You looked in the Adafruit forums right.

https://forums.adafruit.com/viewtopic.php?f=19&t=69115&p=350571

Did you get this to work? I have a pitft 3.5 on an rpi2 and I think it works brilliantly!

Yes. I followed this tutorial https://learn.adafruit.com/running-opengl-based-games-and-emulators-on-adafruit-pitft-displays/pitft-setup

You have to substitute 28r for 35r.

The line: /usr/local/bin/fbcp &

Needs to be added to /etc/rc.local

In config.txt you need to add this line: dtoverlay=pitft35r,rotate=90,speed=80000000,fps=60

and comment out a similar line which has been added by the helper script.

Slightly better video here:

Hello all,

80000000 works fine on my pitft, I can’t discern any glitches, flashes or flickers. There’s little or no input lag. I went with the adafruit settings purely because that’s what they recommended for the 2.8 screens and they seem to work fine with my 3.5. Regarding the fps – I’m not sure why there’s an fps setting however I was happy enough to stick with 60fps as it correlates with the refresh rate of most of the emulators I’m using. I’m not sure why a tft would have both a refresh and an fps setting though.

Raspberry Pi 2
Adafruit 3.5 PiTFT

Same problem, screen is extremely shaky/wavy, and now my HDMI says out of sync(no display on monitor)
BTW: Should this be at this resolution, since the pitxt is a 480
hdmi_cvt=320 240

config.txt
# for more options see http://elinux.org/RPi_config.txt
gpu_mem_256=128
gpu_mem_512=256
gpu_mem_1024=384

overscan_scale=1
gpu_mem=64
dtparam=spi=on

hdmi_force_hotplug=1
hdmi_cvt=320 240 60 1 0 0 0
hdmi_group=2
hdmi_mode=87
# — added by adafruit-pitft-helper Wed Jun 17 10:30:02 CDT 2015 —
[pi1]
device_tree=bcm2708-rpi-b-plus.dtb
[pi2]
device_tree=bcm2709-rpi-2-b.dtb
[all]
dtparam=spi=on
dtparam=i2c1=on
dtparam=i2c_arm=on

# dtoverlay=pitft35r,rotate=90,speed=42000000,fps=20
# — end adafruit-pitft-helper Wed Jun 17 10:30:02 CDT 2015 —

dtoverlay=pitft35r,rotate=90,speed=80000000,fps=60

I would like to fill the LCD as much as possible/biggest so it’s usable.

The only way I found so far is modifying the opt/retropie/configs/mame-mame4all/mame.cfg file;

[config]
artwork=yes
samples=yes
antialias=yes
translucency=yes
skiplines=0
skipcolumns=0
beam=1.0
flicker=0.0
gamma=1.0
frameskip=auto
ror=no
rol=no
flipx=no
flipy=no
samplerate=44200
volume=0
cheat=no
#vector_width=640
vector_width=480
#vector_height=480
vector_height=320

kioskmode=no
force_stereo=no
# Anti-alias the display?
display_smooth_stretch=yes
# Stretches to fill screen (ONLY if display_smooth_stretch is enabled)
display_smooth_stretch_full=yes
display_border=0
# display effect postprocessing: 0 none, 1 scanlines
display_effect=0
#Set a static resolution
#resolution=400×240
#resolution=480×320
resolution=320×250
#resolution=360×228