01:19 <Guest70> I've been trying to use a program called drm-framebuffer to 'cat' to the screen.  https://github.com/embear-engineering/drm-framebuffer  On a different device I can 'cat /dev/urandom | drm-framebuffer -d /dev/dri/card0 -c HDMI-A-1' and the image will show over top of the console until I quit the application.  On a beaglebone black running Debian

01:19 <Guest70> 11 (I tried the IOT image first, then the XFCE image after disabling the lightdm service), the screen goes off when I run that drm-framebuffer command until I quit it.  This is a bit bizarre, does anyone have any idea why?

07:20 <zmatt> Guest70: drm does work on a beaglebone, I've used it successfully myself

07:20 <zmatt> Guest70: does it log any errors?

07:21 <zmatt> or perhaps it configured a weird mode for some reason

07:24 <zmatt> Guest70: looks like it's trying to use a pixel format that's not supported

07:35 <zmatt> it's also using an old legacy api, drmModeAddFB, that infers the pixel format ( https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/drm_fourcc.c#L84 ) from bpp and depth, instead of its replacement drmAddModeFB2 which lets you specify the exact pixel format you want

07:39 <zmatt> by default the formats DRM_FORMAT_RGB565, DRM_FORMAT_BGR888, and DRM_FORMAT_XBGR8888 ... the first one (16-bit color) is recommended since only 16 data lines are actually connected to the HDMI framer so using higher color depth is just a waste of time (and RAM)

07:40 <zmatt> BGR888 and XBGR888 are pretty non-standard formats and rarely supported

07:41 <zmatt> if needed it's possible to make the hdmi framer swap the red and blue channels, which changes the list of supported pixel formats to DRM_FORMAT_BGR565, DRM_FORMAT_RGB888, and DRM_FORMAT_XRGB8888 ... I don't quite remember how, I think there was a DT overlay for it maybe?

08:07 <zmatt> Guest70: drmAddModeFB2 is slightly annoying to call though because you need to supply it with info for four planes even though you'll almost always only use one... the underlying ioctl is actually more convenient to use than its libdrm wrapper: https://pastebin.com/61ftsZTu

13:16 <Guest5> @zmatt: Thanks for all that.  Was guest70 and got booted off.  I will look into that later today.  The device that works is 8-bit colour so I think you've identified the problem.