[PATCH 0/2] rust: honor the maximum DMA segment size

[Matteo Kloiber]

Booting nova-core against a real GSP with CONFIG_DMA_API_DEBUG=y warns
when the ~60 MiB firmware image is mapped as a scatter-gather table:

  DMA-API: nova-core 0000:00:03.0: mapping sg segment longer than
  device claims to support [len=62914560] [max=65536]

There are two separate issues behind it.

Patch 1 has nova-core declare the segment size it actually supports. It
re-decomposes every segment into 4 KiB GSP page-table entries, so it has
no upper bound on segment length.

Furthermore, I also checked what other GPUs do, and most do the same:
set the max segment size u32::MAX. So this should be the correct
behavior for nova as well.

Patch 2 has SGTable::new() honor dma_get_max_seg_size() in addition to
dma_max_mapping_size(). The abstraction is generic, so a driver with a
real hardware segment limit would otherwise silently be handed segments
it cannot express in a single descriptor.

Tested on an RTX 5080 (GB203) passed through to a QEMU guest via VFIO
with CONFIG_DMA_API_DEBUG=y: the warning is gone, and GSP boot and RPC
still work.

Discussed on Zulip:
https://rust-for-linux.zulipchat.com/#narrow/channel/509436-Nova/topic/nova.20core.3A.20DMA-API.3A.20nova-core.200000.3A00.3A03.2E0.3A.20mapping.20sg.20segme/with/618319830

Matteo Kloiber (2):
  gpu: nova-core: declare unlimited DMA max segment size
  rust: scatterlist: honor the device's maximum segment size

 drivers/gpu/nova-core/gpu.rs |  7 +++++++
 rust/helpers/dma.c           |  5 +++++
 rust/kernel/scatterlist.rs   | 12 ++++++++++--
 3 files changed, 22 insertions(+), 2 deletions(-)

base-commit: cee9395acd8043be0644b25c34bfa86623f2b935
-- 
2.51.2