Re: AW: General NAND Flash support?

[Ross Younger <ecos at impropriety dot org dot uk>]

On 17/12/14 03:53, Richard Rauch wrote:
> So, as I understand, full NAND functionality is implemented inside YAFFS
> only?

YAFFS is implemented on top of the eCosCentric NAND layer. The eCos port 
patches YAFFS into the file I/O mechanism where it is available 
alongside FAT, jffs2 and others.

The NAND layer is independent of YAFFS - you can find a recent version 
of its interface at 
http://hg-pub.ecoscentric.com/nand-ecoscentric/file/17169b710e76/packages/io/nand/current/include/nand.h 


> As I understand, beside user data YAFFS is writing additionally control
> information to the Flash (like FAT)?

That is correct. However, YAFFS has been specially designed for NAND 
flash - there is no file allocation table or superblock, instead it is 
log-structured. This is part of its strategy for avoiding wearing out 
individual flash cells, which can be an issue with NAND parts.

> But, when we have only NAND on board, how we can boot from NAND?
> ROM bootstrap loader from uController is expecting simple Binary image.

This is necessarily a board-specific question, but in general terms:

Some CPUs contain a ROM bootstrap that is just clever enough to load a 
small bootstrap from a fixed location on a NAND device. This code in 
turn knows how to load the next stage. You might for example choose to 
put RedBoot there, which you had carefully configured with YAFFS and 
your chosen partition geometry. Your final application image would then 
reside on the YAFFS filesystem and you could even script RedBoot to 
automatically load and execute it.

Alternatively, there are some NAND parts that offer a NOR-like (i.e. 
direct read) interface to limited number of pages. These are intended 
for just this purpose -- to store a minimal bootstrapper.

Regards,

Ross
(Full disclosure: I work for eCosCentric; I wrote their NAND layer and 
did the YAFFS port.)

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