Highmem issues with MMC filesystem

[Jamie Lokier]

Nicolas Pitre wrote:
> Data read from a DMA based block device is corrupted in memory and 
> causing all sorts of misbehavior such as segmentation faults in user 
> space, or EXT2 complaining about broken filesystem metadata.

I doubt it is related, but just in case: I've seen similar on a
no-doubt completely unrelated ARM9 system on now-ancient kernels:
occasional ext3 metadata corruption.

In my case it was due to a bug in the chip's IDE driver, which did:

   1. Setup DMA address register.
   2. Appropriate cache flushes.
   3. Write to remaining DMA registers to start DMA.

It turned out step 1 causes the DMA controller to begin reading from
that address into an internal 128-byte FIFO, before officially
starting DMA.  Moving step 2 before step 1 fixed that.

-- Jamie