With this patch, the driver code gets complemented with DMA support.
The support for master DMA, in turn, cleared the way for using
interrupts to wait for the completion of transfers, which largely
relieves the CPU compared to the polling PIO mode. Consequently, the new
version has a much lower CPU footprint.
In the current version, both modes of operation PIO and DMA are
functional. However, PIO mode is retained for benchmarking purposes only
and will possibly be removed to keep the driver simple. It is disabled
in the driver's 'main.cc'.
The new SD card driver at 'os/src/drivers/sd_card/omap4' allows the use
of an SD card with the Pandaboard as block service. Currently, the
driver is using PIO, no DMA, and no IRQs. The driver can be tested using
the 'os/run/sd_card.run' script.
