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I have two 4 GB RPi4s, and can run any tests if you like.

Or is this about some 1GB barrier DMA limitation?

I did notice VideoCore can only access maximum 1GB of RAM. Related to that?

the issue about the available RAM is this: https://github.com/raspberrypi/linux/commit/cdb78ce891f6c636...

probably some kind of address mapping, but I'm no expert on this stuff ;-)

Saw that, but that doesn't really tell why, and how much effort is required to solve this. Maybe SD stuff is in the VideoCore side, and it simply can't DMA above 1 GB limit, and requires to have below 1GB DMA buffers + copy for transfers to upper memory. Maybe.

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