Fix for c4 spi

[hattorihanso]

Description

This pull requests adds a device tree overlay for the Odroid C4 enabling hardware SPI on pins 19, 21, 23 and 24 on the GPIO header. It doesn't require any other changes, except one additional line in the Makefile.
Besides, it can replace the old device tree meson-sm1-odroid-c4-spidev.dtb (which activates wrong pins and doesn't work due to this) while offering more flexibility.

How Has This Been Tested?

The SPI communication has been successfully tested with a LogicPort analyzer and some custom made FPGA board (SPI readout of a Mamamatsu silicon spectrometer).

• Logic Analyzer: timing of MOSI vs. SCK has been verified, maximum speed is about 12MHz for SCK
• data transfer between C4 and spectrometer FPGA has been tested

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules

[Tonymac32]

Directionally I agree with this, we shouldn't have any "spidev" or other special devices trees in any kernel that supports overlays. If we can get a second check by anyone with a C4 I think this is a good move.

[hattorihanso]

One question not related to that... in case being approved, would it make sense to transfer this patch to 6.11 kernel to make sure it doesn't get lost?

[rpardini]

we shouldn't have any "spidev" or other special devices trees in any kernel that supports overlays

My fault. I can't spew out overlays, as the syntax remains a mystery to me.

would it make sense to transfer this patch to 6.11 kernel to make sure it doesn't get lost?

Indeed. I think exactly the same change can be done to 6.11/edge.
If not done, eventually edge will replace current, and this will be lost.

[hattorihanso]

My fault. I can't spew out overlays, as the syntax remains a mystery to me.

No. Your effort! I took the stuff you did and fiddled into an overlay. I don't understand that syntax (I assume only few people really do), and I had some teachings from other gurus here so I could barely do it.

[adeepn]

would it make sense to transfer this patch to 6.11 kernel to make sure it doesn't get lost

add patch for 6.11 into this PR, please.

[hattorihanso]

would it make sense to transfer this patch to 6.11 kernel to make sure it doesn't get lost

add patch for 6.11 into this PR, please.

Could you please advise how to do this? This is my first PR ever...