Framebuffer size can be too small for all screens in intermediate steps

[TobiasDummschat]

I'm running into an issue when applying a configuration if the current configuration has an output outside the framebuffer of the target configuration. I'll illustrate with ARandR screenshots and show the offending commands afterwards:

Screenshots

Initial configuration:

Target configuration:

The first xrandr command autorandr attempts to run targets only the two upper to screens, but already resizes the framebuffer to the final size even though this will leave the lower screen outside the framebuffer like this:

XRandR commands

A --dry-run shows me that autorandr wants to run these three xrandr commands:

xrandr --fb 6560x2560 --output HDMI-0 --gamma 1.0:1.0:1.0 --mode 2560x1440 --pos 0x0 --rate 59.94 --reflect normal --rotate left --set non-desktop 0 --output DP-3 --gamma 1.0:1.0:1.0 --mode 2560x1440 --pos 1440x560 --primary --rate 59.94 --reflect normal --rotate normal --set non-desktop 0 
xrandr --fb 6560x2560 --output DP-4 --gamma 1.0:1.0:1.0 --mode 2560x1600 --pos 4000x560 --rate 60.01 --reflect normal --rotate normal --set non-desktop 0 
xrandr --fb 6560x2560

The first one fails with this error:

xrandr: specified screen 6560x2560 not large enough for output DP-4 (2560x1600+0+2560)
xrandr: Configure crtc 1 failed

Possible solution

I think the value --fb argument should not be the size of the target configuration, but should be large enough to include all outputs that will be adjusted in later xrandr calls, but have not yet been adjusted. In my case, everything works fine, if I replace the --fb 6560x2560 in the first command with --fb 4000x4160 which is just large enough to hold every output.

I'm guessing that the single-pass option referenced in #88 would also work, but if it doesn't work for many drivers, that's probably not the best solution.

[phillipberndt]

Agreed, using a framebuffer size that fits both old and new configuration for everything but the last command seems like the best approach here.

[TobiasDummschat]

I don't know much about XRandR, but these lines in apply_configuration seem relevant:

L928-933 at the beginning:

fb_dimensions = get_fb_dimensions(new_configuration)
try:
    fb_args = ["--fb", "%dx%d" % fb_dimensions]
except:
    # Failed to obtain frame-buffer size. Doesn't matter, xrandr will choose for the user.
    fb_args = []

L983-986 after the "auxiliary pre-changes":

# Starting here, fix the frame buffer size
# Do not do this earlier, as disabling scaling might temporarily make the framebuffer
# dimensions larger than they will finally be.
base_argv += fb_args

L1021-1025 at the very end:

    # Adjust the frame buffer to match (see #319)
    if fb_args:
        argv = base_argv
        if call_and_retry(argv, dry_run=dry_run) != 0:
            raise AutorandrException("Command failed: %s" % " ".join(map(shlex.quote, argv)))

Given the initial comment that failing to obtain the frame-buffer size doesn't matter as xrandr will choose for us, would it be possible to only set the frame-buffer size at the very end with the last command and keep it flexible in the meantime? That would look like this: #408

[josephholten]

I am encountering the same issue when switching from a config with two monitors with 3820x2140 + 3440x1440 to only one monitor with 1920x1080. Can verify the PR solves the issue for me as well, so would love to see the PR merged. Would gladly help if needed.