Hi
In the process of investigating a boot failure on one of our devices, the
UBI: fixable bit-flip detected at PEB
message was seen with the following behaviour during kernel load in u-boot.
Read [2285568] bytes UBI: fixable bit-flip detected at PEB 415 UBI: schedule PEB 415 for scrubbing UBI: fixable bit-flip detected at PEB 415 UBI: fixable bit-flip detected at PEB 419 UBI: schedule PEB 419 for scrubbing UBI: fixable bit-flip detected at PEB 419 UBI: fixable bit-flip detected at PEB 420 UBI: schedule PEB 420 for scrubbing UBI: fixable bit-flip detected at PEB 420 UBI: fixable bit-flip detected at PEB 419 UBI: fixable bit-flip detected at PEB 420 UBI: fixable bit-flip detected at PEB 419 UBI: fixable bit-flip detected at PEB 420 UBI: fixable bit-flip detected at PEB 419 UBI: fixable bit-flip detected at PEB 420 UBI: fixable bit-flip detected at PEB 419 UBI: fixable bit-flip detected at PEB 420 UBI: fixable bit-flip detected at PEB 419 UBI: fixable bit-flip detected at PEB 420 UBI: fixable bit-flip detected at PEB 419
This repeats until reset.
U boot is a patched version of 2010.06 supplied by the chip vendor. No newer version is available from the vendor to try.
The patches include the init eba/wl swap.
A more detailed log with debugging available follows:
UBI: fixable bit-flip detected at PEB 419 UBI DBG: schedule_erase: schedule erasure of PEB 419, EC 19, torture 0 UBI DBG: erase_worker: erase PEB 419 EC 19 UBI DBG: sync_erase: erase PEB 419, old EC 19 UBI DBG: do_sync_erase: erase PEB 419 UBI DBG: sync_erase: erased PEB 419, new EC 20 UBI DBG: ubi_io_write_ec_hdr: write EC header to PEB 419 UBI DBG: ubi_io_write: write 2048 bytes to PEB 419:0 UBI DBG: ensure_wear_leveling: schedule scrubbing UBI DBG: wear_leveling_worker: scrub PEB 420 to PEB 419 UBI DBG: ubi_io_read_vid_hdr: read VID header from PEB 420 UBI DBG: ubi_io_read: read 2048 bytes from PEB 420:2048 UBI DBG: ubi_eba_copy_leb: copy LEB 6:11, PEB 420 to PEB 419 UBI DBG: ubi_eba_copy_leb: read 126976 bytes of data UBI DBG: ubi_io_read: read 126976 bytes from PEB 420:4096 UBI: fixable bit-flip detected at PEB 420 UBI DBG: ubi_io_write_vid_hdr: write VID header to PEB 419 UBI DBG: ubi_io_write: write 2048 bytes to PEB 419:2048 UBI DBG: ubi_io_read_vid_hdr: read VID header from PEB 419 UBI DBG: ubi_io_read: read 2048 bytes from PEB 419:2048 UBI DBG: ubi_io_write: write 126976 bytes to PEB 419:4096 UBI DBG: ubi_io_read: read 126976 bytes from PEB 419:4096 UBI: fixable bit-flip detected at PEB 419 UBI DBG: schedule_erase: schedule erasure of PEB 419, EC 20, torture 0 UBI DBG: erase_worker: erase PEB 419 EC 20 UBI DBG: sync_erase: erase PEB 419, old EC 20 UBI DBG: do_sync_erase: erase PEB 419 UBI DBG: sync_erase: erased PEB 419, new EC 21 UBI DBG: ubi_io_write_ec_hdr: write EC header to PEB 419 UBI DBG: ubi_io_write: write 2048 bytes to PEB 419:0 UBI DBG: ensure_wear_leveling: schedule scrubbing UBI DBG: wear_leveling_worker: scrub PEB 420 to PEB 419 UBI DBG: ubi_io_read_vid_hdr: read VID header from PEB 420 UBI DBG: ubi_io_read: read 2048 bytes from PEB 420:2048 UBI DBG: ubi_eba_copy_leb: copy LEB 6:11, PEB 420 to PEB 419 UBI DBG: ubi_eba_copy_leb: read 126976 bytes of data UBI DBG: ubi_io_read: read 126976 bytes from PEB 420:4096 UBI: fixable bit-flip detected at PEB 420 UBI DBG: ubi_io_write_vid_hdr: write VID header to PEB 419 UBI DBG: ubi_io_write: write 2048 bytes to PEB 419:2048 UBI DBG: ubi_io_read_vid_hdr: read VID header from PEB 419 UBI DBG: ubi_io_read: read 2048 bytes from PEB 419:2048 UBI DBG: ubi_io_write: write 126976 bytes to PEB 419:4096 UBI DBG: ubi_io_read: read 126976 bytes from PEB 419:4096 UBI: fixable bit-flip detected at PEB 419
Investigation showed that a read with correctable bit errors was done returning -EUCLEAN to the ubi read function.
Having read https://lists.denx.de/pipermail/u-boot/2013-September/161961.html which details a workaround to not return EUCLEAN from the NAND reader unless the number of fixed bits returned was 75% of the total number of correctable bits was exceeded during the read. This was impleneted in this version of ubi in uboot 2010.06 and it does hide the bit-flip infinite issue since this is new NAND FLASH. The original 2010.06 implementation returns EUCLEAN for any number of fixable bit flips and thus causes the PEB move to the best free one (scrub mode in wear_leveling_worker).
This fix is not a root cause fix though. Investigating further led to the following root cause solution. The following is AFAICT.
When the scrubber chooses a PEB to move the from the free balanced tree. This tree is sorted by EC (erase count) and then by PEB number.
The find_wl_entry call uses a max parameter of WL_FREE_MAX_DIFF which is 8192 in this config. So the find_wl_entry function will find a PEB that is better in error count that the current PEB EC. This can easily cause it to find the PEB that was just moved from if it is the lowest numbered PEB in the free tree. Waiting for EC to go above 8192 would take a long time and cause premature aging of the flash PEBs in question.
The easy solution is to change the max parameter to this call to 0 so it finds a PEB with a smaller EC than the one being replaced. This means it wont use the previously discarded PEB as its first choice.
This fix was implemented and fixable bit-flip errors no longer hang/freeze the boot process! UBI erase and reformat was used between re-tests to get consistent results.
Adding the above 75% correctable bitflip threshold is also a good thing as less movement will ensue when the FLASH is new, but as the flash ages, the root cause will once again be invoked causing un-recoverable boot failures.
Note this fault is also in the latest kernel drivers for UBI and may also exist in other wear leveling implementations. The kernel driver issue may be at fault for android devices locking up/freezing sporadically during FLASH read when scrubbing due to a relatively full flash and correctable errors causing ping pong PEB moves.
The question is, is my root cause solution sound or have I missed something?
I know an algo change would probably be better or a way to detect move loops to prevent this from occurring, but this solution does work on all the devices that were failing manufacture tests previously.
Regards
Mark