For SD cards to properly perform wear leveling the fstrim command must be used periodically. When we tried to trim our filesystem, I discovered that the driver wasn't setting the MMC_CAP_ERASE bit. Comments from when this feature was introduced (2010) seem to indicate that various drivers might expect timeouts or other issues that needed to be resolved.
I added the following to atmci_init_slot():
mmc->caps |= MMC_CAP_ERASE;
To my surprise, it seems to "just work". Perhaps the developers can comment on some obvious problem that I've missed? At any rate, I imagine Atmel will want to release someting, as trimming improved write performance and device longevity.