[Bug 92775] [radeon][TTM] Contention when evicting large buffers between VRAM and GTT

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=92775

--- Comment #1 from Michel Dänzer <michel at daenzer.net> ---
(In reply to Shawn Starr from comment #0)
> One solution discussed is to split up the transfer into smaller chunks in
> radeon_ttm.

Specifically, here's how I think a fallback could be implemented in the kernel
driver which can never fail because of fragmentation or resource starvation:

During initialization, reserve some pinned GTT memory for bounce buffers. When
a BO can't be bound to GTT for eviction as in the case reported here, instead
do the eviction directly from VRAM to CPU domain in one or several passes of:
1. Copy part of the BO from VRAM to one of the reserved bounce buffers in GTT
using the GPU.
2. Copy that part of the BO from the bounce buffer to the BO's system RAM pages
using the CPU.

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