Re: Comments on the ConnectDevice API function

[Szymon Janc <szymon.janc@codecoup.pl>]

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.


 
> 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