Bug in mtd_get_device_size()?

[Brian Norris]

+ Richard

On Thu, Feb 28, 2013 at 4:30 AM, Velykokhatko, Sergey
<Sergey.Velykokhatko at mcc-med.de> wrote:
> I got today such case:
>
>       * Kernel 3.8
>
>       * We are using M29F2G16 NAND chip with 4096 blocks, each has 128k
>
>       * Configured with CONFIG_MTD_UBI_BEB_LIMIT=100

This is your problem. See below for more comments.

>       * Our rootfs partition contains 640 EBs
>
>       * At system start comes following error message:
>
> Feb 28 09:23:09 (none) kern.notice kernel: [    0.375000] Creating 6 MTD
> partitions on "atmel_nand":
>
> Feb 28 09:23:09 (none) kern.notice kernel: [    0.375000]
> 0x000000000000-0x000000020000 : "obsolete_bootstrap"
>
> Feb 28 09:23:09 (none) kern.notice kernel: [    0.381000]
> 0x000000020000-0x000000320000 : "kernel_a"
>
> Feb 28 09:23:09 (none) kern.notice kernel: [    0.386000]
> 0x000000320000-0x000000620000 : "kernel_b"
>
> Feb 28 09:23:09 (none) kern.notice kernel: [    0.392000]
> 0x000000620000-0x000005620000 : "rootfs_a"
>
> Feb 28 09:23:09 (none) kern.notice kernel: [    0.399000]
> 0x000005620000-0x00000a620000 : "rootfs_b"
>
> Feb 28 09:23:09 (none) kern.notice kernel: [    0.406000]
> 0x00000a620000-0x000020000000 : "config_data"
>
> ...
>
>
>
> Feb 28 09:23:10 (none) kern.info kernel: [    0.546000] NET: Registered
> protocol family 17
>
> Feb 28 09:23:10 (none) kern.notice kernel: [    0.557000] UBI: attaching
> mtd3 to ubi0
>
> Feb 28 09:23:10 (none) kern.info kernel: [    0.748000] usb 1-2: new
> high-speed USB device number 2 using atmel-ehci
>
> Feb 28 09:23:10 (none) kern.notice kernel: [    0.810000] UBI: scanning is
> finished
>
> Feb 28 09:23:10 (none) kern.warn kernel: [    0.821000] UBI warning:
> print_rsvd_warning: cannot reserve enough PEBs for bad PEB handling,
> reserved 115, need 400
>
> Feb 28 09:23:10 (none) kern.notice kernel: [    0.826000] UBI: attached mtd3
> (name "rootfs_a", size 80 MiB) to ubi0
>
> Feb 28 09:23:10 (none) kern.notice kernel: [    0.826000] UBI: PEB size:
> 131072 bytes (128 KiB), LEB size: 129024 bytes
>
> Feb 28 09:23:10 (none) kern.notice kernel: [    0.826000] UBI: min./max. I/O
> unit sizes: 2048/2048, sub-page size 512
>
> Feb 28 09:23:10 (none) kern.notice kernel: [    0.826000] UBI: VID header
> offset: 512 (aligned 512), data offset: 2048
>
> Feb 28 09:23:10 (none) kern.notice kernel: [    0.826000] UBI: good PEBs:
> 640, bad PEBs: 0, corrupted PEBs: 0
>
> Feb 28 09:23:10 (none) kern.notice kernel: [    0.826000] UBI: user volume:
> 1, internal volumes: 1, max. volumes count: 128
>
> Feb 28 09:23:10 (none) kern.notice kernel: [    0.826000] UBI: max/mean
> erase counter: 399/202, WL threshold: 4096, image sequence number: 548304255
>
> Feb 28 09:23:10 (none) kern.notice kernel: [    0.826000] UBI: available
> PEBs: 0, total reserved PEBs: 640, PEBs reserved for bad PEB handling: 115
>
> Feb 28 09:23:10 (none) kern.notice kernel: [    0.827000] UBI: background
> thread "ubi_bgt0d" started, PID 288
>
> UBI wanted to reserve 400 blocks on MTD with 640. Hm... During debugging I
> found that mtd_get_device_size() gives the size of whole flash memory back
> (4096 blocks * 128kB). I guess that I found small error. With micro patch
> the problem seems to be corrected for my case

Your patch is wrong, actually. mtd_get_device_size() should get "the
size of the entire flash chip", so if mtd is a partition, you *want*
to return the size of its 'master'.

In fact, everything is behaving as expected (by the designers; not by
you). Because your BEB_LIMIT=100, you are reserving 100*size/1024
(that is 9.8% of your total size, or 400 blocks) in *every* partition.
I don't see why you need that high of a limit, though. Check your NAND
data sheet, but the limit defaults reasonably to 20 (or, about 2%).
That would reserve only 80 blocks on your system, and you would not
see these warnings/errors, since you already have 115 blocks reserved.

You might want to read the comments in this commit:

commit ba4087e956d336488c6df9dfca65d1e70cf480f1
Author: Richard Genoud <richard.genoud at gmail.com>
Date:   Tue Jul 10 18:23:41 2012 +0200

    UBI: use the whole MTD device size to get bad_peb_limit

It might be worth revisiting the conclusions of those arguments; it
essentially limits your partitions to a certain minimum size. But as
far as I can tell, the code works as documented.

Brian