From mboxrd@z Thu Jan 1 00:00:00 1970 From: boris.brezillon@free-electrons.com (Boris Brezillon) Date: Fri, 15 Apr 2016 11:39:06 +0200 Subject: [PATCH RESEND] gpmi-nand: Handle ECC Errors in erased pages In-Reply-To: <33206452.D8BR53ndXi@adelgunde> References: <1456059126-25469-1-git-send-email-mpa@pengutronix.de> <7168760.YSQBa3GsdA@adelgunde> <20160415103508.5f6bc8c8@bbrezillon> <33206452.D8BR53ndXi@adelgunde> Message-ID: <20160415113906.7f971cec@bbrezillon> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org Hi Markus, On Fri, 15 Apr 2016 11:35:07 +0200 Markus Pargmann wrote: > Hi Boris, > > On Friday 15 April 2016 10:35:08 Boris Brezillon wrote: > > Hi Markus, > > > > On Fri, 15 Apr 2016 09:55:45 +0200 > > Markus Pargmann wrote: > > > > > On Wednesday 13 April 2016 00:51:55 Boris Brezillon wrote: > > > > On Tue, 12 Apr 2016 22:39:08 +0000 > > > > Han Xu wrote: > > > > > > > > > > Thanks for the feedback. Talking with a coworker about this we may have found a > > > > > > better approach to this that is less complicated to implement. The hardware > > > > > > unit allows us to set a bitflip threshold for erased pages. The ECC unit > > > > > > creates an ECC error only if the number of bitflips exceeds this threshold, but > > > > > > it does not correct these. So the idea is to change the patch so that we set > > > > > > pages, that are signaled by the ECC as erased, to 0xff completely without > > > > > > checking. So the ECC will do all the work and we completely trust in its > > > > > > abilities to do it correctly. > > > > > > > > > > Sounds good. > > > > > > > > > > some new platforms with new gpmi controller could check the count of 0 bits in page, > > > > > refer to my patch https://patchwork.ozlabs.org/patch/587124/ > > > > > > > > > > But for all legacy platforms, IMO, considering bitflip is rare case, set threshold to 0 and > > > > > only check the uncorrectable branch and then correct data sounds better. Setting threshold > > > > > and correcting all erased page may highly impact the performance. > > > > > > > > Indeed, bitflips in erased pages is not so common, and penalizing the > > > > likely case (erased pages without any bitflips) doesn't look like a good > > > > idea in the end. > > > > > > Are erased pages really read that often? > > > > Yes, it's not unusual to have those "empty pages?" checks (added Artem > > and Richard to get a confirmation). AFAIR, UBIFS check for empty pages > > in its journal heads after an unclean unmount (which happens quite > > often) to make sure there's no corruption. > > > > > I am not sure how UBI handles > > > this, does it read every page before writing? > > > > Nope, or maybe it does when you activate some extra checks. > > > > > > > > > > > > > You can still implement this check in software. You can have a look at > > > > nand_check_erased_ecc_chunk() [1] if you need an example, but you'll > > > > have to adapt it because your controller does not guarantees that ECC > > > > bits for a given chunk are byte aligned :-/ > > > > > > Yes I used this function in the patch. The issue is that I am not quite > > > sure yet where to find the raw ECC data (without rereading the page). > > > The reference manual is not extremely clear about that, ecc data may be > > > in the 'auxilliary data' but I am not sure that it really is available > > > somewhere. > > > > AFAIR (and I'm not sure since it was a long time ago), you don't have > > direct access to ECC bytes with the GPMI engine. If that's the case, > > you'll have to read the ECC bytes manually (moving the page pointer > > using ->cmdfunc(NAND_CMD_RNDOUT, column, -1)), which is a pain with > > this engine, because ECC bytes are not guaranteed to be byte aligned > > (see gpmi ->read_page_raw() implementation). > > Once you've retrieved ECC bytes (or bits in this case), for each ECC > > chunk, you can use the nand_check_erased_ecc_chunk() function (just make > > sure you're padding the last ECC byte of each chunk with ones so that > > bitflips cannot be reported on this section). > > Thanks for the information. So I understand that this approach is the > preferred one to avoid any performance issues for normal operation. > > I actually won't be able to fix this patch accordingly for some time. If > anyone else needs this earlier, feel free to implement it. I just did [1] (it applies on top of your patch), but maybe you can test it (I don't have any imx platforms right now) ;). If these changes work, feel free to squash them into your previous patch. Thanks, Boris [1]http://code.bulix.org/bq6yyh-96549 -- Boris Brezillon, Free Electrons Embedded Linux and Kernel engineering http://free-electrons.com