[PATCH] mtd: Add config option to skip scanning bad blocks in partitions

[Ricard Wanderlof]

On Thu, 15 Jan 2015, Daniel Ehrenberg wrote:

> I'm using UBI, so I thought an on-device BBT is redundant and would be
> ignored by UBI. Also, I couldn't figure out how I'm supposed to to
> enable it--the out-of-tree driver I'm using isn't really based on
> nand_base.c; correct me if I'm wrong, but it looks like that's a
> requirement for using the nand_bbt code. Cutting my time in half
> wouldn't be all that great since that's still almost 2 seconds of
> useless work.

What kind of driver are you using? I always though UBI essentially needed 
an mtd device, meaning that if you're using a NAND flash, nand_base.c 
comes in to play, along with a board (hardware) specific driver of course.

The bad block handling mechanism is basically hidden away inside mtd, 
there are mtd API calls to determine if a block is bad or not, and mtd 
handles that according to the configuration for the specific driver, 
either by reading from the flash every time, or using a BBT.

> Where can I read about the other advantages of on-flash BBT when using
> UBI? Maybe I can use this technique for other devices I'm working on
> where nand_bbt could be used.

There are no specific specific UBI-related advantages when using a BBT 
that I know of.

One turning point for me was the realization that at least for some NAND 
chips, it is out of spec to read bad blocks except once, to determine 
which ones are bad by looking at the bad block markers in the individual 
blocks. Hence, using a BBT is a way of fulfilling that requirement. In 
practice, though, the flashes I've come in contact with (SLC up to say 2 
Gbit) don't seem to have any problems in this regard.

Another advantage in a development environment, especially when working 
with upgrade software, is that if something goes wrong that causes blocks 
to be marked bad spuriously, if not using a BBT the new bad blocks are 
marked as such in the same way as factory bad blocks, making it impossible 
to separate the two and undo the bad block markings and revert to the 
factory condition. When using a BBT, new bad blocks are marked in the BBT 
only, so by erasing the BBT and rescanning the bad block markers in the 
OOBs of the blocks the flash can be reset to a known state.

/Ricard
-- 
Ricard Wolf Wanderlöf                           ricardw(at)axis.com
Axis Communications AB, Lund, Sweden            www.axis.com
Phone +46 46 272 2016                           Fax +46 46 13 61 30