Re: NAND technical review

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

Jonathan Larmour wrote:
> Hmm, I guess the key thing here is that in E's implementation most of 
> the complexity has been pushed into the lower layers; at least compared 
> to R's. R's has a more consistent interface through the layers. Albeit 
> at the expense of some rigidity and noticeable function overhead.
> 
> It's not likely E's will be able to easily share controller code, given 
> of course you don't know what chips, and so what chip driver APIs 
> they'll be connected to. But OTOH, maybe this isn't a big deal since a 
> lot of the controller-specific munging is likely to be platform-specific 
> anyway due to characteristics of the attached NAND (e.g. timings etc.) 
> and the only bits that would be sensibly shared would potentially happen 
> in the processor HAL anyway at startup time. What's left may not be that 
> much and isn't a problem in the platform HAL. However the likely 
> exception to that is hardware-assisted ECC. A semi-formal API for that 
> would be desirable.

This is the largest difference in design philosophy between E and R. Is 
it OK if I expand?

NAND chips are all identical in their wire setup. They all have a data 
'bus', and control lines to indicate whether what is on the bus is a 
command, an address, or data.

NAND chips differ in how their command language works, but only so far. 
What is on the market now is 'regular' large-page chips that all speak 
the same command language, and small-page chips that have a somewhat 
different command language. ONFI chips are large-page chips except in 
interrogation at startup and in bad-block marking.

E.g. a page read for a large-page chip (my running example) looks like this:
. write a command 0x00 (READ_START)
. write address bytes of the page(+offset) to be read
. write a command 0x30 (READ_CONFIRM)
. read the data on the bus
. insofar as supported retrieve hw-calculated ECC
For small-page chips the sequence is different because a page's data is 
read in multiple chunks, using READ_1_A (0x00), READ_1_B (0x01), and for 
spare area READ_2 (0x05).

These 2 languages are all the variation there is for NAND chips (plus, 
at another level, 2 timing values for read cycle and write cycle)! The 
wide-ranging differences for devices for NAND are in the controllers.

How controllers work, is that they accept input like 'write a command of 
value 0x..', 'write an address of value 0x.....', etc, and do their job 
on the NAND chip's wires. They cannot really operate at a higher level, 
if only because they must support both small-page and large-page chips 
(and ONFI), and this is the level of common protocol for the chips.

So controller code has to bridge between API calls like page_read and 
the interface of the controller as described above. R's implementation 
presumes that a lot of the code to make this translation is generic: a 
large-page read translates to the controller steps as given above in the 
running example, in any controller implementation. Moreover, the generic 
code handles spare layout: where in the spare is the application's spare 
data folded, where is the ECC, where is the bad-block mark. OTOH, the 
generic code has hooks for handling any ECC that the controller has 
computed in hardware -- how ECC is supported in hardware varies across 
controllers. But the way the ECC check is handled (case in point is 
where a correctible bit error is flagged) is generic again.

So, lots of code can (and will) be shared across controller 
implementations -- whether by code sharing or by code duplication.

Rutger