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Bug 3943

Summary: [PATCH] General SDL_HINT_VIDEO_DOUBLE_BUFFER hint support
Product: SDL Reporter: Manuel Alfayate Corchete <redwindwanderer>
Component: videoAssignee: Brandon Schaefer <brandontschaefer>
Status: RESOLVED FIXED QA Contact: Sam Lantinga <slouken>
Severity: normal    
Priority: P2 CC: brandontschaefer
Version: 2.0.6   
Hardware: All   
OS: All   
Attachments: Patch that adds a general hint for low-latency video in some drivers

Description Manuel Alfayate Corchete 2017-11-06 00:36:44 UTC 
Created attachment 3069 [details]
Patch that adds a general hint for low-latency video in some drivers

Hello,

I have made a new patch that adds a general SDL_HINT_VIDEO_DOUBLE_BUFFER hint, as suggested by Alex Szpakowski.

NOTE that the specific KMSDRM double buffer hint (SDL_HINT_KMSDRM_DOUBLE_BUFFER) was already merged, so this patch removes that hint because the idea is using SDL_HINT_VIDEO_DOUBLE_BUFFER for all drivers that support it. The drivers that currently support it are listed in SDL_hints.h.

Currently, KMSDRM and RaspberryPi drivers support the SDL_HINT_VIDEO_DOUBLE_BUFFER hint, so the SDL_VIDEO_DOUBLE_BUFFER enviroment variable must be set for it to activate.

The KMSDRM code for double buffering is already merged, as I said, so regarding the KMSDRM driver this patch only changes the hint it honors to from SDL_HINT_KMSDRM_DOUBLE_BUFFER to SDL_HINT_VIDEO_DOUBLE_BUFFER, in order to "standarize" it.

This patch includes the RaspberryPi code for double buffering, so the previous unmerged patch (https://bugzilla.libsdl.org/show_bug.cgi?id=3914) is OBSOLETE.
***Merge this instead.***

I would like this merged ASAP because this has to be the last patch I can do in a while (starting studies again, no time for programming).

Thanks
Comment 1 Brandon Schaefer 2017-11-07 15:54:50 UTC 
I dont have my raspi near to test for a few days. It was working on kmsdrm, and we should merge this before the other HINT is released. Wont be easy to remove the other one due to being in a public header once its been released. So Ill just double check all is compiling at lease then merge.
Comment 2 Brandon Schaefer 2017-11-07 16:27:01 UTC 
Im not to familiar with rasberrypi code and its APIs. I dont really see where the other thread is? Does vc_dispmanx_vsync_callback callback from a different thread? Looking it up just seemed to do a dispatch (with locking but seems to happen on the same thread that calls it?). Though I could easily be missing something!
Comment 3 Manuel Alfayate Corchete 2017-11-07 17:03:35 UTC 
(In reply to Brandon Schaefer from comment #2)
> Im not to familiar with rasberrypi code and its APIs. I dont really see
> where the other thread is? Does vc_dispmanx_vsync_callback callback from a
> different thread? Looking it up just seemed to do a dispatch (with locking
> but seems to happen on the same thread that calls it?). Though I could
> easily be missing something!

The vc_dispmanx_vsync_callback function is called from the dispmanx side on every frame: It starts happening when it is passed to vc_dispmanx_vsync_callback(displaydata->dispman_display, RPI_vsync_callback, (void*)wdata);

Thats why you dont see it called explicitly: its automatically called from an internal dispmanx thread outside SDL2. That's how It works.

I agree that merging is urgent.
Comment 4 Brandon Schaefer 2017-11-07 17:12:29 UTC 
That makes sense, just wanted to double check, as I dont have a raspi to dig around right now. Ill double check the code in a few days when I can, Thank you again for the patch!

https://hg.libsdl.org/SDL/rev/2ce56475ad57