KMSDRM video backend doesn't support multiple displays

[SDLBugzilla]

This bug report was migrated from our old Bugzilla tracker.

Reported in version: 2.0.12
Reported for operating system, platform: Linux, ARM

Comments on the original bug report:

On 2020-11-24 13:13:44 +0000, wrote:

Hi,

SDL (SDL_GetNumVideoDisplays) does not recognize the second display on a raspberry pi 4 when running with no X11(kmsdrm video driver). Higher level libraries,like pygame thus can not assign a display for particular output.

More information can be found here.

pygame/pygame#2356

Thanks

On 2020-11-24 13:31:49 +0000, Dan Lawrence wrote:

You can see that the kmsdrm driver is just assuming a single display here, in multiple lines like this one:

dispdata = (SDL_DisplayData *)SDL_GetDisplayDriverData(0); //viddata->devindex);

meanwhile other video drivers that do have multi_display support do things like:

SDL_DisplayData *pData =(SDL_DisplayData*)SDL_GetDisplayDriverData(displayIndex);

Looking back on this thread when the driver was first created:

https://discourse.libsdl.org/t/kms-drm-context-for-the-gl-gles-renderer/21813/11

it seems it was discussed and multiple displays being 'out of scope' was mentioned.

On 2020-11-24 15:37:16 +0000, Sam Lantinga wrote:

Multiple displays are no longer out of scope. I don't have a test setup for this, does anyone have a patch they'd like considered for inclusion that supports this?

[icculus]

Gonna assign this to myself but I don't know when I'll get to it quite yet, in case someone wants to steal it...but I do have a need for this in the relatively near future.

[MakerMatrix]

I have a need for this as well. A simple touch UI on DSI, controlling an application displaying over one or more HDMI screens. X just adds unnecessary bloat and configuration to make it clean since there can be no desktop in this app. The purposely-limited touchscreen is the only HMI.

But, right now, I cannot get KMSDRM to work at all on Pi3b+ (2.0.16) for unknown reasons. Probably PEBKAC.

Edit: perhaps not PEBKAC?: pygame/pygame#2356

[icculus]

I think this is implemented now, but I'll try it on a RPi5 just to verify before we close this.