ipq40xx: wpj428: panic on squashfs error to work around boot limbo

ipq40xx: wpj428: panic on squashfs error to work around boot limbo

Apparently, a few ipq40xx devices have sporadic problems when reading the
flash over SPI. When that happens, the result of the faulty SPI read is
cached and it isn't re-attempted. Depending on when it happens, the router
either panics and reboots or is left in a partially broken state (an
application wont start).
The data on the flash is alright.

This wasn't the case with Openwrt with Linux < 5.x but I wasn't able to
work out which software change was responsible.

Github user karlpip created a patch for testing that disabled the cache
entirely and added logs. Typically, only one or two SPI operations fail at
a time:

  [689200.631152] spi-nor spi0.0: SPI transfer failed: -110
  [689200.631280] spi_master spi0: failed to transfer one message from queue
  [689200.635369] jffs2: Write of 68 bytes at 0x00ffccf4 failed. returned -110, retlen 0
  [689200.642014] jffs2: Not marking the space at 0x00ffccf4 as dirty because the flash driver returned retlen zero

Because reads aren't re-attempted, squashfs can't recover:

  [3171844.279235] SQUASHFS error: Failed to read block 0x2bb912: -5
  [3171844.279284] SQUASHFS error: Unable to read fragment cache entry [2bb912]
  [3171844.283980] SQUASHFS error: Unable to read page, block 2bb912, size 14e6c
  [3171844.291650] SQUASHFS error: Unable to read fragment cache entry [2bb912]
  [3171844.297831] SQUASHFS error: Unable to read page, block 2bb912, size 14e6c

I assume there to be some kind of underlying electrical problem because,
in my experience, this happens a lot more when PoE is used.

NoTengoBattery has made an in-depth investigation:
https://forum.openwrt.org/t/patch-squashfs-data-probably-corrupt/70480

.. and created a patch that evicts the page cache and retries reading:
https://github.com/NoTengoBattery/openwrt/blob/linksys-ea6350v3-mastertrack/target/linux/ipq40xx/patches-5.4/9996-fs_squashfs_improve_squashfs_error_resistance.patch

The patch also works well with the WPJ428 but NoTengoBattery didn't try to
upstream it ("This is not the solution that should be used").

In 2020, I tried and failed to create a working patch that prevents faulty pages to
be cached in the first place. Because I needed a solution, I backported
  "squashfs: add option to panic on errors " (10dde05b89980ef)
which has since become available in Openwrt.

The 'error=panic' option has been tested on a fleet of multiple hundred
WPJ428s over multiple years. Without this patch, devices regularly went
into 'limbo' on reboot or update and required a manual reboot.
Devices with this patch don't. I was initially concerned that the kernel
panic would leave devices with a real corrupted data but I haven't seen a
case of actual corruption since (outside of people turning off the power
during upgrades).

The WPJ428 is the only device I tested this patch on - others might also
benefit.

Reviewed-by: Robert Marko <robimarko@gmail.com>
Signed-off-by: Leon M. Busch-George <leon@georgemail.eu>