cuda : supports running on CPU for GGML_USE_CUBLAS=ON build (ggml-org#3946)

* protyping the idea that supports running on CPU for a GGML_USE_CUBLAS=on build

* doc: add comments to ggml_cublas_loaded()

* fix defined(...)

Author: wsxiaoys

ggml-cuda.cu

@@ -5790,6 +5790,11 @@ static void ggml_cuda_pool_free(void * ptr, size_t size) {
     CUDA_CHECK(cudaFree(ptr));
 }
 
+static bool g_cublas_loaded = false;
+
+bool ggml_cublas_loaded(void) {
+    return g_cublas_loaded;
+}
 
 void ggml_init_cublas() {
     static bool initialized = false;
@@ -5803,7 +5808,12 @@ void ggml_init_cublas() {
         CUDA_CHECK(cudaDeviceSynchronize());
 #endif
 
-        CUDA_CHECK(cudaGetDeviceCount(&g_device_count));
+        if (cudaGetDeviceCount(&g_device_count) != cudaSuccess) {
+            initialized = true;
+            g_cublas_loaded = false;
+            return;
+        }
+
         GGML_ASSERT(g_device_count <= GGML_CUDA_MAX_DEVICES);
         int64_t total_vram = 0;
 #if defined(GGML_CUDA_FORCE_MMQ)
@@ -5851,6 +5861,7 @@ void ggml_init_cublas() {
         // CUBLAS_CHECK(cublasLoggerConfigure(1, 1, 0, nullptr));
 
         initialized = true;
+        g_cublas_loaded = true;
     }
 }
 
@@ -7158,6 +7169,8 @@ static void ggml_cuda_rms_norm(const ggml_tensor * src0, const ggml_tensor * src
 }
 
 bool ggml_cuda_can_mul_mat(const struct ggml_tensor * src0, const struct ggml_tensor * src1, struct ggml_tensor * dst) {
+    if (!g_cublas_loaded) return false;
+
     const int64_t ne10 = src1->ne[0];
 
     const int64_t ne0 = dst->ne[0];
@@ -7843,6 +7856,8 @@ void ggml_cuda_free_scratch() {
 }
 
 bool ggml_cuda_compute_forward(struct ggml_compute_params * params, struct ggml_tensor * tensor) {
+    if (!g_cublas_loaded) return false;
+
     ggml_cuda_func_t func;
     const bool any_on_device = tensor->backend == GGML_BACKEND_GPU
         || (tensor->src[0] != nullptr && (tensor->src[0]->backend == GGML_BACKEND_GPU || tensor->src[0]->backend == GGML_BACKEND_GPU_SPLIT))

ggml-cuda.h

@@ -17,7 +17,12 @@ extern "C" {
 
 #define GGML_CUDA_MAX_DEVICES       16
 
+// Always success. To check if CUDA is actually loaded, use `ggml_cublas_loaded`.
 GGML_API void   ggml_init_cublas(void);
+
+// Returns `true` if there are available CUDA devices and cublas loads successfully; otherwise, it returns `false`.
+GGML_API bool   ggml_cublas_loaded(void);
+
 GGML_API void * ggml_cuda_host_malloc(size_t size);
 GGML_API void   ggml_cuda_host_free(void * ptr);