Re: NAND technical review

[Rutger Hofman <rutger@cs.vu.nl>]

I should have stated this in my first mail...

I am not at all qualified to say anything about E's work, because I 
didn't have time to do any kind of review of it. So, I will mainly limit 
myself to comments on things that concern R's work, and where I say 
anything on E it will be based on the E's mails on the list.

Jonathan Larmour wrote:
[snip]
> A device number does seem to be a bit limiting, and less deterministic. 
> OTOH, a textual name arguably adds a little extra complexity.

This will be straightforward to change either way.

> I note Rutger's layer needs an explicit init call, whereas yours DTRT using a constructor, which is good.

I followed flash v2 in this. If the experts think a constructor is 
better, that's easy to change too.

> Does your implementation _require_ a BBT in its current implementation? 
> For simpler NAND usage, it may be overkill e.g. an application where the 
> number of rewrites is very small, so the factory bad markers may be 
> considered sufficient.

This is a bit hairy in my opinion, and one reason is that there is no 
Standard Layout for the spare areas. One case where a BBT is forced: my 
BlackFin NFC can be used to boot from NAND, but it enforces a spare 
layout that is incompatible with MTD or anybody. It is even incompatible 
with most chips' specification that the first byte of spare in the first 
page of the block is the Bad Block Marker. BlackFin's boot layout uses 
this first byte in a way that suits it, and it may be 0 -- which would 
otherwise mean Bad Block.

Also, what to do if a block grows bad during usage, and that block 
doesn't allow writing a marker in its spare area? BBT seems a solution.

>> (b) Dynamic memory allocation
>>
>> R's layer mandates the provision of malloc and free, or compatible
>> functions. These must be provided to the cyg_nand_init() call.
> 
> That's unfortunate - that limits its use in smaller boot loaders - a key 
> application.

Well, it is certainly possible to calculate statically how much space 
R's NAND layer is going to use, to allocate that statically, and write a 
tiny function to hand it out piecemeal at the NAND layer's request. 
There is no call to free() here except at shutdown, so nothing 
malloc-like is necessary. (An exception is in the debug handling, see 
below.)

>> E's doesn't; instead it declares a small number of static buffers.
> 
> I assume everything is keyed off CYGNUM_NAND_PAGEBUFFER, and there are 
> no other variables. Again I'm thinking of the scenario of single 
> firmware - different board revs. Can you confirm?
> 
>> Andrew Lunn opined on 6/3/09 that R's requirement for malloc is not a 
>> major
>> issue because the memory needs of that layer are well-bounded; I think I
>> broadly agree, though the situation is not ideal in that it forces 
>> somebody
>> who wants to use a lean, mean eCos configuration to work around.
> 
> The overhead of including something like malloc/free in the image may 
> compare badly with the amount of memory R's needs to allocate in the 
> first place. I also note that if R's implementation has program verifies 
> enabled it allocates and frees a page _every_ time. If nothing else this 
> could lead to heap fragmentation.

Program verifies should be considered a very deep debugging trait. 
Still, another possible implementation for this page buffer would be on 
the stack (not!), or in the controller struct. That would grow then by 
8KB + spare.

[snip]

>> - R's model shares the command sequence logic amongst all chips,
>> differentiating only between small- and large-page devices. (I do not 
>> know
>> whether this is correct for all current chips, though going forwards 
>> seems
>> less likely to be an issue as fully-ONFI-compliant chips become the 
>> norm.)
> 
> Hmm. Nevertheless, this is a concern for me with R's. I'm concerned it 
> may be too prescriptive to be robustly future-proof.

Well, there is no way I can see into the future, but I definitely think 
that the wire command model for NAND chips is going to stay -- it is in 
ONFI, after all. Besides, all except the 1 or 2 most pioneering museum 
NAND chips use it too. There are chips that use a different interface, 
like SSD or MMC or OneNand, but then these chips come with on-chip bad 
block management, wear leveling of some kind, and are completely 
different in the way they must be handled. I'd say E's and R's 
implementations are concerned only with 'raw' NAND chips.

> One could say that makes it a more realistic emulation. But yes I can 
> see disadvantages with a somewhat rigid world view. Thinking out loud, I 
> wonder if Rutger's layer could work with something like Samsung OneNAND.

See my comment above. The datasheet on e.g. KFM{2,4}G16Q2A says: 
"MuxOneNAND™‚ is a monolithic integrated circuit with a NAND Flash array 
using a NOR Flash interface."

> Incidentally I note Rutger has a "Samsung" ECC implementation, whereas 
> you support Samsung K9 chips, but use the normal ECC algorithm. Did 
> Samsung change their practice?

The ECC algorithm is not something that is related to chips. It is 
either software, or it is in the controller's ECC hardware and may need 
software support. Controller EEC hardware seems to use one of two public 
algorithms that are known as 'Toshiba ECC' and 'Samsung ECC'.

> I would certainly appreciate feedback from anyone who has used R's 
> layer. What you say would seem to imply that both small page and OFNI 
> are untested in R's layer.

That is correct. I would love some small-page testing. I have seen no 
ONFI chips on the market yet, so testing will be future work for both E 
and R.

> I'd need feedback from Rutger as to what level of testing has been done 
> with his.

I ran YAFFS tests, some took more than an hour to complete on my 
BlackFin. But for serious testing, see Jurgen Lambrecht's mail.

Rutger