From 1e8b17e7fc47283b8e34429b40a63069d9d5e1c0 Mon Sep 17 00:00:00 2001
From: berkkirik <berk.kirik@outlook.com>
Date: Fri, 24 Apr 2026 11:06:54 +0300
Subject: [PATCH] fix: default to auto-detected device so CPU-only machines
 don't crash

The CLI --device flag defaulted to "cuda", which crashed with a raw
PyTorch traceback ("Found no NVIDIA driver on your system ...") on
machines without a GPU. Users had to discover --device cpu themselves.

Add an "auto" mode that picks the best available backend (cuda if
detected, otherwise cpu) and make it the default. Users who explicitly
pass --device cuda still get the original loud failure on non-CUDA
machines, which is the correct behavior when they ask for cuda by name.

- opf/_common/device.py (new): resolve_device("auto"|...) helper.
- opf/_cli/common.py: flip --device default to "auto", expand help text.
- opf/_core/runtime.py, opf/_train/runner.py: call resolve_device()
  where device names turn into torch.device objects.

Stderr on auto-fallback:
  info: no CUDA device detected; falling back to CPU
        (pass --device cuda to override).

Fixes #12
---
 opf/_cli/common.py    |  9 +++++++--
 opf/_common/device.py | 30 ++++++++++++++++++++++++++++++
 opf/_core/runtime.py  |  3 ++-
 opf/_train/runner.py  |  3 ++-
 4 files changed, 41 insertions(+), 4 deletions(-)
 create mode 100644 opf/_common/device.py

diff --git a/opf/_cli/common.py b/opf/_cli/common.py
index 31224e1..b75f043 100644
--- a/opf/_cli/common.py
+++ b/opf/_cli/common.py
@@ -55,8 +55,13 @@ def add_device_arg(parser: object) -> None:
     parser.add_argument(
         "--device",
         type=str,
-        default="cuda",
-        help="Device to run on",
+        default="auto",
+        help=(
+            "Device to run on. 'auto' (default) picks cuda if a GPU is "
+            "available, otherwise cpu. Pass 'cuda' or 'cpu' explicitly to "
+            "override or to get a loud error when the requested backend is "
+            "unavailable."
+        ),
     )
 
 
diff --git a/opf/_common/device.py b/opf/_common/device.py
new file mode 100644
index 0000000..f2b162c
--- /dev/null
+++ b/opf/_common/device.py
@@ -0,0 +1,30 @@
+"""Device-name resolution helpers shared by CLI entrypoints."""
+
+from __future__ import annotations
+
+import sys
+
+import torch
+
+AUTO_DEVICE: str = "auto"
+
+
+def resolve_device(device_name: str) -> torch.device:
+    """Resolve a user-supplied device name into a concrete ``torch.device``.
+
+    ``"auto"`` selects the best available device: CUDA if a GPU is detected,
+    otherwise CPU. Any other value is passed through to ``torch.device`` as-is
+    so that explicit requests like ``"cuda"`` or ``"cpu"`` still fail loudly
+    when the underlying backend is unavailable.
+    """
+    if device_name == AUTO_DEVICE:
+        if torch.cuda.is_available():
+            return torch.device("cuda")
+        print(
+            "info: no CUDA device detected; falling back to CPU "
+            "(pass --device cuda to override).",
+            file=sys.stderr,
+            flush=True,
+        )
+        return torch.device("cpu")
+    return torch.device(device_name)
diff --git a/opf/_core/runtime.py b/opf/_core/runtime.py
index 2c3034e..5ed4f1a 100644
--- a/opf/_core/runtime.py
+++ b/opf/_core/runtime.py
@@ -19,6 +19,7 @@
     REDACTED_OUTPUT_LABEL,
     REDACTED_OUTPUT_PLACEHOLDER,
 )
+from .._common.device import resolve_device
 from .._common.env import get_env_bool
 from .decoding import ViterbiCRFDecoder
 from .._common.label_space import resolve_label_space_from_config
@@ -215,7 +216,7 @@ def load_inference_runtime(
     if output_mode not in OUTPUT_MODES:
         raise ValueError(f"Unsupported output_mode: {output_mode!r}")
     _validate_checkpoint_dir(checkpoint)
-    device = torch.device(device_name)
+    device = resolve_device(device_name)
     checkpoint_config = _load_checkpoint_config(checkpoint)
     n_ctx = _resolve_n_ctx(checkpoint_config, n_ctx_override, device)
     encoding_name = checkpoint_config.get("encoding")
diff --git a/opf/_train/runner.py b/opf/_train/runner.py
index b00e23f..274fa20 100644
--- a/opf/_train/runner.py
+++ b/opf/_train/runner.py
@@ -16,6 +16,7 @@
 from .args import parse_args
 from .._api import resolve_checkpoint_path
 from .._common.constants import SCHEMA_VERSION
+from .._common.device import resolve_device
 from .._common.label_space import (
     resolve_checkpoint_label_space,
     resolve_label_space_from_config,
@@ -588,7 +589,7 @@ def main(argv: Sequence[str] | None = None, *, prog: str | None = None) -> int:
                 progress_interval_s = parsed_interval
 
     checkpoint = resolve_checkpoint_path(args.checkpoint)
-    device = torch.device(args.device)
+    device = resolve_device(args.device)
 
     # Default to Triton-backed MoE kernels on non-CPU devices unless callers
     # explicitly opt out. CPU uses torch ops by default so Triton stays optional.