FASTVIDEO_VSA_SM100A=1 (default off) sends no-grad tile-64 forwards through
fastvideo_kernel.block_sparse_attn_sm100a (the Blackwell block-sparse
forward merged in #1719, which handles per-q-tile NON-uniform q2k_num rows
and zero-count rows). Route preconditions: module importable,
is_supported() (sm_100 device, bf16, head_dim 128, even tile count), no
grad tracking; any failure with the env set logs one warning and falls
back to the Triton-64 kernels, which also keep the entire grad path
unchanged.
The bool block map is compacted with the same map_to_index the Triton bool
entry uses, so the sm_100a kernel sees H3's true NON-uniform per-row counts
(prefix query tiles dense, video tiles prefix+top-k).
The FastH3 example surfaces the route as --vsa-kernel {triton,sm100a}
(default triton), which sets the env before pipeline boot so spawned GPU
workers inherit it; documented in the basic README.
CPU route-selection tests: default-off, engage-on-env, grad fallback,
warn-once fallback for missing module/unsupported geometry, no-grad-context
detection.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
