mtd: rawnand: atmel: ECC error after update to kernel 6.6

[Zixun LI]

Hi,

After updating our SAM9G25 device from kernel 3.16 to 6.6, we have
encountered UBIFS failures with ECC errors:

  ubi0 warning: ubi_io_read: error -74 (ECC error) while reading
126976 bytes from PEB 8:4096, read only 126976 bytes, retry

On the old system (kernel 3.16), nandtest passed successfully:

  nandtest -p 1 -l 0x40000 /dev/mtd2
  ECC corrections: 1
  ECC failures : 0
  Bad blocks : 0
  BBT blocks : 0
  00020000: checking...
  Finished pass 1 successfully

But on the new system (kernel 6.6), nandtest reports many errors:

  nandtest -p 1 -l 0x40000 /dev/mtd2
  ECC corrections: 1667
  ECC failures : 1205
  Bad blocks : 1
  BBT blocks : 0
  00000000: reading (1 of 4)...
  218 bit(s) ECC corrected at 00000000

After some diagnostics, I used devmem2 to compare SMC and PMECC
registers between the two kernels. All values match except for the
PMECC_CLK register, which is 2 in kernel 3.16 [1] and 0 in kernel 6.6
[2]. It appears the clock setting is missing since the kernel 4.14
refactor.

According to the SAM9G25 datasheet this field must be programmed with 2.

Manually setting PMECC_CLK to 2 (devmem2 0xffffe010 w 2) resolves the
nandtest issue.

Is the clock setting moved to elsewhere after the refactor ?

Best regards,
Zixun LI

[1] https://github.com/torvalds/linux/blob/19583ca584d6f574384e17fe7613dfaeadcdc4a6/drivers/mtd/nand/atmel_nand.c#L1058
[2] https://github.com/torvalds/linux/blob/ffc253263a1375a65fa6c9f62a893e9767fbebfa/drivers/mtd/nand/raw/atmel/pmecc.c#L772