Re:USB Made Simple (2008)
by MichaelZuo on Tuesday October 17, 2023 @09:45AM
(#37914963)
How would you implement such a mechanism without introducing a lot of overhead?
Re:USB Made Simple (2008)
by sampullman on Tuesday October 17, 2023 @10:31AM
(#37915710)
Where's the overhead? Depending on your system the whole thing could be zero copy down to the DMA buffer. It's just a read/write protocol with a separate channel for commands.
Re:USB Made Simple (2008)
by MichaelZuo on Tuesday October 17, 2023 @12:04PM
(#37917208)
I'm not an expert on implementing Bluetooth in real life products, but something tells me it will be a bit more complex then that.
Re:USB Made Simple (2008)
by sampullman on Wednesday October 18, 2023 @04:46AM
(#37926247)
At the HCI level the Bluetooth protocol is sane enough, it's straightforward to provide a zero-copy interface to the L2CAP layer if you're writing your own host stack.
A product I worked on in the past used a serial connection over Bluetooth to Android/iOS apps. In order to communicate to an Apple device, the layers roughly looked like:
Radio -> L2CAP -> RFCOMM -> iAP2
The RFCOMM/iAP2 part were redundant. It's understandable that Apple wanted to certify devices, but implementing that as a thin layer over L2CAP and making the profiles on top optional would have made the system much simpler. There may have been historical reliability concerns on the Bluetooth side that justifies what they ended up with, I'm not sure.
The problem is that the cross product of hardware on the sending and receiving sides leaves sufficient gaps that you will be banging your head against the wall trying to figure out why a profile isn't working and it's because Vendor's part doesn't implement the spec properly.
I sometimes wonder what a world where every radio is a software-defined radio and we can all just update our full Bluetooth implementation alongside our OS. Instead it's the worst case of "actually another CPU with closed source blob firmware" and so we're absolutely left stranded.
I think what is being advocated here is moving profiles outside the implementation, into libraries we can port around, or e.g. the kernel somewhere, and I think that is indeed a step in the right direction.