4 ms·

There really isn't such a thing as "the actual framebuffer". Instead the display hardware can do composition during scanout from a set of buffers at a set of po

There really isn't such a thing as "the actual framebuffer". Instead the display hardware can do composition during scanout from a set of buffers at a set of positions with varying capabilities. These buffers then just being arbitrary dmabufs. It doesn't give a damn if you give it 2 buffers and one contains a mouse cursor and the other everything else or if you give it 2 buffers and one is everything including the mouse and the other is a video, allowing complete power collapse of the GPU rendering units. Often they support more than 2 of these as well, and with color conversions, 1D & 3D LUTs, and a handful of other useful properties. Mobile SoCs in particular, like your typical mid/high end snapdragon, actually have upwards of a dozen overlay planes. This is how Android manages to almost never hit GPU composition at all. On desktop linux all of these go through the drm/kms APIs.

arghwhat 2y ago
Well, GPUs from the big players do give a damn as they tend to have surprisingly limited plane count and capabilities. It is often just a single primary, cursor and overlay plane, sometimes the latter is shared across all outputs, and sometimes what the plane can do depends on what the plane it overlaps with is doing. Mobile chips are as you mention far ahead in this space, with some having outright arbitrary plane counts.
- kllrnohj 2y ago
  Although desktop GPU plane counts are much more limited, it's not as restricted as you're portraying. Here's the AMD SoC in the Steam Deck, for example: https://github.com/ValveSoftware/gamescope/blob/master/src/docs/Steam%20Deck%20Display%20Pipeline.png https://github.com/ValveSoftware/gamescope/blob/master/src/d... Even though it's only 3 planes, they are relatively feature-rich still. In a typical desktop UI that would indeed be primary, cursor, and video planes. But if the system cursor is hidden, such as in a game, that frees up a plane that can be used for something else - such as the aforementioned game.
  - arghwhat 2y ago
    What you are showing is just a standard color pipeline, which is the bare minimum for color management. On AMD in particular, the cursor plane must match several aspects of any plane it overlaps with, including transform and color pipeline IIRC. The AMD SoC in my laptop (much newer than the steam deck) only exposes two overlay planes to share among all 4 display controllers. Intel used to have a single overlay plane per display. The Raspberry Pi 5 on the other hand intentionally limited the exposed overlay planes to "just" 48, as it can be as many as you have memory for. You can peek at the reported capabilities of various devices here: https://drmdb.emersion.fr/ https://drmdb.emersion.fr/

arghwhat 2y ago

Well, GPUs from the big players do give a damn as they tend to have surprisingly limited plane count and capabilities. It is often just a single primary, cursor and overlay plane, sometimes the latter is shared across all outputs, and sometimes what the plane can do depends on what the plane it overlaps with is doing. Mobile chips are as you mention far ahead in this space, with some having outright arbitrary plane counts.

kllrnohj 2y ago
Although desktop GPU plane counts are much more limited, it's not as restricted as you're portraying. Here's the AMD SoC in the Steam Deck, for example: https://github.com/ValveSoftware/gamescope/blob/master/src/docs/Steam%20Deck%20Display%20Pipeline.png https://github.com/ValveSoftware/gamescope/blob/master/src/d... Even though it's only 3 planes, they are relatively feature-rich still. In a typical desktop UI that would indeed be primary, cursor, and video planes. But if the system cursor is hidden, such as in a game, that frees up a plane that can be used for something else - such as the aforementioned game.
- arghwhat 2y ago
  What you are showing is just a standard color pipeline, which is the bare minimum for color management. On AMD in particular, the cursor plane must match several aspects of any plane it overlaps with, including transform and color pipeline IIRC. The AMD SoC in my laptop (much newer than the steam deck) only exposes two overlay planes to share among all 4 display controllers. Intel used to have a single overlay plane per display. The Raspberry Pi 5 on the other hand intentionally limited the exposed overlay planes to "just" 48, as it can be as many as you have memory for. You can peek at the reported capabilities of various devices here: https://drmdb.emersion.fr/ https://drmdb.emersion.fr/