After I released my latest kernel with 48MHz support for accessing MMC and SD cards, a couple of readers noticed that the card performance was surprisingly low (around 4MB/s) which was way slower than I saw when I did testing way back when. Lucky, I kept those old results around and the fastest card I have was reporting 12.9MB/s and it too is now doing 4.something. That’s not good. The really weird part is that the transfer rate seems to *increase* when I put load on the CPU! Obviously, there’s a dimishing returns behaviour here but certainly if I put ~20% load on the CPU, I see the transfer rate increase by about 50%.
So, given that, it seems like there’s something funky going on with the cpu power conservation and it’s not speeding up when it needs to to maximise the data transfers.
I’ve filed bug 2838 for this problem.
In the meantime, my 48MHz patch is rather accademic as you’re never going to reach the hardware limits.
My n810 is now in the mail and I’ll report if I see any difference in behaviour.