Fix wrong _IOC_TYPE on musl

[jirutka]

When compiling on Alpine Linux x86_64 with Alpine's rust and target x86_64-alpine-linux-musl:

error[E0308]: mismatched types
  --> libv4l-rs-917e1f4ffd90b94a/d7ac716/src/v4l2/api.rs:23:24
   |
23 |         v4l2_ioctl(fd, request, argp)
   |                        ^^^^^^^ expected `u64`, found `i64`

Ref maximbaz/wluma#43

[raymanfx]

Hmm, I'm not sure this is the correct thing to do, since the conditionals were introduced as part of armv7 musl support here: 874cf29. So it looks like we need to distinguish between ... x86{_64} and ARM as well? Or maybe only 32 bit musl wants signed integers?

[jirutka]

That’s interesting because it currently fails to build even on armv7 musl…

error[E0308]: mismatched types
  --> target/patch/d7ac716/src/v4l2/api.rs:33:26
   |
33 |         v4l2_mmap(start, length as u64, prot, flags, fd, offset)
   |                          ^^^^^^^^^^^^^ expected `u32`, found `u64`
   |
help: you can convert a `u64` to a `u32` and panic if the converted value doesn't fit
   |
33 |         v4l2_mmap(start, (length as u64).try_into().unwrap(), prot, flags, fd, offset)
   |                          +             +++++++++++++++++++++
error[E0308]: mismatched types
  --> target/patch/d7ac716/src/v4l2/api.rs:36:28
   |
36 |         v4l2_munmap(start, length as u64)
   |                            ^^^^^^^^^^^^^ expected `u32`, found `u64`
   |
help: you can convert a `u64` to a `u32` and panic if the converted value doesn't fit
   |
36 |         v4l2_munmap(start, (length as u64).try_into().unwrap())
   |                            +             +++++++++++++++++++++
   Compiling mccs-db v0.1.2

I don’t have experience with writing cross-platform bindings in Rust, so I don’t know what’s the correct approach here, except setting up CI to continuously test against all supported targets.

[raymanfx]

I'll do some cross-compiling experiments by myself and get back to you.

[raymanfx]

Okay, so these are my findings:

• You are using the v4l-sys feature which links against libv4l dynamically and thus uses a different code path than what was used by the other musl armv7 user: Fix compilation error against musl #14
• Compiling against x86_64-alpine-linux-musl is fine with the default features (v4l2-sys), but not when linking against libv4l (no-default-features, v4l-sys).
• The same goes for the armv7-unknown-linux-musleabihf target.

Other interesting bits:

• Linux ioctl(..) looks like this: int ioctl(int fd, unsigned long request, ...) (doc)
• musl ioctl(..) looks like this: int ioctl (int, int, ...) (src)
• libv4l defines its own v4l2_ioctl wrapper as: int v4l2_ioctl (int fd, unsigned long int request, ...) (src)

Yikes.

I pushed a commit to the next branch to fix this: 0f3f0df. Could you please test that with your setup and let me know what happens?