Add basic support for the ESP32

[taktoa]

Motivation for this change

Created a basic cross-compilation toolchain for the ESP32 microcontroller.

To test, try running flash.bash after cloning https://github.com/taktoa/esp32-baremetal (assuming you have an ESP32 microcontroller connected on /dev/ttyUSB0 and your current user is in the dialout group). Then do:

$ stty -F /dev/ttyUSB0 180:4:18b2:8a20:3:1c:7f:15:4:0:1:0:11:13:1a:0:12:f:17:16:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0
$ cat /dev/ttyUSB0

and you should see a spam of "hello, world!".

I tested with the SparkFun ESP32 Thing, for other boards YMMV, though I don't know of any reason why it might not work.

CC: @Ericson2314 @cleverca22 @bgamari

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[lheckemann]

I absolutely love the idea of building my ESP32 firmware with nix! I have some little implementation niggles, but thank you for this!

[lheckemann]

I'm not sure I like the idea of introducing all this very specific stuff in the generic gcc8 expression. At a glance, this looks to me like all these changes can be achieved using overrides? And in that case I think it would be better to have a separate expression file containing these overrides, and using the overridden version to define the stdenv for this platform.

[lheckemann]

Sorry for the delay; it is indeed not trivially clear where this should go — maybe @Ericson2314 and/or @matthewbauer can provide some input, being familiar with stdenv/bootstrapping stuff?

[lheckemann]

Possibly here:

nixpkgs/pkgs/stdenv/cross/default.nix

Lines 56 to 68 in 04cea8b

	cc = if crossSystem.useiOSPrebuilt or false
	then buildPackages.darwin.iosSdkPkgs.clang
	else if crossSystem.useAndroidPrebuilt or false
	then buildPackages."androidndkPkgs_${crossSystem.ndkVer}".clang
	else if targetPlatform.isGhcjs
	# Need to use `throw` so tryEval for splicing works, ugh. Using
	# `null` or skipping the attribute would cause an eval failure
	# `tryEval` wouldn't catch, wrecking accessing previous stages
	# when there is a C compiler and everything should be fine.
	then throw "no C compiler provided for this platform"
	else if crossSystem.useLLVM or false
	then buildPackages.llvmPackages_8.lldClang
	else buildPackages.gcc;

[Mic92]

I am also for putting this behind a different attribute and use overrides.

[Ericson2314]

@lheckemann I think it's fine. See the very similar changes for VC4 and Redox for GCC 6. If we could break up the GCC derivation like we do with the LLVM family of projects, we could do better, but we haven't done that yet so I think GCC is just kinda doomed to be messier.

[lheckemann]

Likewise, I think an override would be nicer here.

[lheckemann]

It could go in a separate file, taking "regular" newlib as an arg and overriding it, then put into place like here:

nixpkgs/pkgs/top-level/all-packages.nix

Lines 12495 to 12513 in 04cea8b

	libcCrossChooser = name:
	# libc is hackily often used from the previous stage. This `or`
	# hack fixes the hack, *sigh*.
	/**/ if name == "glibc" then targetPackages.glibcCross or glibcCross
	else if name == "bionic" then targetPackages.bionic or bionic
	else if name == "uclibc" then targetPackages.uclibcCross or uclibcCross
	else if name == "avrlibc" then targetPackages.avrlibcCross or avrlibcCross
	else if name == "newlib" && stdenv.targetPlatform.isMsp430 then targetPackages.msp430NewlibCross or msp430NewlibCross
	else if name == "newlib" && stdenv.targetPlatform.isVc4 then targetPackages.vc4-newlib or vc4-newlib
	else if name == "newlib" then targetPackages.newlibCross or newlibCross
	else if name == "musl" then targetPackages.muslCross or muslCross
	else if name == "msvcrt" then targetPackages.windows.mingw_w64 or windows.mingw_w64
	else if stdenv.targetPlatform.useiOSPrebuilt then targetPackages.darwin.iosSdkPkgs.libraries or darwin.iosSdkPkgs.libraries
	else if name == "libSystem" then targetPackages.darwin.xcode
	else if name == "nblibc" then targetPackages.netbsdCross.libc
	else if name == "wasilibc" then targetPackages.wasilibc or wasilibc
	else if name == "relibc" then targetPackages.relibc or relibc
	else if stdenv.targetPlatform.isGhcjs then null
	else throw "Unknown libc ${name}";

[bgamari]

Indeed, this looks great.

Also relevant: https://github.com/bgamari/esp32.nix.

[Mic92]

Good to see esp-idf working, setting up the SDK was such a pain!

[Mic92]

I also found that we have some tutorials in the wiki: https://nixos.wiki/wiki/ESP-IDF#See_also
Maybe this can be updated to use esp32.nix instead.

[Ericson2314]

Someone wanna rebase this? I would have merged it but for the conflicts, being more laxed than the others.

[taktoa]

Sorry, I'm kind of busy right now, but if anyone has the time and interest I'd welcome help.

[Ericson2314]

I think a few other people were interested, so 🤞.

[plietar]

I have a new rebased version of this PR, along with some minor improvements. What’s the best way to proceed? Should I open a new PR for it?

[Mic92]

I have a new rebased version of this PR, along with some minor improvements. What’s the best way to proceed? Should I open a new PR for it?

sure.

[taktoa]

Feel free to open a new PR; I'll close this one

[plietar]

Opened #112401.