I did some further investigations...
1. MMC access are generally terribly slow - 45-50 kB/s writes, 60-65
kB/s reads. Expected value at least in hundreds kB/s - normal cards have
maximum throughput at least 1 MB/s, premium cards up to 5-10 MB/s!
2. USB accesses performed at the same time as MMC ones generate MMC
errors.
3. Reducing MMC clock frequency ("mmci.fmax=1900000") reduces number of
errors, for the price of slowing down (even more!) transfers.
4. Two possible sources of problem are: a bug in driver or underlying
hardware (VE motherboard peripherals FPGA) problem, with the latter more
probable (as the driver is used - presumably with success - in field).
Due to lack of time here, I'll have to pass the issue to ARM support
team.
--
USB/mmc rootfs conflict - vexpress
https://bugs.launchpad.net/bugs/632798
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