Manuel Alfayate Corchete
The KMSDRM backend was doing things wrong because of some small (but important) misconceptions on how KMS/DRM works: to implement a largely broken non-vsync refresh mechanism, the SwapWindow() function was issuing new pageflips before previous ones had completed, thus causing EBUSY returns, buffer mismanagement, etc... resulting in general breakage on vsync disabling from apps, that would not allow vsync to work again without KMSDRM video re-initialization.
To further clarify, on most DRM drivers async pageflips are NOT working nowadays, so all issued pageflips will complete on next VBLANK, NOT ASAP (calling drmModePageFlip() with the DRM_MODE_PAGE_FLIP_ASYNC flag will return error).
The old code was assuming that can just issue a synchronous (=on VBLANK) pageflip and then pass a 0 timeout to the pull() function so we do not wait for the pageflip event, thinking that this will lead to correct non-vsynced screen updates from the program: That is plain wrong.
Each pageflip has to be waite before issuing a new one, ALWAYS. And if we do not support ASYNC pageflips on the DRM driver level, then we are forced to wait for the next VBLANK. There is no way around it.
I have also added many comments on the KMSDRM code. This is needed for future reference for me or others who may need to look at this code: KMS/DRM terminology regarding what SYNC and ASYNC mean in pageflip terms, and where to do certain things and why, is not trivial. It is not desirable or possible to invest time on researching the same concepts every time there is need to dive into this code. So please leave all these comments in the patch.
Malte Kie?ling
At the moment i get following warnings from kmsdrm:
* in SDL_kmsdrmvideo.c KMSDRM_DestroySurfaces is return type int, but thats never returned or checked against
* in SDL_kmsdrmvideo.c KMSDRM_DestroySurfaces the variable viddata is not used
* in SDL_kmsdrmopengles.c KMSDRM_GLES_LoadLibrary a cast to NativeDisplayType is missing
I attached a patch for them :)
Malte Kie?ling
Since https://hg.libsdl.org/SDL/rev/f908bd722523 / bug 4966 i cannot build SDL anymore. The error i get is, essentially, caused by -Werror=declaration-after-statement in SDL_kmsdrmvideo.c and SDL_kmsdrmopengles.c
Anthony Pesch
* Remove triple buffering support. As far as I can tell, this goes against the libdrm API; the EGL implementations themselves control the buffering. Removing it isn't absolutely necessary as it seemingly works on the Pi at least, but I noticed this while doing my work and explained my reasoning in the commit.
* Replace the crtc_ready logic which allocates an extra bo to perform the initial CRTC configuration (which is required before calling drmModePageFlip) with a call to drmModeSetCrtc after the front and back buffers are allocated, avoiding this allocation.
* Standardized the SDL_*Data variable names and null checks to improve readability. Given that there were duplicate fields in each SDL_*Data structure, having generic names such as "data" at times was very confusing.
* Removed unused fields from the SDL_*Data structures and moves all display related fields out of SDL_VideoData and into SDL_DisplayData. Not required since the code only supports a single display right now, but this was helpful in reading and understanding the code initially.
* Implement KMSDRM_GetDisplayModes / KMSDRM_SetDisplayMode to provide dynamic modeset support.
These changes have been tested on a Raspberry Pi 4 and a Dell XPS laptop with an HD 520.
As an update, I went back over the triple buffer changes and left them in. I didn't entirely get the code originally, I had just seen it calling KMSDRM_gbm_surface_lock_front_buffer twice for a single swap and had removed it because I was paranoid of bugs stemming from it while working on the modeset changes.
I've made a few small changes to the logic that had thrown me off originally and rebased the changes:
* The condition wrapping the call to release buffer was incorrect.
* The first call to KMSDRM_gbm_surface_lock_front_buffer has been removed. I don't understand why it existed.
* Added additional comments describing what was going on in the code (as it does fix the buffer release pattern of the original code before it).
If KMSDRM_drmModeSetCrtc is called when the swap interval is
set to 0, the driver behaves as though vertical sync is engaged by
limiting framerate to the refresh rate, but performance is much worse
than with vertical sync enabled.
Resolve this issue by ensuring that the Crtc is only set up once,
and KMSDRM_drmModePageFlip is called, albeit without any followup
queueing or waiting for flips.
Icenowy Zheng
One front buffer is locked in GLES_SetupCrtc() and overrides the next_bo just locked in KMSDRM_GLES_SwapWindow, then the next_bo gets lost and is not released even when quitting the video.
It may leads to problems with GLES drivers that doesn't clean up GBM correctly if there's any bo left (e.g. the Mali Utgard r6p2 blob). In the case of Mali Utgard r6p2 blob, the DRM device file is still hold by the blob, and if you try to SDL_Quit to let another program to run (this is done by EmulationStation), the new program will fail to open DRM device.
Manuel
I noticed that, at least on Intel GPU hardware, passing SDL_RENDERER_PRESENTVSYNC would result on a static console instead of the program graphics.
That was due to the fact that calling drmModePageFlip() only works if we have previously set up CRTC to one of the GBM buffers with a drmModeSetCrtc() call, so now it's done and things work as expected.
The KMSDRM_GLES_SetupCrtc() call is done only one time, only when needed (when egl_swapinterval is not 0: when it's 0, there's no need for it because we flip by calling drmModePageFlip() anyway).
The place where KMSDRM_GLES_SetupCrtc() call is done may look strange, but it's right: it needs EGL completely ready because it needs to call eglSwapBuffers() internally to work (see more comments about it in the code).
Manuel
The attached patch adds support for KMS/DRM context graphics.
It builds with no problem on X86_64 GNU/Linux systems, provided the needed libraries are present, and on ARM GNU/Linux systems that have KMS/DRM support and a GLES2 implementation.
Tested on Raspberry Pi: KMS/DRM is what the Raspberry Pi will use as default in the near future, once the propietary DispmanX API by Broadcom is overtaken by open graphics stack, it's possible to boot current Raspbian system in KMS mode by adding "dtoverlay=vc4-kms-v3d" to config.txt on Raspbian's boot partition.
X86 systems use KMS right away in every current GNU/Linux system.
Simple build instructions:
$./autogen.sh
$./configure --enable-video-kmsdrm
$make
