SDHostBRCME88C - sdport-based driver for RPI4 emmc2

[idigdoug]

sdport-based miniport driver for RPI4's emmc2 controller.

• Supports UHS-DDR50
• Supports DMA (via bounce buffer)

Intended to help with issue #20

[binarymaster]

Just to note, it seems the directory for eMMC2 controller driver already exists in the source tree:

https://github.com/raspberrypi/windows-drivers/tree/master/drivers/sd/bcm2711/bcmemmc2

[mariobalanica]

Thanks for working on this!

I'm not sure how it would be best to merge it, considering the issues you've mentioned here: #20 (comment), and the already existing bcmemmc2 INF. It could be merged in a separate branch, and once it's confirmed to be as stable as sdbus, replace bcmemmc2 with it then finally merge into master.
@christopherco thoughts?

bcm2836sdhc isn't particularly useful without a WLAN driver, but if someone starts writing one, the interrupt conflicts will become an issue.

I've briefly tested the driver without bcm2836sdhc being present and haven't seen any issues. Even unloading works fine in the latest insider builds.

[idigdoug]

I was assuming this would be merged and treated like all of the other drivers that are in the repo but not actually ready to be in the default image. That would make it easier for people to test and fix bugs than having it in a separate branch.

[mariobalanica]

Oh, forgot about the non-working drivers. Good point.
We can merge it into master, but there should be a README in the "drivers/sd/bcm2711" directory explaining why we have 2 drivers for the same controller.

[christopherco]

Thanks for adding this EMMC2 driver. Went through all but the SlotExtension code and made some comments. Will read through the slot extension tomorrow.