USDHC driver failure and imx8mm platfrom support

[Jean-Adrien Domage]

Dear Genodians,

I need help with the clock controller module in the platform driver for 
an imx8mm-based SoC. We must configure the clock controller module to 
communicate with an eMMC device.

We have worked with the Compulab imx8mm IoT gateway for a year. So far, 
it has worked exceptionally well. We received a board from Compulab with 
a new revision and an updated Linux BSP a few weeks ago.

Unfortunately, the new version of that board broke the eMMC driver. I 
have not yet been able to get eMMC working with the new driver.

We ported the eMMC driver from the older Linux BSP for the older version 
of the board. That version of the driver does not work with the new 
board because of a voltage validation sequence failure. I suspect this 
is because we assumed only minimal modifications are needed in the 
platform driver to enable imx8mm from the existing imx8mq base. I 
recently discovered that the CCM register definition differs quite 
substantially between imx8mm and imx8mq. You can see this discrepancy in 
section 5.1.8.1 ("CCM_analog memory map definition") from the SoC 
reference manual of the imx8mq and imx8mm SoC.

It looks like the older board worked out of pure luck because U-Boot 
sets these CCM registers correctly, and the platform driver does not 
alter them in a way that breaks the expected behavior.

Here are my thoughts on solving the eMMC driver of the new board:
There appears to be no way around rewriting `ccm.cc` and `ccm.h` of the 
`platform_drv` for the imx8mm SoC to set the CCM registers correctly.

However, I'm not 100% convinced that it would help with my eMMC problem.

1. This approach seems straightforward fix the CCM registers to set the 
proper clock settings and gating. Can someone confirm?

2. To someone with experience with the `sd_card_drv` imx8mq, do you 
think fixing the CCM would also solve any voltage validation sequences?

3. The new Linux eMMC driver tries to reconfigure the `pinctrl` for the 
controller multiple times. Is it currently impossible to do that from a 
component with base-hw kernel using the Linux DDE? Only the 
`platform.cc` from the `bootstrap` boot module seems to be able to set 
that configuration. Setting that configuration within the `bootstrap` 
module is how we currently achieve it on the older board.

I have pushed my imx8mm changes to the following repository:
https://github.com/a-dmg/genode-imx/tree/1558-merge_imx_and_imx8mm_repos

This repo works with Genode 22.08 on the older revision of the Compulab 
imx8mm IoT gateway. I created a run script called 
`sd_card_imx8mm_iot_gate.run` to verify this.

Cheers!
Jad

----
Jean-Adrien Domage
Software Engineer
Gapfruit AG
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