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diff --git a/Documentation/gpu/amdgpu/display/mpo-overview.rst b/Documentation/gpu/amdgpu/display/mpo-overview.rst new file mode 100644 index 000000000..0499aa92d --- /dev/null +++ b/Documentation/gpu/amdgpu/display/mpo-overview.rst @@ -0,0 +1,242 @@ +======================== +Multiplane Overlay (MPO) +======================== + +.. note:: You will get more from this page if you have already read the + 'Documentation/gpu/amdgpu/display/dcn-overview.rst'. + + +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. + +.. note:: 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 <https://gitlab.freedesktop.org/pq/color-and-hdr/-/blob/main/doc/power.rst>`__. + +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). + +.. note:: 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 'Documentation/gpu/amdgpu/display/dcn-overview.rst', 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. + +Plane Restrictions +================== + +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 + +Cursor Restrictions +=================== + +Before we start to describe some restrictions around cursor and MPO, see the +below image: + +.. kernel-figure:: mpo-cursor.svg + +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. + +Use Cases +========= + +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 + +.. note:: 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: + +.. kernel-figure:: single-display-mpo.svg + +.. note:: 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: + +* ``COLOR_RANGE``, ``COLOR_ENCODING`` + +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_RANGE``, +``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: + +- ``kms_plane@plane-panning-bottom-right-pipe-*-planes`` +- ``kms_plane@plane-panning-bottom-right-suspend-pipe-*-`` +- ``kms_plane@plane-panning-top-left-pipe-*-`` +- ``kms_plane@plane-position-covered-pipe-*-`` +- ``kms_plane@plane-position-hole-dpms-pipe-*-`` +- ``kms_plane@plane-position-hole-pipe-*-`` +- ``kms_plane_multiple@atomic-pipe-*-tiling-`` +- ``kms_plane_scaling@pipe-*-plane-scaling`` +- ``kms_plane_alpha_blend@pipe-*-alpha-basic`` +- ``kms_plane_alpha_blend@pipe-*-alpha-transparant-fb`` +- ``kms_plane_alpha_blend@pipe-*-alpha-opaque-fb`` +- ``kms_plane_alpha_blend@pipe-*-constant-alpha-min`` +- ``kms_plane_alpha_blend@pipe-*-constant-alpha-mid`` +- ``kms_plane_alpha_blend@pipe-*-constant-alpha-max`` + +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: + +.. kernel-figure:: multi-display-hdcp-mpo.svg + +Let's discuss some of the hardware limitations we have when dealing with +multi-display with MPO. + +Limitations +~~~~~~~~~~~ + +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: + +.. kernel-figure:: multi-display-hdcp-mpo-less-pipe-ex.svg + +- 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). + +.. note:: These scaling limitations might vary from ASIC to ASIC. + +* **Size Limitation** + +The minimum MPO size is 12px. |