Future work for i.MX8 uSDHC driver

[Ivan-Velickovic]

#157 adds initial support for the i.MX8 based MMC block device.

The main motivation for this is to support experimentation regarding hot-plugging and so we do not implement all driver features you would expect with a uSDHC driver.

Below is various future work we may or may not implement. It is likely for the minor things and high-speed mode to be implemented eventually.

• Version 1 SD cards (the initialisation flow)
• Version 2 SDSC / SDXC cards (only because I haven't implemented support for calculating card capacity)
• High-speed mode
• Hook it up to a clock driver
• Figure out if there is an optimal value for 'watermark level'. Right now we set it as the maximum (U-Boot does the same thing).
• Properly initialise/reset VEND_SPEC register.
• Voltage Negotiation (anything but 3.3V)
• Card detect does not work (see followup comment)
• When reading/writing blocks, we always set the block length with a uSDHC command. Don't think it should be necessary more than once so we should be able to omit it.
• Properly set block config read_only flag.

[Ivan-Velickovic]

Whoops, wrong repository (fixed now).

[midnightveil]

Regarding card detection:

Neither the MaaxBoard nor the i.MX8MM-EVK board's (nominally-iMX8-specific) uSDHC controller fire the CINSIEN/CRMIEN IRQs for card insertion / removal, as per the standard SD-PHY specifications. However:

The iMX8MM-EVK specifies the cd-gpios property in its device tree, and is correctly updates to reflect the card status:

# in
u-boot=> gpio status
Bank GPIO1_:
GPIO1_15: input: 0 [x] [email protected]
# out
u-boot=> gpio status
Bank GPIO1_:
GPIO1_15: input: 1 [x] [email protected]

In contrast, the MaaXboard does not set the cd-gpios property in its device tree, and so U-Boot/Linux don't know about unplug/replug. However, there is still a GPIO linked up to the card detect pin:

# from running `gpio status -a` in uboot
$ diff ~/plug.txt ~/unplug.txt
8c8
< GPIO1_6: input: 0 [ ]
---
> GPIO1_6: input: 1 [ ]

For this feature to work, we need a GPIO driver and to listen for interrupts on those pins, as per the device tree for mmc controllers.