disable_overscan is not having any effect in current raspbian firmware

[hhromic]

According to documentation [1], the disable_overscan option should disable overscan processing on the GPU, however setting it to 1 doesn't seem to do anything.

If I set the overscan_{left,right,top,bottom} options to visible values such as 40, I get the expected black borders around the framebuffer when disable_overscan is commented or 0, however setting it to 1 doesn't remove the black borders as expected. The GPU is still processing an overscan.

I'm using a computer monitor over HDMI for testing and it is using DMT for video mode. Maybe the disable_overscan option doesn't work on DMT modes? shouldn't be the case.

Also, even after setting disable_overscan to 1, vcgencmd still reports the option to be 0, making me believe it's being ignored by the firmware.

[1] https://www.raspberrypi.org/documentation/configuration/config-txt/video.md

[popcornmix]

disable_overscan=1 removes the default overscan settings.
These are:
48 pixels on each edge for HD CEA modes
32 pixels on each edge for SD CEA modes
0 for DMT modes (which displays don't overscan).

You can still apply additional overscan settings which are added to the default.

So the behaviour you describe is as expected.

[hhromic]

A-ha! So, in reality, disable_overscan=1 is actually just a convenience shortcut for setting overscan_{left,right,top,bottom}=0 ?
If so, then yes, that would explain all the behaviour I have been experiencing and is not a bug indeed.
However, I think the documentation should be updated then to better explain what this option actually does.

I was mislead on thinking that the setting disables (as its name implies) all sort of overscan processing, hence I expected that even setting the individual margins wouldn't matter. If I understand well, overscan is always applied (just with 0 pixels when "disabled"), does this mean there is always a penatly on scaling the video output? Or this is only costly with overscan_scale=1?

Thanks in advance for all the clarifications.

[popcornmix]

Overscan (without overscan_scale=1) doesn't cause any additional scaling - the framebuffer size is reduced by the amount of overscan, so pixels are unscaled.
overscan_scale=1 is generally not recommended and will result in elements being scaled.

[hhromic]

Thanks for the clarification! I will close this issue now.
If you agree that the documentation should be improved for the disable_overscan setting (to explain it actually sets the four other margins to 0), let me know and I can send a PR for that.

[JamesH65]

@hhromic Feel free to send a PR to the docs repo.

[hhromic]

@JamesH65 submitted now, thanks!