hidapi/libusb: maintain in-memory cache of vendor/product strings

[slouken]

The get_usb_string call is rather expensive on some USB devices, so we cache the vendor/product strings for future lookups (e.g. when hid_enumerate is invoked again later).

This way, we only need to ask libusb for strings for devices we haven't seen since before we started.

This is an important performance improvement when frequently iterating devices using libusb.

[Youw]

What about serial_number? Shouldn't it be cached as well?

[Youw]

And the caching probably needs to be device-specific, not vid/pid-specific. I have two USB/HID devices on my table right now: both have the same vid/pid but different Manufacturer and Product Name (FW is localized for different countries).

[Youw]

I also think none of it would be required when https://github.com/libusb/libusb/pull/1258 is completed.

[slouken]

And the caching probably needs to be device-specific, not vid/pid-specific. I have two USB/HID devices on my table right now: both have the same vid/pid but different Manufacturer and Product Name (FW is localized for different countries).

Good point.

[mcuee]

Unfortunately https://github.com/libusb/libusb/pull/1258 is closed without being merged, due to some API discussions.

• https://github.com/libusb/libusb/pull/1258

[Youw]

But the idea to get is done is still alive. It is just a matter of getting it done. I'd rather spend time making that than improving the change in this PR.

[mcuee]

But the idea to get is done is still alive. It is just a matter of getting it done. I'd rather spend time making that than improving the change in this PR.

Good point. I agree with you here.

In that case, do you want to close this PR as not planned?

[Youw]

I don't have a strong oppinion. It doesn't bother me having it open.