Reverse MXFP8 quantization row raster

[sraman-rgb]

Description

Please include a brief summary of the changes, relevant motivation and context.

Fixes # (issue)

Type of change

• Documentation change (change only to the documentation, either a fix or a new content)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Infra/Build change
• Code refactoring

Changes

Please list the changes introduced in this PR:

• Change A
• Change B

Checklist:

• I have read and followed the contributing guidelines
• The functionality is complete
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[greptile-apps]

Greptile Summary

This PR reverses the Y-axis row raster order for MXFP8 quantization kernels, making them process row blocks from bottom-to-top rather than top-to-bottom. Within each block, stages are also processed in reverse, with all global offsets, scale indices, and bounds checks updated consistently.

• quantize_mxfp8.cuh: Remaps blockIdx.y → block_id_Y = gridDim.y - 1 - blockIdx.y, and introduces logical_stage = STAGES - 1 - stage for all TMA loads, scale writes, and output stores.
• grouped_layout.cuh: Adds linear_block_id_to_reverse_y_cta_coords to convert linear block IDs to reversed-Y CTA coordinates, and updates advance_to_next_job to take work_blocks_Y.
• group_quantize_mxfp8.cuh: Uses the new helper for initial block assignment and persistent-kernel scheduling, with prefetch and processing loops updated to use logical_stage-based offsets.
• New test: Validates that both the generic and grouped quantization paths produce bitwise-identical results after the raster reversal.

Confidence Score: 4/5

Safe to merge; the reverse-raster logic is applied consistently across block Y coordinates, stage offsets, scale indices, bounds checks, and dbias offsets in both the single-tensor and grouped kernel paths.

All offsets derived from the Y block index are consistently updated, and the advance_to_next_job signature change is complete across all call sites. The new test validates bitwise correctness for the generic TMA path and grouped persistent-kernel path, though the specialized cast-only path (column count divisible by 128) is not directly tested.

quantize_mxfp8.cuh and group_quantize_mxfp8.cuh carry the most complex multi-stage pipeline logic; the prefetch priming loop and the stage processing loop both had to be updated in sync, which they are.

Important Files Changed

Filename	Overview
transformer_engine/common/cast/mxfp8/quantize_mxfp8.cuh	Introduces reversed Y-raster via block_id_Y and logical_stage; all derived offsets are consistently updated including the initial TMA prefetch.
transformer_engine/common/cast/core/grouped_layout.cuh	Adds linear_block_id_to_reverse_y_cta_coords helper and updates advance_to_next_job signature; logic is straightforward and correct.
transformer_engine/common/cast/mxfp8/group_quantize_mxfp8.cuh	Uses new reverse-Y helper for initial block assignment and advance_to_next_job; prefetch and processing loops apply logical_stage correctly.
tests/pytorch/mxfp8/test_mxfp8_quantize_raster.py	Tests generic and grouped paths with N=96; padded-column reference approach is sound but the specialized cast-only path is not covered.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A["Launch MXFP8 kernel"] --> B["Compute block_id_Y = gridDim.y - 1 - blockIdx.y"]
    B --> C["block_offset_Y = block_id_Y x CHUNK_DIM_Y"]
    C --> D["Initial prefetch: rows at (STAGES-1)*BUFF_DIM_Y"]
    D --> E["Stage loop: logical_stage = STAGES-1 - stage"]
    E --> F["stage_offset_Y = logical_stage x BUFF_DIM_Y"]
    F --> G["TMA store @ block_offset_Y + stage_offset_Y"]
    G --> J{More stages?}
    J -- Yes --> E
    J -- No --> K["advance_to_next_job via linear_block_id_to_reverse_y_cta_coords"]
    K --> L{More jobs?}
    L -- Yes --> B
    L -- No --> M["Done"]

Loading

%%{init: {'theme': 'base', 'themeVariables': {"darkMode": true, "background": "#0d1117", "primaryColor": "#21262d", "primaryTextColor": "#e6edf3", "primaryBorderColor": "#8b949e", "lineColor": "#8b949e", "textColor": "#e6edf3", "edgeLabelBackground": "#161b22", "actorBkg": "#21262d", "actorBorder": "#8b949e", "actorTextColor": "#e6edf3", "actorLineColor": "#8b949e", "signalColor": "#8b949e", "signalTextColor": "#e6edf3", "noteBkgColor": "#373320", "noteBorderColor": "#d4a72c", "noteTextColor": "#f0e6c0", "labelBoxBkgColor": "#21262d", "labelBoxBorderColor": "#8b949e", "labelTextColor": "#e6edf3", "loopTextColor": "#e6edf3", "activationBkgColor": "#30363d", "activationBorderColor": "#8b949e"}}}%%
flowchart TD
    A["Launch MXFP8 kernel"] --> B["Compute block_id_Y = gridDim.y - 1 - blockIdx.y"]
    B --> C["block_offset_Y = block_id_Y x CHUNK_DIM_Y"]
    C --> D["Initial prefetch: rows at (STAGES-1)*BUFF_DIM_Y"]
    D --> E["Stage loop: logical_stage = STAGES-1 - stage"]
    E --> F["stage_offset_Y = logical_stage x BUFF_DIM_Y"]
    F --> G["TMA store @ block_offset_Y + stage_offset_Y"]
    G --> J{More stages?}
    J -- Yes --> E
    J -- No --> K["advance_to_next_job via linear_block_id_to_reverse_y_cta_coords"]
    K --> L{More jobs?}
    L -- Yes --> B
    L -- No --> M["Done"]

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[greptile-apps]

No coverage for column-count that takes the specialized cast-only path

The comment at line 38 explicitly notes that N=96 is chosen to avoid the specialized rowwise cast-only path. However, the reverse-raster changes in quantize_mxfp8.cuh (via block_id_Y and logical_stage) also affect that specialized path since block_offset_Y and scales_block_offset_Y_rowwise/colwise are computed before any path divergence. A test with a column count that is a multiple of 128 (e.g., 128 or 256) would exercise the specialized path and strengthen confidence that both paths produce correct values after the raster change.

Note: If this suggestion doesn't match your team's coding style, reply to this and let me know. I'll remember it for next time!

[greptile-apps]

The module-level recipe_available / reason_for_no_recipe assignments run at import time; if the import is collected by pytest without a GPU, the te.is_mxfp8_available call could produce misleading error messages before any skipif guard fires. Assigning inside a helper called lazily is the safer pattern used in other test files in this repo.

Suggested change

	recipe_available, reason_for_no_recipe = te.is_mxfp8_available(return_reason=True)
	def _get_mxfp8_availability():
	return te.is_mxfp8_available(return_reason=True)
	recipe_available, reason_for_no_recipe = _get_mxfp8_availability()