USERPTR

USERPTR is a memory model supported by the Video4Linux2 (V4L2) API in Linux. In this model, the application allocates its own user-space buffers and provides pointers to the driver when queuing buffers for capture or output. The driver then uses those user-space buffers as the I/O targets, rather than allocating and managing buffers in kernel space or using DMA-BUF sharing.

How it works in practice varies by device and driver, but the common pattern is that the application allocates buffers in user space, locks or pins the pages to prevent them from being swapped out, and passes the user-space addresses to the driver via V4L2 buffer ioctls. The driver may map or pin the corresponding pages and perform DMA to or from the user buffers. Because the memory is in user space, the driver must access it safely from kernel space, which typically involves pinning pages and handling potential page faults or memory migrations during I/O.

Advantages of USERPTR include lower kernel memory usage for drivers and potentially simpler buffer management for some applications, particularly on systems with limited kernel memory or when a user-space memory allocator is already in use. Disadvantages include the need for explicit memory locking and pinning by the application, potential security and stability concerns from letting a driver access arbitrary user memory, and generally more complex synchronization and portability concerns across drivers. Performance can be highly sensitive to memory fragmentation, paging behavior, and the overhead of pinning pages.

USERPTR is one of several V4L2 memory models, alongside MMAP and DMABUF. It remains useful in certain embedded or specialized workflows, but many modern pipelines favor DMABUF or kernel-managed buffers for better isolation and compatibility.