Multiplane Overlay (MPO)¶
You will get more from this page if you have already read the ‘Display Core Next (DCN)’.
Multiplane Overlay (MPO) allows for multiple framebuffers to be composited via fixed-function hardware in the display controller rather than using graphics or compute shaders for composition. This can yield some power savings if it means the graphics/compute pipelines can be put into low-power states. In summary, MPO can bring the following benefits:
- Decreased GPU and CPU workload - no composition shaders needed, no extra buffer copy needed, GPU can remain idle.
- Plane independent page flips - No need to be tied to global compositor page-flip present rate, reduced latency, independent timing.
Keep in mind that MPO is all about power-saving; if you want to learn more about power-save in the display context, check the link: Power.
Multiplane Overlay is only available using the DRM atomic model. The atomic model only uses a single userspace IOCTL for configuring the display hardware (modesetting, page-flipping, etc) - drmModeAtomicCommit. To query hardware resources and limitations userspace also calls into drmModeGetResources which reports back the number of planes, CRTCs, and connectors. There are three types of DRM planes that the driver can register and work with:
DRM_PLANE_TYPE_PRIMARY: Primary planes represent a “main” plane for a CRTC, primary planes are the planes operated upon by CRTC modesetting and flipping operations.
DRM_PLANE_TYPE_CURSOR: Cursor planes represent a “cursor” plane for a CRTC. Cursor planes are the planes operated upon by the cursor IOCTLs
DRM_PLANE_TYPE_OVERLAY: Overlay planes represent all non-primary, non-cursor planes. Some drivers refer to these types of planes as “sprites” internally.
To illustrate how it works, let’s take a look at a device that exposes the following planes to userspace:
- 4 Primary planes (1 per CRTC).
- 4 Cursor planes (1 per CRTC).
- 1 Overlay plane (shared among CRTCs).
Keep in mind that different ASICs might expose other numbers of planes.
For this hardware example, we have 4 pipes (if you don’t know what AMD pipe means, look at ‘Display Core Next (DCN)’, section “AMD Hardware Pipeline”). Typically most AMD devices operate in a pipe-split configuration for optimal single display output (e.g., 2 pipes per plane).
A typical MPO configuration from userspace - 1 primary + 1 overlay on a single display - will see 4 pipes in use, 2 per plane.
At least 1 pipe must be used per plane (primary and overlay), so for this hypothetical hardware that we are using as an example, we have an absolute limit of 4 planes across all CRTCs. Atomic commits will be rejected for display configurations using more than 4 planes. Again, it is important to stress that every DCN has different restrictions; here, we are just trying to provide the concept idea.
AMDGPU imposes restrictions on the use of DRM planes in the driver.
Atomic commits will be rejected for commits which do not follow these restrictions:
- Overlay planes must be in ARGB8888 or XRGB8888 format
- Planes cannot be placed outside of the CRTC destination rectangle
- Planes cannot be downscaled more than 1/4x of their original size
- Planes cannot be upscaled more than 16x of their original size
Not every property is available on every plane:
- Only primary planes have color-space and non-RGB format support
- Only overlay planes have alpha blending support
Before we start to describe some restrictions around cursor and MPO, see the below image:
The image on the left side represents how DRM expects the cursor and planes to be blended. However, AMD hardware handles cursors differently, as you can see on the right side; basically, our cursor cannot be drawn outside its associated plane as it is being treated as part of the plane. Another consequence of that is that cursors inherit the color and scale from the plane.
As a result of the above behavior, do not use legacy API to set up the cursor plane when working with MPO; otherwise, you might encounter unexpected behavior.
In short, AMD HW has no dedicated cursor planes. A cursor is attached to another plane and therefore inherits any scaling or color processing from its parent plane.
