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[45.11.60.42]) by smtp.gmail.com with ESMTPSA id y16sm2910700ejq.82.2020.05.27.07.10.21 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 27 May 2020 07:10:21 -0700 (PDT) From: Szymon Janc To: Luiz Augusto von Dentz , Martin Woolley Cc: "Linux-bluetooth@vger.kernel.org" Subject: Re: Comments on the ConnectDevice API function Date: Wed, 27 May 2020 16:10:19 +0200 Message-ID: <11527909.O9o76ZdvQC@ix> Organization: CODECOUP In-Reply-To: References: MIME-Version: 1.0 Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="us-ascii" Sender: linux-bluetooth-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-bluetooth@vger.kernel.org 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 > Sent: 22 May 2020 18:49 > To: Martin Woolley > 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 > 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&data=02%7C01%7C%7Cfafcb0f940054867612508d7fe7866fb%7Ce4e0fec5fc6c4dd > 6ae374bdb30e156b9%7C0%7C0%7C637257665400787125&sdata=Iy%2FwWkxs%2FyW3gL2 > 39FLWdoDRGa0apb63WxMhYwRoneM%3D&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