Editing Direct Rendering Manager (section)

==== {{Anchor|PRIME}} DMA Buffer Sharing and PRIME ====
The ''DMA Buffer Sharing API'' (often abbreviated as DMA-BUF) is a [[Linux kernel]] internal [[Application programming interface|API]] designed to provide a generic mechanism to share [[Direct memory access|DMA]] buffers across multiple devices, possibly managed by different types of device drivers.{{r|Corbet 2012 dmabuf}}{{r|Clark Semwal 2012}} For example, a [[Video4Linux]] device and a graphics adapter device could share buffers through DMA-BUF to achieve [[zero-copy]] of the data of a video stream produced by the first and consumed by the latter. Any Linux [[device driver]] can implement this API as exporter, as user (consumer) or both.

This feature was exploited for the first time in DRM to implement PRIME, a solution for [[GPU offloading]] that uses DMA-BUF to share the resulting framebuffers between the DRM drivers of the discrete and the integrated GPU.{{r|Peres 2014|p=13}} An important feature of DMA-BUF is that a shared buffer is presented to user space as a [[file descriptor]].{{r|drmbook-mm}}{{r|Peres Ravier 2013|p=17}} For the development of PRIME two new ioctls were added to the DRM API, one to convert a local GEM handle to a DMA-BUF file descriptor and another for the exact opposite operation.

These two new ioctls were later reused as a way to fix the inherent unsafety of GEM buffer sharing.{{r|Peres Ravier 2013|p=17}} Unlike GEM names, file descriptors can not be guessed (they are not a global namespace), and Unix operating systems provide a safe way to pass them through a [[Unix domain socket]] using the SCM_RIGHTS semantics.{{r|drmbook-mm}}{{r|Pinchart 2013|p=11}} A process that wants to share a GEM object with another process can convert its local GEM handle to a DMA-BUF file descriptor and pass it to the recipient, which in turn can get its own GEM handle from the received file descriptor.{{r|Peres Ravier 2013|p=16}} This method is used by [[Direct Rendering Infrastructure|DRI3]] to share buffers between the client and the X Server{{r|Edge 2013}} and also by [[Wayland (display server protocol)|Wayland]].