Re: [PATCH] mtd: nand: add option to erase NAND blocks even if detected as bad.

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

On Fri, 12 May 2017 05:34:10 -0300
Mario Rugiero <mrugiero@gmail.com> wrote:

> 2017-05-12 5:24 GMT-03:00 Boris Brezillon <boris.brezillon@free-electrons.com>:
> > On Fri, 12 May 2017 05:16:08 -0300
> > Mario Rugiero <mrugiero@gmail.com> wrote:
> >  
> >> 2017-05-12 5:12 GMT-03:00 Richard Weinberger <richard.weinberger@gmail.com>:  
> >> > Mario,
> >> >
> >> > On Fri, May 12, 2017 at 7:39 AM, Mario J. Rugiero <mrugiero@gmail.com> wrote:  
> >> >> Some chips used under a custom vendor driver can get their blocks
> >> >> incorrectly detected as bad blocks, out of incompatibilities
> >> >> between such drivers and MTD drivers.
> >> >> When there are too many misdetected bad blocks, the device becomes
> >> >> unusable because a bad block table can't be allocated, aside from
> >> >> all the legitimately good blocks which become unusable under these
> >> >> conditions.
> >> >> This adds a build option to workaround the issue by enabling the
> >> >> user to free up space regardless of what the driver thinks about
> >> >> the blocks.  
> >> >
> >> > Hmm, this sounds like a gross hack.  
> >> It is, but I see no other solution. The NAND chips were used in an
> >> incompatible way by a hack-n-slash driver made by allwinner, and
> >> trying to load them with a proper MTD driver fails miserably if this
> >> is not done.
> >> If anyone can propose a better solution I'll more than happily implement it.
> >> I'm open to suggestions, and of course I'm open to rejection of my
> >> patches if needed.  
> >
> > u-boot provides the nand.scrub command, which does exactly what you're
> > looking for. And no, I don't think it's a good idea to allow erasing
> > bad blocks, at least not by default.
> >
> > If we really want to support this feature in linux, this should be
> > explicitly enabled through debugfs.  
> If I do this, does it stand a chance at getting upstream?
> If so, I'll have it done soon.
> Note however that the build option is disabled by default. I get that
> there should also be one runtime option, disabled by default, exposed
> through debugfs. Does that sound right?
> >  
> >> >  
> >> >> Example usage: recovering NAND chips on sunxi devices, as explained
> >> >> here: http://linux-sunxi.org/Mainline_NAND_Howto#Known_issues  
> >> >
> >> > What this wiki suggests is not wise.
> >> > How can you know which blocks are really bad and which not?  
> >> You don't, at least not without an even grosser hack implementing read
> >> support for their incompatible format.
> >> Would that be better? I might attempt it if desired.  
> >
> > No, please don't do that, at least not in the kernel. If you really
> > want to parse the old format, you should develop a tool that reads NAND
> > pages in raw mode, stores the list of bad blocks somewhere and then
> > re-use this list to select which blocks should be forcibly erased.
> >
> > Not sure it's worth the pain :-).  
> It's worth the pain to me. I'm dealing with a bit rotten 3.4 based
> pile of cr*p on production because of this. Whatever I have to do to
> get those machines running the mainline kernel is worth it.

No, I meant, doing that vs scrubbing the NAND. Note that MLC support is
not reliable in mainline, so I'd strongly discourage to use a mainline
kernel right now, unless you have an SLC NAND.