[U-Boot] Chain loading an u-boot from an u-boot

[Helmut Raiger]

Hi,

     to give you some background why we would want to do something 
(strange) like this:

- we have a hardware design bug
- we have a few hundred i.MX31 TT-01 devices in the field
- the i.MX31 rom boot loader is only capable of using 1bit HW-ECC 
(loading the first page (2k) from the NAND)
- the NAND chip specifies a requirement of 1bit ECC for the first 128kB 
(PEB) and 4bit ECC for the rest
- our current u-boot uses 1-bit HW-ECC, the kernel uses UBIFS and 1bit 
HW-ECC
- we face increasing bit errors in the field in the PEBs used by u-boot.

Using UBIFS in the kernel mitigates the requirements of 4bit ECC for the 
whole NAND because it moves
PEBs when bit errors show up. The real problem is the area where u-boot 
is located (currently approx. 450kB,
including UBIFS, USB ethernet support and more ..).

So the idea was:

- use a small u-boot (<128kB) in the first PEB of the NAND (written with 
1bit HW-ECC) that supports 4bit BCH
- let it load a second u-boot (<512kB) from the next 4 PEBs (written 
with 4bit BCH)
- jump to the second u-boot and load the kernel from an UBI volume using 
1bit HW-ECC again

I did all that and it seemed to work just fine, but jumping to the 
second u-boot almost always
crashes the system. In detail we do:

- romboot loads the SPL (2kb)
- SPL loads the first u-boot stage (which relocates and runs nicely)
- the first u-boot 'boots' the second u-boot by loading it from the NAND
- the second u-boot is loaded to the link address minus 2kB (for SPL)
- this is the same for the first and the second u-boot  (link address 
0x87e00000 - 0x800 = 0x87dff800)
- it jumps to 0x87e00000 omitting the SPL for the second u-boot
- the second u-boot should relocated itself again

The second u-boot is verified in RAM with crc32 and it is valid.

I've tested many configuration and found, that it only works if both 
u-boots are identical:

- different builds of the same code work (different build date, but same 
code)
- different configurations never work
- it does not matter if cashes are turned on or off
- I skipped the relocation of the second u-boot (actually not necessary) 
to no avail

I also tried u-boot standalone applications which always work (after 
fixing a bug in u-boot r8<->r9 for gd),
again independent of cashes.
Using different configuration I never get any serial output of the 
second u-boot (board info) or debugging stuff.
If I set a breakpoint in the second u-boot (after relocation) and 
continue from there it works until it tries
to get the SPI clock (mxc_get_clock() when accessing CCM_CCMR) for the 
PMIC access.
If I throw in a mxc_dump_clocks() earlier it hangs there.

I'm pretty much running out of ideas, so any pointers are appreciated.

Thx
Helmut


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