Re: NAND technical review

[Jonathan Larmour <jifl at jifvik dot org>]

[ Sorry for getting back to this late - I wanted to continue with Ross 
before he went on holiday ]

Rutger Hofman wrote:
> Jonathan Larmour wrote:
>
> > 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.

Noted, thanks.

> > 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.

Flash v2 doesn't use a constructor for legacy reasons and only because of 
some last minute discussions before the v3 release which couldn't reach a 
conclusion about constructor priority, given things like SPI flash. 
cyg_flash_init() is going to be properly eliminated in due course.

These issues don't really affect your layer so much as you don't have any 
legacy burden, so moving straight to a constructor is better.

> > 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.

I infer that your layer can cope with that? I didn't see the handling for 
that in io_nand_chip_bad_block.c.

Is your BBT compatible with Linux MTD? Including your use of a mirror?

> 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.

Well I was making the explicit assumption that it wasn't rewritten very 
often in the lifetime of the device. Think of things like in-field 
firmware upgrades.

> > > (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. 

If you know what it's going to be (at most), it could just be allocated 
statically and just used directly surely? That's got the lowest overheads.

E's implementation had a good idea of a CDL variable for the maximum 
supported block size. Then individual HALs or driver packages can use a 
CDL 'requires' to ensure it's >= the block size of the chips really in use.

> > > 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. 

I'm not sure about that. Experience with NOR Flash has shown that despite 
promises of error reporting in the datasheets, sometimes the only way to 
be sure of data integrity is an explicit verify step. It's up to the user, 
but I would consider it to have more use than just for debugging a driver.

> 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.

Or a single one for all chips maybe (since chances of clashes seem pretty 
small, so just protected with a mutex). And only if the program verify 
option is enabled of course. As per above, the page buffer size could be 
derived from the configuration, with appropriate CDL.

> [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.

I don't entirely disagree. But people do have a habit of inventing new 
things, particularly if it allows them to differentiate their products 
from their competitors.

> 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."

OneNAND isn't like SSD or MMC which essentially provide a block interface 
and an advanced controller hiding the details of NAND. It isn't like NOR 
flash because you can't address the entire array - as shown by the fact it 
only has a 16-bit address bus. Instead with OneNAND you get an SRAM buffer 
as a "window" into the NAND array. There are commands to load data from 
NAND pages into the SRAM buffers, or write them back. It has onboard ECC 
logic, but it has a very different way of controlling the NAND. You do get 
access to both data and spare areas too.

You can consider this the sort of thing I mean when I say that 
manufacturers can come up with interesting things which break rigid 
assumptions of how you talk to NAND chips. So my concern is not (just) 
that your layer can't support OneNAND, but it couldn't support anything 
which also had a different interface.

Obviously you already support small versus large page, which require 
different protocols, but they are still relatively similar in how they're 
controlled. Would it even be possible to sensibly extend your generic 
layer to support something like OneNAND? Without having a large number of 
kludges?

> > 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.

Ross said that the Samsung K9 is pretty similar to ONFI, other than how 
you read the device ID etc. Is your layer equally close?

Thanks,

Jifl
--
--["No sense being pessimistic, it wouldn't work anyway"]-- Opinions==mine