README.md

SD Host drivers for Raspberry Pi 4

The Raspberry Pi 4 contains 3 SD Host controllers. The "BCM2711 ARM Peripherals" datasheet calls them SDHOST, EMMC, and EMMC2.

SDHOST - old, unusual design, uses bcm2836\rpisdhc driver (an sdport.sys miniport driver). The RPi4 UEFI does not currently expose this device. In theory it could be enabled and connected to GPIO pins to run an external SD slot.
EMMC - newer controller, supports SD memory and SDIO, uses bcm2836\bcm2836sdhc driver (an sdport.sys miniport driver). UEFI currently exposes this as ACPI\ARASAN and connects it to the WiFi SDIO card by default.
EMMC2 - new for RPi4, supports only SD memory. UEFI currently exposes this as ACPI\BRCME88C and connects it to the Micro-SD slot by default.

There are two drivers available for the EMMC2 controller.

bcm2711\bcmemmc2 contains configuration files that load the Windows in-box sdbus.sys driver to run the EMMC2 controller.
- Advantage: sdbus.sys is stable and reliable.
- Disadvantage: sdbus.sys uses more CPU for slower data transfers because it can only use PIO (cannot use EMMC2's nonstandard DMA) and can only use 25 MB/s transfer speeds (cannot switch the Raspberry Pi's voltage regulator to the 1.8v needed for UHS signaling).
bcm2711\brcme88c contains the SDHostBRCME88C.sys miniport driver that works with the Windows in-box sdport.sys port driver to run the EMMC2 controller.
- Advantage: SDHostBRCME88C.sys uses less CPU for faster data transfers because it can use DMA and can use 50 MB/s transfer speeds.
- Disadvantage: SDHostBRCME88C.sys is probably less reliable than sdbus.sys. SDHostBRCME88C.sys is new and not yet fully tested, plus sdport.sys is not as well-maintained by Microsoft and has some known issues.

For now, most users will probably want to use the bcm2711\bcmemmc2 driver while work continues on testing/improving the bcm2711\brcme88c driver.