Re: [PATCH RESEND] gpmi-nand: Handle ECC Errors in erased pages

[Boris Brezillon <boris.brezillon@free-electrons.com>]

Hi Markus,

On Fri, 15 Apr 2016 11:35:07 +0200
Markus Pargmann <mpa@pengutronix.de> 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 <mpa@pengutronix.de> wrote:
> > 
> > > On Wednesday 13 April 2016 00:51:55 Boris Brezillon wrote:
> > > > On Tue, 12 Apr 2016 22:39:08 +0000
> > > > Han Xu <han.xu@nxp.com> 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