The mainline kernel includes a panel driver for the 7" touchscreen. A device tree diff to enable it exists. An overlay was planned, but it's still nowhere to be found. Plus, the touch input and backlight drivers are not included in the mainline kernel.

Turns out, it's really easy to compile the input/backlight drivers out of tree! They don't depend on anything that's not upstreamed.

This repo includes copies of these modules and a device tree.

make modules
sudo make install

sudo insmod /lib/modules/$(uname -r)/extra/rpi-ft5406.ko.gz
sudo insmod /lib/modules/$(uname -r)/extra/rpi_backlight.ko.gz

dtc -I dts -O dtb -o upstream.dtb upstream.dts

The included device tree is for the Pi 3 B, if you're on 2 or whatever, you'll need to make your own:

• Decompile the upstream dtb for your Pi (dtc -I dtb -O dts ...)
• Add labels:
  • firmware { -> firmware: firmware {
  • gpio@7e200000 { -> gpio: gpio@7e200000 {
  • dsi@7e700000 { -> dsi1: dsi@7e700000 {
• Add the i2c_dsi: i2c section and modify the dsi1: dsi@7e700000 section (don't forget to remove status = "disabled";) like in that diff, using appropriate gpio numbers (they're shown there as &i2c_dsi blocks for various Pis' separate files, just take the gpio numbers and modify them inline in the i2c_dsi: i2c section)
• Add the rpi_ft5406 and rpi_backlight sections from my upstream.dts
• Finally, compile!

Look at the diff linked above, it has different numbers for different Pis.

Depends on your distro and way of booting.

For netbooting from U-Boot, something like this:

tftp ${kernel_addr_r} /Image
tftp ${fdt_addr_r} /upstream.dtb
tftp ${ramdisk_addr_r} /initramfs-linux.img
booti ${kernel_addr_r} ${ramdisk_addr_r}:${filesize} ${fdt_addr_r}

For loading from the firmware, in config.txt:

device_tree=upstream.dtb