MMC performance

[Pierre Ossman]

I've added SGIO support to my driver now hoping that it would resolve 
the piss-poor performance I've been getting. Didn't do much difference 
though.

Read operations are fairly fast. It queues 8kB at a time. A bit small 
perhaps, but still decent.

Writing, however, only sends a single sector at a time. The queue 
process eats up half of the CPU time on my machine during a write. And 
since MMC cards have to clear a whole bunch of sectors before a write 
shouldn't you send as many sectors as possible to them?

Since I don't have another controller to compare with I don't really 
know if the problem is in my code, the MMC layer, the block layer or the 
filesystem.

I'm going to dig around a bit more but some pointers are welcome. At 
least which layer I should be looking at.

Rgds
Pierre
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