Bluetooth: L2CAP_BR: Support dynamic PSM allocation

[lylezhu2012]

In the function bt_l2cap_br_server_register(), the PSM cannot be dynamic allocated. And only pre-set PSM is supported.

Improve the function bt_l2cap_br_server_register() to support the dynamic PSM allocation if the passed PSM is zero.

[jhedberg]

I think it might be a little bit cleaner if the PSM was returned through a pointer parameter:

err = bt_l2cap_br_allocate_psm(&server->psm);
if (err) {
	return err;
}

Then you don't need the explicit typecast from int to uint16_t. Thoughts?

[lylezhu2012]

Well. But I think the positive value of err is used in current scenario, right?

[jhedberg]

Yes, then you don't "take advantage" of the positive values anymore, which is the way most APIs that return int errors behave. I'm just saying that from an API design, usability and intuitiveness perspective I feel like it's clearer to split out the returned object or value into a separate pointer parameter and keep the return value purely for error reporting. If you compare my above snippet to the current code it's also less lines of code in addition to eliminating the explicit typecast. C doesn't have any safeguards when doing explicit typecasts, i.e. it happily accepts whatever you ask it, so it's often better if you can write code that doesn't need any such typecasts to begin with.

[lylezhu2012]

OK. Updated.