KMSDRM: Garbled video with testgles2 on NVIDIA Tegra about sdl HOT 5 CLOSED

Excellent, thanks for merging 🏆

from sdl.

I've been trying to debug this problem and stepping through the KMSDRM and EGL code. Changing strides and such doesn't seem to have any effect at all.

However once thing I've noticed is that this platform (NVIDIA Tegra) reports having two displays, one of which seems to be a 640x480 display, possibly some internal or MIPI thing. If I look at the way the picture is garbled I'm beginning to suspect that somewhere in SDL the displays get mixed up and perhaps EGL is created using the dimensions of one screen while the GBM is using the other.

P.S. --fullscreen does not seem to be working:

ERROR: Could not lock front buffer on GBM surface
ERROR: eglSwapBuffers failed

from sdl.

I've finally found the problem on this particular platform, after testing between drm*, GBM and EGL call differences between a known working app (https://github.com/embtom/kmscube) and SDL.

There seem to be three different variants of the drmModeAddFB call: drmModeAddFB, drmModeAddFB2 and drmModeAddFB2WithModifiers. SDL uses the first variant as can be seen at https://github.com/libsdl-org/SDL/blob/release-2.30.x/src/video/kmsdrm/SDL_kmsdrmvideo.c#L360

When I replace that call with drmModeAddFB2WithModifiers instead and use the GBM bo modifiers, the test cases start working:

uint32_t format, strides[4] = {0}, handles[4] = {0}, offsets[4] = {0}, flags = 0;

w = KMSDRM_gbm_bo_get_width(bo);
h = KMSDRM_gbm_bo_get_height(bo);
format = KMSDRM_gbm_bo_get_format(bo);

uint64_t modifiers[4] = {0};
modifiers[0] = gbm_bo_get_modifier(bo);
const int num_planes = gbm_bo_get_plane_count(bo);
for (int i = 0; i < num_planes; i++) {
        strides[i] = gbm_bo_get_stride_for_plane(bo, i);
        handles[i] = gbm_bo_get_handle(bo).u32;
        offsets[i] = gbm_bo_get_offset(bo, i);
        modifiers[i] = modifiers[0];
}

if (modifiers[0]) {
        flags = DRM_MODE_FB_MODIFIERS;
}

ret = drmModeAddFB2WithModifiers(viddata->drm_fd, w, h, format, handles, strides, offsets, modifiers, &fb_info->fb_id, flags);

The GBM modifier seems to be set at 0x3000000000fe014 so I'm suspecting that the modifier must be used when creating the frame buffer. It doesn't seem that strange, as these modifiers come from GBM itself.

Swapping in the above code seems to fix the display issue, tested with release-2.30.x. If helpful I could open a PR for this issue.

(Inversely, when I changed kmscube to use drmModeAddFB2 or drmModeAddFB it showed the same display problem as SDL.)

🙊 I've also noticed that the fullscreen flag seems to be failing the test case, this seems to be due to the fullscreen logic causing a dirty surface and recreation at https://github.com/libsdl-org/SDL/blob/release-2.30.x/src/video/kmsdrm/SDL_kmsdrmopengles.c#L110 but I suspect something is not released properly as eglSwapBuffers will fail with GL_FALSE from that point on. Should probably keep this for a different issue, but just wanted to note it down for reference.

from sdl.

Sure, a PR would be helpful, thanks! Good sleuthing. :)

from sdl.

Great, a pull request was made in #10117. One thing to note is that drmModeAddFB2WithModifiers and the newer gbm* modifier API calls seems to be a newer calls so this might break backwards compatibility with old versions of those libraries.

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