Support opening files for writing

[z3ntu]

On Milos (SM7635) the ADSP seems to want to open a file for writing, and the sensors via libssc and iio-sensor-proxy don't work.

Tried to open /mnt/vendor/persist/sensors/registry/registry/DIR for writing

Briefly looking through hexagonrpcd/apps_std.c I don't quite understand how to get started on that, at least "mocking" that we're writing successfully. Can I just remove the check, or how does file access work with hexagonrpc?

(See also https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/7405#note_519034)

[flamingradian]

File access is done through the apps_std interface with the relevant code in hexagonrpcd/apps_std.c and hexagonrpcd/interfaces/apps_std.def. I'm not sure if you already tried to remove the check, but the next step would be to implement apps_std_fwrite.

FastRPC remote interfaces can be invoked from a different procesor (e.g. ADSP can invoke apps_std on CPU/AP, CPU/AP can invoke remotectl and sometimes chre_slpi on ADSP). Remote interfaces contain remote methods, which each have a parameter specification and a constant numeric ID. Remote method invocations send the handle to the remote interface, the remote method ID, 0-255 input buffers (parameters), and 0-255 output buffers (return values).

When the DSP invokes the apps_std_fwrite remote method, HexagonRPCD (hexagonrpcd/listener.c) finds its metadata in the apps_std_procs array (indexed by remote method ID), does input validation, and calls the implementation with the plain buffer arrays. A stub implementation would only need to hardcode the data in outbufs.

The remote method metadata can be found in the source code of Qualcomm's implementation (see the remote method ID then check _skel_method_23): https://github.com/qualcomm/fastrpc/blob/development/src/apps_std_skel.c#L1389. There isn't an automatic conversion for the parameter specification.