[ECOS] NAND for STM32, YAFFS and UFFS

Ross Younger wry@ecoscentric.com
Wed Jul 1 18:05:00 GMT 2009


Hi Simon,

> I added NAND support for the STM3210E-EVAL board today. I currently use 
> Ross's framework [...]

Glad to hear it, thanks for giving the code a try :-)

> which is rather important for me at the moment. The port consists of the 
> HAL part as well as a new ST NANDxxxx3A NAND driver. Both were started
> as copies of Ross's code.

This was my clear intention from the start; whilst clone+hack is often 
frowned upon, in this sort of situation it keeps the individual driver size 
down. Perhaps over time we might put together some more generic chip drivers.

> What is still missing is interrupt mode and 
> the usage of STM32's onboard ECC computation unit.

Interrupt mode adds a nontrivial overhead; if your application isn't 
multi-threaded, it won't win you anything, and in any event it the overhead 
will make your accesses slower overall (though won't hit the CPU in 
polling). As ever, the trade-off is yours to make, which is why it's an 
explicit CDL option.

I have been benchmarking parts of my setup this week and found - rather 
unsurprisingly - that the overheads are pretty punishing. The chip on the EA 
board takes 25us to read a page, 300-700us to write a page, and 2-3ms to 
erase a block. I'm finding that interrupt mode on that board is adding about 
a millisecond to every NAND call, which adds up fast. (There are 65,536 
pages on this chip, and an exhaustive scan of a nearly-full YAFFS fs has to 
read most of them.)

It's clear that it's just not worth doing interrupt mode for reads on this 
particular board; writes maybe, and erases are clearly worth it. I will be 
disabling interrupt mode for reads in the EA2468 driver very shortly and I 
would advise you to consider whether the situation on the STM3210E is 
similar 8-)


As for ECC, it should hopefully be straightforward for you to add an ECC 
"driver" which will plug right on in in place of the Linux MTD algorithm. 
The MTD ECC code is 3 bytes ECC per 256 bytes of data; if the onboard is a 
different size, you will also need to add a new OOB layout definition in 
nand_oob.c. (I got mine by reading the Linux MTD code.) If you don't need to 
interoperate with anything else, the MTD alg may be suitable, but of course 
if code space is really tight for you, you might prefer to use the onboard 
ECC and add a CDL option to not compile nand_ecc_mtd.c.


> I wanted to do some quick tests with YAFFS, as my initial port of the 
> UFFS filesystem is not yet done, and I wanted to get some rough numbers 
> of what resources are needed to run YAFFS. Unfortunately it turned out 
> that the YAFFS port does not handle small page NANDs, as one is found on 
> the STM32 evalboard. Further investigation showed that YAFFS1 was using 
> small page NANDs and YAFFS2 is used on 1k++ page NANDs. The eCos port is 
> based on YAFFS2 I guess, so I think this should be renamed properly. Or 
> was there a plan to support both YAFFS1 and YAFFS2?

There's a bit of a potential for terminology confusion here, so I shall 
digress briefly in the hopes that it's useful for people not following this 
debate so closely... In Aleph One's CVS repo, there are two distinct modules 
`yaffs' and `yaffs2'. The former only supports small-page, and is obsolete; 
the latter is effectively a patched version which supports both small- and 
large-page devices. Confusingly, internally to the code, `YAFFS1' refers to 
the small-page support and `YAFFS2' the large-page. Personally, I consider 
there to be one YAFFS with two tentacles.


I have been working only with the yaffs2 source base. However, as I have 
only been working with its large-page support (YAFFS2), I have compiled out 
the small-page (YAFFS1) support meantime, and only written large-page NAND 
hooks. This leaves a small gap waiting to be bridged.

Off the top of my head, what's needed to make the small-page code work is:
* to add a switch somewhere to tell eyaffs_mount() to set up the device 
struct as YAFFS1 instead of YAFFS2;
* to provide the couple of YAFFS1 NAND access functions which I haven't yet 
(prototypes for which exist in ecos-yaffs-nand.c);
* to unset CONFIG_YAFFS_NO_YAFFS1 in ecos-yaffs.h (perhaps make 
YAFFS1/YAFFS2 modes properly CDL-switchable);
* to add in the "missing" files from the yaffs2 distribution (just 
yaffs_packedtags1.[ch], I think).


As it happens I believe we do have an STM3210E somewhere here, so I will 
probably end up looking into this myself in due course, but it rather looks 
like I'm going to be distracted by commercial concerns for the next couple 
of weeks :-(


You mention that RAM is tight, I should raise a flag about YAFFS memory 
usage. My benchmarking this week provided a rather alarming result. Buried 
in YAFFS's own documents is the suggestion that one should budget two bytes 
of RAM per page of NAND for YAFFS's internal data structures. Now, the chip 
on the EA board is 1024 blocks x 64 pages per block, i.e. 65536 pages total 
(x 2kbytes each, for a 128Mbyte chip) - so by the rule of thumb one would 
expect to budget 128Kbytes. Worryingly, after I had written 10,000 small 
files into YAFFS, my heap usage as discerned from mallinfo() was around 
1.6Mbyte! (I need to investigate this more; it's next on my plate after 
sorting out the timings. However I am pretty sure there is no memory leak, 
as the heap usage went to zero after umount.) I am in the process of 
composing a post to the YAFFS mailing list raising this and will see what 
the folks there say.


> After some first investigation of the UFFS source code it seems that it 
> needs quite a bit of work to make it work properly with eCos and not 
> have hacks all over the place. But it seems that YAFFS does still use 
> too much resources for our usage, so I think I need to work on a simpler 
> one and UFFS still looks like a good start ...

By the way, I'm still working on that proposal I mentioned here the other 
day to let RedBoot use NAND as its FIS (by pretending it's a funny flavour 
of NOR). While that won't be directly of use for you, it might prove 
adaptable as a very very simple FS. But, so many things to do, so little time...


Regards,


Ross

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