Re: Comments on the ConnectDevice API function

[Luiz Augusto von Dentz <luiz.dentz@gmail.com>]

Hi Szymon,

On Wed, May 27, 2020 at 7:10 AM Szymon Janc <szymon.janc@codecoup.pl> wrote:
>
> Hi Martin,
>
> On Wednesday, 27 May 2020 15:41:12 CEST Martin Woolley wrote:
> > Hi Luiz
> >
> > thanks for your response. Much appreciated. I must confess this is the first
> > time I've used the BlueZ APIs directly (via D-Bus).
>
> > I'm all for hiding implementation details where possible in APIs and making
> > the API itself take care of conditional aspects if possible, but these were
> > just my $0.02, no more than that. I wasn't aware of the background or the
> > philosophy, so thanks for that insight as well.
>
> > Having the API take care of the AlreadyExists error by providing a
> > connection to the already discovered device, transparently does seem a nice
> > touch for the application developer but certainly not essential.
>
> This API was added only for qualification purposes (there are some GAP tests
> specified in a way that upper tester is not doing discovery) and it shouldn't
> be used for 'normal' usage.

Btw, we could perhaps have a different flag from such API e.g.
qualification-only or perhaps add an entry on main.conf would be
better in this regard since it may mean that we never really adopt
them as stable APIs.

>
>
> > All the best
> >
> > Martin
> >
> > -----Original Message-----
> > From: Luiz Augusto von Dentz <luiz.dentz@gmail.com>
> > Sent: 22 May 2020 18:49
> > To: Martin Woolley <mwoolley@bluetooth.com>
> > Cc: Linux-bluetooth@vger.kernel.org
> > Subject: Re: Comments on the ConnectDevice API function
> >
> > Hi Martin,
> >
> > On Fri, May 22, 2020 at 1:25 AM Martin Woolley <mwoolley@bluetooth.com>
> > wrote:
> > >
> > >
> > > Hello
> > >
> > >
> > >
> > > I've recently been working with BlueZ via D-Bus and have a situation which
> > > requires me to be able to connect to a device whose Bluetooth device
> > > address is known, but without first scanning. This is a link layer state
> > > transition with the specification allows.
> >
> > >
> > >
> > > BlueZ currently supports this via an API adapter function called
> > > ConnectDevice, whose status is currently "experimental". From my
> > > experience of using this function, it seems to behave like this:
> >
> > >
> > >
> > > If the BlueZ instance has not scanned yet, so that the target device is
> > > not known to it, the ConnectDevice call results in scanning taking place
> > > and then if the target device is found, it is connected to. Success!
> >
> > >
> > >
> > > But if scanning has previously been performed, regardless of the state of
> > > the actual device (e.g. advertising and ready to accept connections), an
> > > exception is thrown with a message whose text value is "Already Exists".
> >
> > >
> > >
> > > I was wondering if I could influence the design of the API before the
> > > ConnectDevice experimental status is removed?
> >
> > >
> > >
> > > I would like to suggest that there should be no need for a special API to
> > > connect directly to a device without first scanning. Why burden the
> > > application developer needing to call it just in case this condition
> > > applies, catching the BlueZ exception ("Already Exists") and responding
> > > by then calling the normal Connect API?
> >
> > I guess the intention was to have the application use the intended API for
> > devices already present _before_ calling ConnectDevice, so before entering
> > the address manually the application would enumerate the existing devices
> > and figure out if that was already present.
>
> >
> > > An alternative would be to accommodate this special case (not scanned
> > > before) in the implementation of the standard device Connect(bdaddr)
> > > function or if that makes no sense because Device objects must correspond
> > > to previously discovered, physical devices, then at least the adapter
> > > ConnectDevice function could take care of the two possible paths and
> > > simplify matters for the application developer.
> >
> > I guess you probably know this but just in case someone look at the archives
> > it is better that we make some things clearer, while the core spec allows
> > connecting without scanning D-Bus are intend to be a higher level API thus
> > why ConnectDevice was not really necessary for a long time and we just
> > introduced it for qualification purpose or when there are multiple adapter
> > where one acts as scanner. Also ever since the introduction of privacy
> > (random addresses) APIs that takes addresses becomes rather complicated to
> > be used directly, and there exists ways to scan for a specific address with
> > pattern filtering:
> > https://nam12.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgit.kerne
> > l.org%2Fpub%2Fscm%2Fbluetooth%2Fbluez.git%2Ftree%2Fdoc%2Fadapter-api.txt%23n
> > 122&amp;data=02%7C01%7C%7Cfafcb0f940054867612508d7fe7866fb%7Ce4e0fec5fc6c4dd
> > 6ae374bdb30e156b9%7C0%7C0%7C637257665400787125&amp;sdata=Iy%2FwWkxs%2FyW3gL2
> > 39FLWdoDRGa0apb63WxMhYwRoneM%3D&amp;reserved=0
> > That said I don't oppose to remove Already Exists error, but we should be
> > very clear that the use of such API should only be recommended with users
> > input and does not substitute the likes of Device.Connect.
>
> > --
> > Luiz Augusto von Dentz
>
>
> --
> pozdrawiam
> Szymon Janc
>
>


-- 
Luiz Augusto von Dentz