Hi Boris, On Friday 15 April 2016 11:39:06 Boris Brezillon wrote: > 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) ;). Great, thank you :). I just tested the patch and it works for me. The erased page bitflips are still detected and fixed. I will send a new version then. Best Regards, Markus > > If these changes work, feel free to squash them into your previous > patch. > > Thanks, > > Boris > > [1]http://code.bulix.org/bq6yyh-96549 > > -- Pengutronix e.K. | | Industrial Linux Solutions | http://www.pengutronix.de/ | Peiner Str. 6-8, 31137 Hildesheim, Germany | Phone: +49-5121-206917-0 | Amtsgericht Hildesheim, HRA 2686 | Fax: +49-5121-206917-5555 |