Hi Luiz, Den tors 31 jan. 2019 kl 18:03 skrev Luiz Augusto von Dentz <luiz.dentz@xxxxxxxxx>: > > Hi Emil, > > On Thu, Jan 31, 2019 at 6:19 PM Emil Lenngren <emil.lenngren@xxxxxxxxx> wrote: > > > > Hi, > > > > I was looking through the quite lengthy discussion at > > https://github.com/WebBluetoothCG/web-bluetooth/issues/238 on the > > issue that in Web-Bluetooth, only a single "write value" API is > > available, causing Web-Bluetooth to decide on its own if Write With > > Response or Write Without Response should be used, in case both are > > supported by the characteristic. > > > > But in the Bluetooth spec about Write Without Response: > > > > "This sub-procedure is used to write a Characteristic Value to a > > server when the client knows the Characteristic Value Handle and the > > client does not need an acknowledgement that the write was > > successfully performed." > > > > Basically, it says it's up to the client/application to decide if an > > acknowledgement is needed or not, and hence it's the app that should > > decide if Write With or Without Response should be used. The "client" > > can't mean a bluetooth stack here since it can of course not know if > > an acknowledgement is needed or not. > > There is a property indicating if write without response is supported > though, but you are right regarding that not excluding regular write > so at that point the client would have a choice whether to use it or > not. > > > I noticed that according to gatt-api.txt, BlueZ has the same > > limitation in the WriteValue method, in that the stack chooses the > > write type "arbitrarily" if both write types are supported (or really > > the Write With Response is chosen, which might cause unwanted > > latency). Therefore I suggest that an option should be added to the > > WriteValue method, for example "write-without-response" (bool) to > > force Write Without Response. > > It gets a bit trickier if the attribute is in fact a control point in > which case perhaps only write-without-response really works, anyway > control points are better off using AcquireWrite. > > > Note how iOS has a write type parameter to the write method, and > > Android has a write type property you set before you execute the > > write. > > > > I see that it might be possible to achieve the same result with > > AcquireWrite -> write to socket -> release but that wouldn't be a good > > solution for bluetooth stacks built on top of BlueZ that would like to > > differentiate between the two write types (such as Web-Bluetooth) > > since AcquireWrite can fail, for example if two apps write the value > > at the same time (I guess the lock is exclusive?). It also seems like > > unnecessary overhead to open and close sockets. > > AcquireWrite is to be used when the app needs exclusive access, like > control points such as those commonly used for things like DFU, I > don't think that is your intent here (or is it?) so I guess adding an > option for WriteValue is probably better. Note though that obviously > one cannot use such a flag with things like e.g. offset as that is not > supported which makes the API a little trickier to use but I guess > that ok given that setting flags is optional. No DFU etc. wasn't really the intention here. I guess most (all?) people don't use the offset parameter. The reason the offset parameter exists in the Prepare Write Request is so that it's possible to write a long value in several chunks I guess. Anyway, the solution is to simply disallow offset != 0 and write-without-response=true at the same time. By the way, I see "Reliable Write" is also forced/first choice if the characteristic supports that (even though I think nobody uses it?). The downside of using Reliable Write over a simple Write Request is that it requires more packets/overhead so I was thinking that maybe, to cover all cases, instead of having a bool "write-without-response", it should be a "write-type" option which can take the values "reliable-write", "write-with-response" or "write-without-response" (or use automatic logic like today if the option is not specified). What do you think? /Emil
