N25Q256A 13E40

[Marek Vasut]

Hi guys!

Let's talk about our favourite chip :) I just got my hands on N25Q256A . Since 
there was quite a flurry of patches for this and similar chips and I got it 
working on a different chip with exactly the same IDs and type, I'd would expect 
linux to work with this one too. Guess what ... ;-)

This particular incarnation of N25Q256A completely ignores the 4-byte ERASE 
(0xdc) command. Apparently, if I look closely, it's N25Q256A 13E40 variant and 
seeing the Note #14 (datasheet page 30 / below Table 18. Command set), quoting:

14.This command is only for part numbers N25Q256A83ESF40x and N25Q256A83E1240x.

they are not supported on this part. Someone surely did some hard thinking 
inventing such a crappy part. I can surely cook a patch, but I wonder what 
direction we should take here. We can switch this chip into 4-byte mode by 
0xb7/0xe9 opcodes, which would in turn break BootROMs which depend on the SPI 
NOR to be in 3-byte mode upon reboot. We can program the BAR register before 
erase, which will do the same.

Sigh, if you have any idea, that'd be nice to hear.

Thanks!