RPI OpenGL PWM display driver

Question

So I'm building a system based on a raspberry pi 4 running Linux (image created through buildroot) driving a Led matrix (64x32 RGB connectors) and I'm very confused about the software stack of linux. I'd like to be able to use OpenGL capabilities on a small resolution screen that would then be transfered to a driver that would actually drive the Led matrix.

I've read about DRM, KMS, GEM and other systems and I've concluded the best way to go about it would be to have the following working scheme:

User space:      App
                  | OpenGL
                  v
Kernel space:    DRM -GEM-> Led device driver
                                  |
                                  v
Hardware:                     Led Matrix

Some of this may not make a lot of sense since the concepts are still confusing to me.

Essentially, the app would make OpenGL calls that would generate frames that could be mapped to buffers on the DRM which could be shared with the Led device driver which would then drive the leds in the matrix.

Would something like this be the best way about it?

I could just program some dumb buffer cpu implementation but I'd rather take this as a learning experience.

Answer 1

OpenGL renders into a buffer (called "framebuffer" that is usually displayed onto the screen. But rendering into an off screen buffer (as the name implies) does not render onto the screen but into an array, which can be read by C/C++. There is one indirection on modern operating systems. Usually you have multiple windows visible on your screen. Therefore the application can't render onto the screen itself but into a buffer maneged by the windowing system, which is then composited into one final image. Linux uses Wayland, multiple Wayland clients can create and draw into the Wayland compositor's buffers.

If you only want to display your application just use a off screen buffer.

If you want to display another application read it's framebuffer by writing your own Wayland compositor. Note this may be hard (I've never done that) if you want to use hardware acceleration.