Re: NAND technical review

[Ross Younger <wry at ecoscentric dot com>]

Jonathan Larmour wrote:
> Can you at least shed light on the API changes (by cut'n'pasting
> relevant sections of headers/code even if not the whole thing)?

I have changed the device interface (nand_device.h) by carving up the read
and write page functions into three. (The prototype changes are the same for
both, with only the obvious semantic differences when writing, so I'll only
paste in read for the sake of brevity.)

Reading a page used to be an all-in-one call:

int (*read_page) (cyg_nand_device *dev, cyg_nand_page_addr page,
          void * dest, size_t size, void * spare, size_t spare_size);

Now, the NAND layer calls "begin" once to set up the read:

    int (*read_begin)(cyg_nand_device *dev, cyg_nand_page_addr page);

... "stride" one or more times to actually transfer data

    int (*read_stride)(cyg_nand_device *dev, void * dest, size_t size);

... and then "finish" once to do the spare area and any finishing up that
may be necessary (e.g. send the "program confirm" command, unlock the device).

    int (*read_finish)(cyg_nand_device *dev,
                       void * spare, size_t spare_size);

The ECC interface (nand_ecc.h, not well documented) has also expanded
slightly. I had had just a 'calc' call, but have now added an 'init' call so
that any device-specific registers can be tweaked. The interaction is
perhaps best sketched out as pseudocode; here's what the NAND library looks
like in a call to read a page:

  dev->read_begin(page number);
  while (there are still bytes to send) {
    ecc->init(); // may be a no-op
    dev->read_stride(ecc->datasize bytes);
    if (ecc is hardware)
      ecc->calc(the block we've just read);
  }
  dev->read_finish(spare data);
  if (ecc is software)
    ecc->calc(the whole thing);
  ecc->repair(the whole block, looping as necessary, comparing the
calculated ECC against what's in the spare area);


I have renamed the device interface struct and macros on the grounds of
"change the interface, change the name" (but not the ECC interface, because
nothing outside that package had used it before now).


>> [-funroll-loops]
> OOI you could add it to the per-package CFLAGS (unless you meant you
> already did and the bloat was noticeable even just there).

Good point, I forgot about that. Will try when I get back around to the
go-faster stripes.


Ross

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