[U-Boot] SPI flash writing

[Gerlando Falauto <gerlando.falauto@keymile.com>]

Hi everyone,

[I took the liberty to Cc: Mike and Simon as they have provided patches 
in the area]

I struggled for a while trying to update a Kirkwood-based board to the 
latest u-boot (with Keymile's patches).

As it turned out, our update procedure:

sf probe 0;sf erase 0 50000;sf write ${load_addr_r} 0 ${filesize}

mistakenly expects a maximum size of 0x50000 (327680) bytes for
u-boot.kwb. Sadly, the latest u-boot trunk results in a binary size for 
that board which is dangerously close to that limit. Hence, after adding 
some innocent lines of code, the update procedure could brick the board 
(for no evident reason and with no error message whatsoever) if the 
binary size crosses that boundary.

It turns out somebody else also picked up this "magic" number:
http://lacie-nas.org/doku.php?id=uboot#update_u-boot_mainline

And others have bricked their board, most likely for the same reason:
http://www.trimslice.com/forum/viewtopic.php?f=16&t=462

Also, something bad could happen if you make a mistake in the opposite 
direction (use too big a number for the write size):
http://sequanux.org/pipermail/lacie-nas/2012-March/000378.html

 From what I can understand, writing into a sector which has not been 
erased first is an acceptable behaviour of the flash interface, it will 
just set to zero whatever bits are not zero already, without reporting 
any error whatsoever.

Even though any change we introduce now would only apply to upgrades 
FROM future versions, I think it might be worth fixing this somehow.
I believe several things could be easily done here:

1) a "+" syntax to the "sf update" command so it can be used with 
${filesize} as a parameter, and/or some "read,replace,erase,overwrite" 
block mechanism for the last (incomplete) block

2) an out-of-boundary-check againts the flash size so at least a warning 
is issued when you use too big a size value

3) a command line option ("sf write -v" and/or to "sf update -v"), or an 
entirely new command (like "sf writeverify", "sf updateverify") to read 
back after writing so to double-check what really ended up being written 
to the flash before it's too late.

I'm willing to implement them, but I wanted to hear your thoughts first.

Thanks,
Gerland