Re: [PATCH 2/4] mtd: nand: implement two pairing scheme

George Spelvin
2016-06-12T09:23:36Z

>> It also applies an offset of +1, to avoid negative numbers and the
>> problems of signed divides.

> It seems to cover all cases.

I wasn't sure why you used a signed int for the interface.

(Another thing I thought of, but am less sure of, is packing the group
and pair numbers into a register-passable int rather than a structure.
Even 2 bits for the group is probably the most that will ever be needed,
but it's easy to say the low 4 bits are the group and the high 28 are
the pair. Just create a few access macros to pull them apart.

This was inspired by Linus's hash_len abstraction, recently moved to
<linux/stringhash.h>)

>> (or you could add an mtd->write_per_erase field).

> Okay. Actually I'd like to avoid adding new 'conversion' fields to the
> mtd_info struct. Not sure we are really improving perfs when doing that,
> since what takes long is the I/O ops between the flash and the
> controller not the conversion operations.

Well, yes, but you may need to do conversion ops for in-memory cache
lookups or searching for free blocks, or wear-levelling computations,
all of which may involve a great many conversions per actual I/O.

(In hindsight, I'd wish for writesize and write_per_erase, and not
store erasesize explicitly. Not only is the multiply more efficient,
but it abolishes the error of an erase size which is not a multiple of
the write size by making it impossible.)

> Can we have a boolean to make it clearer?
>
> bool lastpage = ((page + 1) * mtd->writesize) == mtd->erasesize;

An improvement IMHO. You can use the same name in all four functions
to make the equivalence clear.

> Also, the page update is quite obscure for people that did not have the
> explanation you gave above. Can we make it

> /*
> * The first and last pages are not surrounded by other pages,
> * and are thus less sensitive to read/write disturbance.
> * That's why NAND vendors decided to use a different distance
> * for these 2 specific case, which complicates a bit the
> * pairing scheme logic.

Um... this is, as far as I can tell, complete nonsense.

I realize you know this about a thousand times better than I do, so
I'm hesitant to make such a strong statement, but one thing that I do
know is that paired pages are stored in the exact same transistors.
The pairing is purely a logical addressing distance. The physical
distance is exactly zero.

The qustion is why they chose this particular *logival* addressing
scheme, and I believe the reason is write bandwidth for the common case
of streaming writes to consecutive pages.

The obvious thing to do is pair consecutive even and odd pages (pages 0 and 1,
then 2 and 3, then...), but that makes it hard to pipeline programming of the
two pages. You can't start programming page 1 until page 0 is finished.

The next obvious thing is stride-2: 0<->2, 1<->3, 4<->6, 5<->7, etc.

This is done in some MLC chips. See p. 98 of this Micron data sheet:
http://pdf.datasheet.directory/datasheets-0/micron_technology/MT29F32G08CBACAWP_C.pdf
which has a stride-4 pairing. 0..3 pair with 4..8, then 9..11 with
12..15, and so on.

However, it's desirable to alternate group-0 and group-1 pages, since
the write operations are rather different and even take different amounts
of time. Alternating them makes it possible to:
1) Possibly overlap parts of the writes that use different on-chip resources,
2) Average the non-overlapping times for minimum jitter.

This leads naturally to the stride-3 solution. You want to minimize the
stride because you can read both pages in a pair with one read disturbance,
and the shorter the distance, the more likely you'll want both pages
(and the less buffering you'll need to make both available).

Stride-3 does have those two awkward edge cases, and changing the
stride is simply the simplest way to special-case them.

> Thanks for your valuable review/suggestions.
>
> Just out of curiosity, why are you interested in the pairing scheme
> concept? Are you working with NANDs?

Not at present, but I do embedded hardware and might some day.

Also, the data sheets are a real PITA to find. I have yet to
see an actual data sheet that documents the stride-3 pairing scheme.