My particular hardware target is a MinnoboardMax Turbot Dual-E, but I believe this question is generic to Intel Atom/BayTrail processors on other boards.
The SoC has a "multifunction serial port" (SSP) that can be used as a SPI bus interface. It can be attached by ACPI or by PCI (in the later case, the PCI id is 0x8086:0x0f0e). And within the Linux kernel, there is existing driver support (low_speed_spidev, spi_pxa2xx_pci, spi_pxa2xx_platform) which allows access to the hardware.
I would like to access the hardware from UEFI, and I'm finding very little documentation on how to access or program this hardware. I don't care very much whether I have to set it into ACPI or PCI mode in the BIOS, either one is OK for my purposes. I'd love to find an existing driver, but after half a day of searching I'm pretty convinced it's not out there. (Happy to be proven wrong on that.)
In lieu of that, can anyone point me to any code examples that would help to bootstrap this? I've tried looking at the Linux and BSD source code, but they both involve so many extra layers for generality/portability that I'm having trouble sorting out the portions that relate to this particular device, and the portions that simply support a generic driver model.
Edit: Post originally mistakenly said AHCI.