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Hi there,

While doing broad tests on ths KMSDRM backend, I found out that vanilla-conquer (https://github.com/TheAssemblyArmada/Vanilla-Conquer) shows abysmal performance on KMSDRM, resulting in very bad framerates compared to the exact same binary running on the SDL2 X11 backend.

-There are no errors returned by any of the the KMSDRM functions (meaning that drmModePageFlip() is always returning OK, so we are not missing any frames that the games sends, or so it looks).

-In both backends (X11 and KMSDRM) CPU usage is the same, ~25% on my X86_64 laptop.

-In both backends, the SDL internal rendering path seems to be the same, as SDL reports the SAME content in the SDL_RendererInfo for the SDL_Renderer it creates using X11 or KMSDRM:

(gdb) p info
$1 = {name = 0x7ffff7f56b21 "opengl", flags = 14, 
  num_texture_formats = 8, texture_formats = {372645892, 
    376840196, 370546692, 374740996, 842094169, 1448433993, 
    842094158, 825382478, 0, 0, 0, 0, 0, 0, 0, 0}, 
  max_texture_width = 16384, max_texture_height = 16384}


-The engine creates it's renderer here:

https://github.com/TheAssemblyArmada/Vanilla-Conquer/blob/34072fea51a9ba7c5fb78d82f14ab94f180055c0/common/video_sdl2.cpp#L175

It blits from an 8bpp surface
https://github.com/TheAssemblyArmada/Vanilla-Conquer/blob/34072fea51a9ba7c5fb78d82f14ab94f180055c0/common/video_sdl2.cpp#L490

to a 32bpp surface:
https://github.com/TheAssemblyArmada/Vanilla-Conquer/blob/34072fea51a9ba7c5fb78d82f14ab94f180055c0/common/video_sdl2.cpp#L494

It does this blitting here:
https://github.com/TheAssemblyArmada/Vanilla-Conquer/blob/34072fea51a9ba7c5fb78d82f14ab94f180055c0/common/video_sdl2.cpp#L565


...but it gets done ultimately, so I don't see how that would be affected by the backend.



Any ideas on what could going on here are welcome. Thanks!