a LEB > because of bitflips (probably because it's expensive and tends to wear > the block even faster) :-/. The destination PEB is tortured if we fail > to read the VID header back, which is definitely not a guarantee that > other data are readable or do not contain too much bitflips. > > There's definitely something to improve there. Hi Boris, The flash in use is a Macronix MX30LF1G18AC and uses ONFI mode. My understanding of the problem is that when a block is read (say kernel+initrd) and one of the PEBs reads ok but with corrected bit errors, scrub mode is enabled. It then finds a suitable PEB to copy it to which it does. It then verifies this copy and also detects a corrected bit error, and frees the PEB it copied it from as it read ok, but with corrected errors. It then finds a suitable PEB to copy it to, and finds the original PEB that it moved it from! Does the whole copy and readback verify with corrected errors. This continues forever (or until the PEB does not verify which could be a while). Naturally the block read never completes. This is the behaviour I observed in the older driver with lots of print debugging. This may not be the behaviour in the current master, but I suspect it is. Some way of detecting this loop in a scrubbing session would be optimal, but seems complex to do from my examination of the UBI scrubber. But it shouldnt require a persisted header change. Regards Mark