Draft: Add LLC bypass region for Carfield

[CyrilKoe]

The region at [0x2_8000_0000 ; 0x3_0000_0000[ can be used to bypass the LLC when the LLC is enabled.
This is useful to speedup device DMA when LLC is enabled (of course requires managing LLC flushes in software).
Note that the LLC already has an AXI demux - mux which bypasses the LLC by default. Thus there is no need to change anything in the LLC or its internal address map.

Maintainer's Note: TODO:

• Get Makefile: Add nonfree repo to fix model fetching hyperbus#27 merged (@thommythomaso)
• Get LLC partitioning merged... somehow? (General LLC PR merge sweep?)
• Retarget to main
• Make bypass range configurable (defaults can be parameters)

[paulsc96]

Would you be interested in retargeting this to main? I cannot adopt the LLC partitioning change as is (#74 seems to be stuck in serious limbo) but I am generally interested in this feature for mainline, e.g. for efficient VGA frame buffer accesses.

What I am not sure about is whether it is flexible enough. A configurable range would be nice.

[alex96295]

For me, yes; it was not merged in Carfield as I was in the process of bumping Cheshire first (PR308, which is pending as it waits for this PR to be merged)

I agree that the LLC partitioning as-is cannot be merged, but this features could be placed on top of LLC main

[paulsc96]

Then that sounds like a path forward. Kinda. Updated the PR message.

There is an LLC PR for partitioning (pulp-platform/axi_llc#10), but either something is bizarrely wrong with it (5k+ changes) or that is just not getting merged. Either way, it was never undrafted.