[FFmpeg-devel] [PATCH] Allow using primary CUDA device context

[Oleg Dobkin]

Add AVCUDADeviceContextFlags to control the creation of CUDA device
context for the hardware CUDA decoder.

The current values are 0 (default behavior) - new context will be
created for each decoder, and 1 - primary CUDA context will be used.

There are several reasons for using primary device context instead of
creating a new one:

 - This is the recommended way to handle device contexts (see
https://docs.nvidia.com/cuda/cuda-driver-api/group__CUDA__CTX.html#group__CUDA__CTX_1g65dc0012348bc84810e2103a40d8e2cf)

 - Memory allocations, kernels and other state are associated with the
current device context. Currently, the context is not accessible from
FFmpeg API, so, technically, the memory created by the hardware decoder
(the video frame) can't be safely read.

Signed-off-by: Oleg Dobkin <olegd at anyvision.co>
---
 libavutil/hwcontext_cuda.c | 20 +++++++++++++++-----
 libavutil/hwcontext_cuda.h |  7 +++++++
 2 files changed, 22 insertions(+), 5 deletions(-)

diff --git a/libavutil/hwcontext_cuda.c b/libavutil/hwcontext_cuda.c
index cca39e9fc7..608ea57569 100644
--- a/libavutil/hwcontext_cuda.c
+++ b/libavutil/hwcontext_cuda.c
@@ -281,8 +281,12 @@ static void cuda_device_uninit(AVHWDeviceContext *device_ctx)
     if (hwctx->internal) {
         CudaFunctions *cu = hwctx->internal->cuda_dl;
         if (hwctx->internal->is_allocated && hwctx->cuda_ctx) {
-            CHECK_CU(cu->cuCtxDestroy(hwctx->cuda_ctx));
+            if (hwctx->flags == DCF_CREATE_CONTEXT)
+                CHECK_CU(cu->cuCtxDestroy(hwctx->cuda_ctx));
+            else
+                CHECK_CU(cu->cuDevicePrimaryCtxRelease(hwctx->cuda_device));
             hwctx->cuda_ctx = NULL;
+            hwctx->cuda_device = NULL;
         }
         cuda_free_functions(&hwctx->internal->cuda_dl);
     }
@@ -322,7 +326,6 @@ static int cuda_device_create(AVHWDeviceContext *device_ctx,
 {
     AVCUDADeviceContext *hwctx = device_ctx->hwctx;
     CudaFunctions *cu;
-    CUdevice cu_device;
     CUcontext dummy;
     int ret, device_idx = 0;
 
@@ -338,18 +341,25 @@ static int cuda_device_create(AVHWDeviceContext *device_ctx,
     if (ret < 0)
         goto error;
 
-    ret = CHECK_CU(cu->cuDeviceGet(&cu_device, device_idx));
+    ret = CHECK_CU(cu->cuDeviceGet(&hwctx->cuda_device, device_idx));
     if (ret < 0)
         goto error;
 
-    ret = CHECK_CU(cu->cuCtxCreate(&hwctx->cuda_ctx, CU_CTX_SCHED_BLOCKING_SYNC, cu_device));
+    hwctx->flags = flags;
+
+    if (flags == DCF_CREATE_CONTEXT)
+        ret = CHECK_CU(cu->cuCtxCreate(&hwctx->cuda_ctx, CU_CTX_SCHED_BLOCKING_SYNC, hwctx->cuda_device));
+    else
+        ret = CHECK_CU(cu->cuDevicePrimaryCtxRetain(&hwctx->cuda_ctx, hwctx->cuda_device));
+
     if (ret < 0)
         goto error;
 
     // Setting stream to NULL will make functions automatically use the default CUstream
     hwctx->stream = NULL;
 
-    CHECK_CU(cu->cuCtxPopCurrent(&dummy));
+    if (flags == DCF_CREATE_CONTEXT)
+        CHECK_CU(cu->cuCtxPopCurrent(&dummy));
 
     hwctx->internal->is_allocated = 1;
 
diff --git a/libavutil/hwcontext_cuda.h b/libavutil/hwcontext_cuda.h
index 81a0552cab..bab5eefe54 100644
--- a/libavutil/hwcontext_cuda.h
+++ b/libavutil/hwcontext_cuda.h
@@ -34,6 +34,11 @@
  * AVBufferRefs whose data pointer is a CUdeviceptr.
  */
 
+enum AVCUDADeviceContextFlags {
+    DCF_CREATE_CONTEXT = 0,
+    DCF_USE_PRIMARY_CONTEXT = 1
+};
+
 typedef struct AVCUDADeviceContextInternal AVCUDADeviceContextInternal;
 
 /**
@@ -43,6 +48,8 @@ typedef struct AVCUDADeviceContext {
     CUcontext cuda_ctx;
     CUstream stream;
     AVCUDADeviceContextInternal *internal;
+    CUdevice cuda_device;
+    enum AVCUDADeviceContextFlags flags;
 } AVCUDADeviceContext;
 
 /**
-- 
2.17.1