Oct 11, 2017

Force-enable HDMI to specific mode in linux framebuffer console

Bumped into this issue when running latest mainline kernel (4.13) on ODROID-C2 - default fb console for HDMI output have to be configured differently there (and also needs a dtb patch to hook it up).

Old vendor kernels for that have/use a bunch of cmdline options for HDMI - hdmimode, hdmitx (cecconfig), vout (hdmi/dvi/auto), overscan_*, etc - all custom and non-mainline.

With mainline DRM (as in Direct Rendering Manager) and framebuffer modules, video= option seem to be the way to set/force specific output and resolution instead.

When display is connected on boot, it can work without that if stars align correctly, but that's not always the case as it turns out - only 1 out of 3 worked that way.

But even if display works on boot, plugging HDMI after boot never works anyway, and that's the most (only) useful thing for it (debug issues, see logs or kernel panic backtrace there, etc)!

DRM module (meson_dw_hdmi in case of C2) has its HDMI output info in /sys/class/drm/card0-HDMI-A-1/, which is where one can check on connected display, dump its EDID blob (info, supported modes), etc.

cmdline option to force this output to be used with specific (1080p60) mode:

video=HDMI-A-1:1920x1080@60e

More info on this spec is in Documentation/fb/modedb.txt, but the gist is "<ouput>:<WxH>@<rate><flags>" with "e" flag at the end is "force the display to be enabled", to avoid all that hotplug jank.

Should set mode for console (see e.g. fbset --info), but at least with meson_dw_hdmi this is insufficient, which it's happy to tell why when loading with extra drm.debug=0xf cmdline option - doesn't have any supported modes, returns MODE_BAD for all non-CEA-861 modes that are default in fb-modedb.

Such modelines are usually supplied from EDID blobs by the display, but if there isn't one connected, blob should be loaded from somewhere else (and iirc there are ways to define these via cmdline).

Luckily, kernel has built-in standard EDID blobs, so there's no need to put anything to /lib/firmware, initramfs or whatever:

drm_kms_helper.edid_firmware=edid/1920x1080.bin video=HDMI-A-1:1920x1080@60e

And that finally works.

Not very straightforward, and doesn't seem to be documented in one place anywhere with examples (ArchWiki page on KMS probably comes closest).

Member of The Internet Defense League