mmap error with alsa::direct::pcm::Status on Raspberry Pi

[supercurio]

Hi @diwic!
Following on #33 (comment)

Here is a test program:
https://github.com/supercurio/asrc-rs-research/blob/master/src/alsa_direct_status_test.rs

Runs on a amd64 desktop Ubuntu 17.04 with all ALSA cards.

Fails with Error("mmap (of driver memory)", Sys(ENXIO))', libcore/result.rs:945:5 on Raspberry Pi 3+ 4.14.30-v7+ capture and playback with:

• HDMI out
• USB audio (AUREON XFIRE8.0 HD)
• I2S HifiBerry Digi+ I/O

[diwic]

Found it - apparently this mmap of control and status is only available on X86, ALPHA and PowerPC :-(

[diwic]

Ok, so I've made a SyncPtrStatus struct for the other archs. I should probably do something generic across all ways to read status...but for now this will do...

[supercurio]

Cool! I'm looking at how to update the alsa_direct_status_test.rs code and just pushed a commit.
It ends up like that: https://github.com/supercurio/asrc-rs-research/blob/5d4b34e487414863086086a9c12fe0284f0c04a5/src/alsa_direct_status_test.rs#L98

alsa-rs/src/pcm_direct.rs

Line 631 in aa33d98

let ss = unsafe { SyncPtrStatus::sync_ptr(pcm_to_fd(&pcm).unwrap(), false, None, None).unwrap() };

relies on pcm_to_fd so I made it public to try here.

On both x84_64 and arm, it returns state: Prepared htstamp: 0

[diwic]

The syscall is made when you call sync_ptr, you need to do this every time you need a new snapshot of the current status. (This is unlike Status which reads directly from memory that the kernel keeps updated.)

relies on pcm_to_fd so I made it public to try here.

Or just copy-paste pcm_to_fd into your own code as it relies on public stuff only. It's merely a two-line helper.

[supercurio]

Right!
The test program, relying on SyncPtrStatus and following your suggestions now works on Raspberry Pi 3+, Pi Zero, and also ODROID-C2 which doesn't support mmap I/O!
Looks good 👍 closing!