Picture-in-Picture (PIP) playback - Underlay strategy¶
Video playback should be done using the “primary plane as underlay” MPO strategy. This is a 2 planes configuration:
- 1 YUV DRM Primary Plane (e.g. NV12 Video)
- 1 RGBA DRM Overlay Plane (e.g. ARGB8888 desktop). The compositor should prepare the framebuffers for the planes as follows: - The overlay plane contains general desktop UI, video player controls, and video subtitles - Primary plane contains one or more videos
Keep in mind that we could extend this configuration to more planes, but that is currently not supported by our driver yet (maybe if we have a userspace request in the future, we can change that).
See below a single-video example:
We could extend this behavior to more planes, but that is currently not supported by our driver.
The video buffer should be used directly for the primary plane. The video can be scaled and positioned for the desktop using the properties: CRTC_X, CRTC_Y, CRTC_W, and CRTC_H. The primary plane should also have the color encoding and color range properties set based on the source content:
The overlay plane should be the native size of the CRTC. The compositor must draw a transparent cutout for where the video should be placed on the desktop (i.e., set the alpha to zero). The primary plane video will be visible through the underlay. The overlay plane’s buffer may remain static while the primary plane’s framebuffer is used for standard double-buffered playback.
The compositor should create a YUV buffer matching the native size of the CRTC.
Each video buffer should be composited onto this YUV buffer for direct YUV
scanout. The primary plane should have the color encoding and color range
properties set based on the source content:
COLOR_ENCODING. However, be mindful that the source color space and
encoding match for each video since it affect the entire plane.
The overlay plane should be the native size of the CRTC. The compositor must draw a transparent cutout for where each video should be placed on the desktop (i.e., set the alpha to zero). The primary plane videos will be visible through the underlay. The overlay plane’s buffer may remain static while compositing operations for video playback will be done on the video buffer.
This kernel interface is validated using IGT GPU Tools. The following tests can be run to validate positioning, blending, scaling under a variety of sequences and interactions with operations such as DPMS and S3:
Multiple Display MPO¶
AMDGPU supports display MPO when using multiple displays; however, this feature behavior heavily relies on the compositor implementation. Keep in mind that usespace can define different policies. For example, some OSes can use MPO to protect the plane that handles the video playback; notice that we don’t have many limitations for a single display. Nonetheless, this manipulation can have many more restrictions for a multi-display scenario. The below example shows a video playback in the middle of two displays, and it is up to the compositor to define a policy on how to handle it:
Let’s discuss some of the hardware limitations we have when dealing with multi-display with MPO.
For simplicity’s sake, for discussing the hardware limitation, this documentation supposes an example where we have two displays and video playback that will be moved around different displays.
- Hardware limitations
From the DCN overview page, each display requires at least one pipe and each MPO plane needs another pipe. As a result, when the video is in the middle of the two displays, we need to use 2 pipes. See the example below where we avoid pipe split:
- 1 display (1 pipe) + MPO (1 pipe), we will use two pipes
- 2 displays (2 pipes) + MPO (1-2 pipes); we will use 4 pipes. MPO in the middle of both displays needs 2 pipes.
- 3 Displays (3 pipes) + MPO (1-2 pipes), we need 5 pipes.
If we use MPO with multiple displays, the userspace has to decide to enable multiple MPO by the price of limiting the number of external displays supported or disable it in favor of multiple displays; it is a policy decision. For example:
- When ASIC has 3 pipes, AMD hardware can NOT support 2 displays with MPO
- When ASIC has 4 pipes, AMD hardware can NOT support 3 displays with MPO
Let’s briefly explore how userspace can handle these two display configurations on an ASIC that only supports three pipes. We can have:
- Total pipes are 3
- User lights up 2 displays (2 out of 3 pipes are used)
- User launches video (1 pipe used for MPO)
- Now, if the user moves the video in the middle of 2 displays, one part of the video won’t be MPO since we have used 3/3 pipes.
- Scaling limitation
MPO cannot handle scaling less than 0.25 and more than x16. For example:
If 4k video (3840x2160) is playing in windowed mode, the physical size of the window cannot be smaller than (960x540).
These scaling limitations might vary from ASIC to ASIC.
- Size Limitation
The minimum MPO size is 12px.