Running flutter-pi on PiTFT 2.8" doesn't work

[MrCustomizer]

I'm trying to show Flutter apps on my PiTFT 2.8" capacitive display. I've connected the PiTFT to a Raspberry Pi 3 Model B and have followed this guide to set it up in HDMI mirror mode. I chose this mode as it is mentioned in the guide that only this mode supports OpenGL and flutter-pi seems to be dependent on OpenGL.
Trying to start the flutter gallery app with flutter-pi results in the following error:

[flutter-pi] Could not query DRM device list: No such file or directory

I'm guessing it tries to list /dev/dri, but this directory does not exist in my current setup.

Is it theoretically possible to run Flutter apps on this PiTFT and if so, what can I do to help find the cause of the problem? I've attached the output of raspinfo.
raspinfo.txt

[ardera]

/dev/dri should not be empty, thats the directory of connected GPU devices. Did you make sure you have the fake-KMS gl driver enabled in raspi-config? Maybe something in the TFT setup turned it back to legacy

Also, can you give me the complete output of raspinfo?

I have the suspicion that with "OpenGL", they implicitly meant "OpenGL using the legacy driver". The fbcp tool they use to mirror the output also talks to the firmware directly.

FYI: The legacy driver is unsupported since it's even buggier than the fake-KMS one. Both I and some other dude failed to get it working (textures disappearing, etc), there's a thread on the raspberry pi forums about this.

[MrCustomizer]

/dev/dri should not be empty, thats the directory of connected GPU devices. Did you make sure you have the fake-KMS gl driver enabled in raspi-config? Maybe something in the TFT setup turned it back to legacy

Good guess! I ran the setup script for the TFT multiple times with different settings. One (or all) of these runs must have turned it back.
After switching to fake-KMS again, it started working. Thank you very much.
There seems to be only one problem left: The screen resolution isn't set correctly. Is there a way to force a resolution? I haven't found one yet. This is the start of the output, when running flutter_gallery with flutter-pi:

[flutter-pi] WARNING: display didn't provide valid physical dimensions.
             The device-pixel ratio will default to 1.0, which may not be the fitting device-pixel ratio for your display.
===================================
display mode:
  resolution: 1280 x 240
  refresh rate: 59Hz
  physical size: 0mm x 0mm
  flutter device pixel ratio: 1.000000
===================================

The resolution is set to 1280x240, but my PiTFT has a resolution of 320x240.

[ardera]

Can you try adding the following lines to your /boot/config.txt?

hdmi_cvt=320 240 60 1
hdmi_group=2
mide_mode=87
hdmi_drive=2
hdmi_force_mode

See also this reference on the config.txt video options.

Btw, do you have a HDMI monitor connected to the Pi (to use the mirroring)? It may work without a HDMI monitor with hdmi_force_hotplug in config.txt (which essentially fakes that a HDMI monitor is connected). With some software support for framebuffer devices inside flutter-pi, it may even work without hdmi_force_hotplug.

I also recommend invoking flutter-pi with the --dimensions option to give it the real physical dimensions of your PiTFT, otherwise the content may be scaled incorrectly.

[MrCustomizer]

Most of the options were already set. I only had to add hdmi_drive=2 and hdmi_force_mode=1 (btw. you probably meant hdmi_mode instead of mide_mode, right?). Sadly, these settings didn't change anything.

I've started experimenting with the line hdmi_cvt=320 240 60 1 (which has some more parameters on my machine: hdmi_cvt=320 240 60 1 0 0 0). Reducing the width from 320 to 80 has the effect, that the display of the console on the TFT is pretty much distorted, but when starting flutter-pi, the horizontal resolution is changed:

[flutter-pi] WARNING: display didn't provide valid physical dimensions.
             The device-pixel ratio will default to 1.0, which may not be the fitting device-pixel ratio for your display.
===================================
display mode:
  resolution: 320 x 240
  refresh rate: 51Hz
  physical size: 0mm x 0mm
  flutter device pixel ratio: 1.000000
===================================

As the displayed vertical resolution has been correct from the start, I'm guessing there is something wrong with the interpretation of the horizontal resolution.

Btw, do you have a HDMI monitor connected to the Pi (to use the mirroring)?

Currently not. hdmi_force_hotplug is already set on my machine.

I also recommend invoking flutter-pi with the --dimensions option

Thanks. I will do that.

[ardera]

which has some more parameters on my machine: hdmi_cvt=320 240 60 1 0 0 0

The additional parameters are not necessary. If they're not specified, they are initialized to their default values (which is 0 for these specific params)

hdmi_cvt seems to not like setting resolutions that low. I think it may work using hdmi_timings:

hdmi_force_hotplug=1
hdmi_timings=320 0 8 32 40 240 1 3 4 6 0 0 0 40 0 4048000 1
hdmi_group=2
hdmi_mode=87
hdmi_force_mode=1

Though, if you actually connect a HDMI monitor to the HDMI port, it won't show anything since the Pi doesn't work with pixel clocks as low as 4MHz. (The 2nd-last parameter of hdmi_timings).
This btw also enforces a screen refresh-rate of 40Hz, since fbcp copies the buffers to the TFT at 40Hz too, so it doesn't make sense to use anything more than that. (You could probably modify fbcp to copy it faster than that, though, and modify the hdmi timings accordingly)

[MrCustomizer]

I have to admit, I don't know what you did here, but it works! 🤣 I guess I will dig into the specifics of these settings, later. As I don't want to connect a monitor to the HDMI port, that's a perfectly suitable solution for me. Thanks a lot.
I'm closing this issue now as everything seems to be working to my satisfaction.