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From:   Emil Lenngren <emil.lenngren@gmail.com>
Date:   Thu, 31 Jan 2019 19:09:31 +0100
Message-ID: <CAO1O6sdTqHFuDOp9DDasK_aHQ7Ld8YHJtsgcZQLDq+1PebZRwQ@mail.gmail.com>
Subject: Re: Flag for specifying write type to WriteValue in gatt-api.
To:     Luiz Augusto von Dentz <luiz.dentz@gmail.com>
Cc:     Bluez mailing list <linux-bluetooth@vger.kernel.org>
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Hi Luiz,

Den tors 31 jan. 2019 kl 18:55 skrev Luiz Augusto von Dentz
<luiz.dentz@gmail.com>:
>
> Hi Emil,
>
> On Thu, Jan 31, 2019 at 7:46 PM Emil Lenngren <emil.lenngren@gmail.com> wrote:
> >
> > Hi Luiz,
> >
> > Den tors 31 jan. 2019 kl 18:03 skrev Luiz Augusto von Dentz
> > <luiz.dentz@gmail.com>:
> > >
> > > Hi Emil,
> > >
> > > On Thu, Jan 31, 2019 at 6:19 PM Emil Lenngren <emil.lenngren@gmail.com> 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?
>
> I would have named it just type since it is for WriteValue we should
> not need to repeat the write term on the flags, so Id would go for
> type="reliable" (reliable-write), "command" (write-without-response),
> "request" (write-with-response). Also, I assume this would force the
> operation no matter what the flags indicate so people can work around
> if the regular WriteValue don't work for some reason, perhaps the
> service is not really adhering to the spec or it is a vendor service
> just not setting the properties properly.
>
Yes, that sounds great!

/Emil