Heads-up: new mmc/sd driver for BeagleBone

[Ian Lepore]

FYI, a few days ago (as of r254559) I checked in a new mmc/sd driver for
BeagleBone (White and Black).  We now use the standard sdhci driver, and
performance is noticibly better.  The new driver supports 8-bit mode for
MMC cards.  Right now it uses PIO mode only (but still performs better
even so).  I intend to add DMA support soon.

The old driver is still available, and if for some reason you suspect
trouble with the new driver, you can quickly switch back by editing
src/sys/arm/ti/am335x/files.am335x, comment out the ti_sdhci line and
uncomment ti_mmchs and rebuild your kernel.

These numbers were with a Lexar class-6 8gb SD card...

ti_mmchs driver:

   root at bb:~ # dd if=/dev/mmcsd0s2b of=/dev/null bs=128k count=100
   13107200 bytes transferred in 6.178682 secs (2121359 bytes/sec)

   root at bb:~ # dd of=/dev/mmcsd0s2b if=/dev/zero bs=128k count=100
   13107200 bytes transferred in 59.523976 secs (220200 bytes/sec)

ti_sdhci driver:

   root at bb:~ # dd if=/dev/mmcsd0s2b of=/dev/null bs=128k count=100
   13107200 bytes transferred in 0.847354 secs (15468389 bytes/sec)

   root at bb:~ # dd of=/dev/mmcsd0s2b if=/dev/zero bs=128k count=100
   13107200 bytes transferred in 1.287745 secs (10178413 bytes/sec)

-- Ian