[PATCH 02/10] mtd: nand: gpmi: Utilize hardware to detect bitflips in erased blocks

[Sascha Hauer]

On Wed, Dec 06, 2017 at 10:27:13AM +0100, Boris Brezillon wrote:
> Hi Sascha,
> 
> On Wed,  6 Dec 2017 10:19:17 +0100
> Sascha Hauer <s.hauer at pengutronix.de> wrote:
> 
> > The GPMI nand has a hardware feature to ignore bitflips in erased pages.
> > Use this feature rather than the longish code we have now.
> > Unfortunately the bitflips in erased pages are not corrected, so we have
> > to memset the read data before passing it to the upper layers.
> 
> There's a good reason we didn't use the HW bitflip detection in the
> first place: we currently have no way to report the number of corrected
> bitflips in an erased page, and that's a big problem, because then UBI
> does not know when it should re-erase the block.

Ah, ok.

> 
> Maybe we missed something when the initial proposal was done and
> there's actually a way to retrieve this information, but if that's not
> the case, I'd prefer to keep the existing implementation even if it's
> slower and more verbose.

I'm not so much concerned about the verbosity of the code but rather
about the magic it has inside. I have stared at this code for some time
now and still only vaguely know what it does.

We could do a bit better: We can still detect the number of bitflips
using nand_check_erased_ecc_chunk() without reading the oob data.
That would not be 100% accurate since we do not take the oob data into
account which might have bitflips aswell, but still should be good
enough, no?

Sascha

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