Hi all, I'm using bluez 5.3's dbus API (also running bluetoothd with
the --experimental option) and wanted to check is it possible to
connect to a BLE device and discover its GATT services without pairing
to the device?
I'm using the dbus APIs and see that the device Pair() function docs
call out that it will perform GATT service discovery. This matches
what I see in practice--when I turn on an adapter's discovery with
StartDiscovery() I start to see nearby BLE devices added to the bluez
dbus root (with the org.bluez.Device1 interface). I can connect to
those devices, but only when I call the device Pair() function do I
see all the GATT services, characteristics, descriptors, etc. added to
the bluez dbus root (using the org.bluez.GattService1,
org.bluez.GattCharacteristic1, etc. interfaces).
However using just the dbus interface is it possible to trigger the
discovery of all those GATT service, characteristic, etc. objects
without pairing with a device? I.e. is there an explicit GATT service
discovery command for a connected device?
Let me know if the question isn't clear, and thanks for any help foks
can provide.
-Tony
On Fri, Apr 17, 2015 at 10:23 PM, Arman Uguray <[email protected]> wrote:
> Hi Tony,
>
>> On Fri, Apr 17, 2015 at 9:00 PM, Tony DiCola <[email protected]> wrote:
>> On Fri, Apr 17, 2015 at 12:04 PM, Tony DiCola <[email protected]> wrote:
>>> Hi all, I'm using bluez 5.3's dbus API (also running bluetoothd with
>>> the --experimental option) and wanted to check is it possible to
>>> connect to a BLE device and discover its GATT services without pairing
>>> to the device?
>>>
>>> I'm using the dbus APIs and see that the device Pair() function docs
>>> call out that it will perform GATT service discovery. This matches
>>> what I see in practice--when I turn on an adapter's discovery with
>>> StartDiscovery() I start to see nearby BLE devices added to the bluez
>>> dbus root (with the org.bluez.Device1 interface). I can connect to
>>> those devices, but only when I call the device Pair() function do I
>>> see all the GATT services, characteristics, descriptors, etc. added to
>>> the bluez dbus root (using the org.bluez.GattService1,
>>> org.bluez.GattCharacteristic1, etc. interfaces).
>>>
>>> However using just the dbus interface is it possible to trigger the
>>> discovery of all those GATT service, characteristic, etc. objects
>>> without pairing with a device? I.e. is there an explicit GATT service
>>> discovery command for a connected device?
>>>
>>> Let me know if the question isn't clear, and thanks for any help foks
>>> can provide.
>>>
>>> -Tony
>>
>> Just to follow up and close the loop, I went back and realized I
>> actually am getting all the GATT services, characteristics, etc.
>> without pairing. I must have had an issue with what I was trying
>> earlier and see now after just a device connect call the GATT
>> information shows up a short while after connecting.
>>
>> One quick question, is there any easy way to check if a device has
>> finished its service discovery? Right now I can watch the bluez
>> ObjectManager's InterfacesAdded signal to see exactly when GATT
>> objects are created, or even just poll the bluez dbus object hiearchy
>> for their existance. Just curious if there's a more explicit way like
>> a signal or changed property to know when a device's service discovery
>> completes. Thanks!
>>
>
> It's pretty straightforward to know when all the attributes of an
> individual service have been discovered; basically, once all the
> GattCharacteristic1 objects under a service have been exported, the
> "Characteristics" property of the service (GattService1) will be
> updated and you'll receive a PropertiesChanged signal for it.
>
> Finding out that the complete list of services have been discovered
> and the objects have been exported is a bit trickier. Usually, the
> "UUIDs" property on org.bluez.Device1 will be populated before a
> connection with services obtained via advertising/inquiry. Upon
> connection and when GATT/SDP service discovery has completed (and
> after the service objects have been exported), the UUIDs property will
> update with the full list of services. Here you could do something
> like tracking InterfacesAdded signals and comparing the newly added
> services to the initial UUID list.
>
> I would be open to adding a signal/property to notify applications
> when service discovery completes though this hasn't really been an
> issue so far. Usually applications can be built in a very event based
> manner and update their state on a per-service basis as service
> hierarchies get added and removed.
>
> I hope this helps!
> Arman
Thanks for the advice! Yeah I ended up going with a somewhat simple
approach to look for the existence of certain services and
characteristics I care about beneath the connected device's dbus
hierarchy. I could get fancier and construct the tree of services,
child characteristics, child descriptors, etc. and look for a specific
hierarchy to consider service discovery complete, but I'd rather keep
it simple for now and just check for the presence of certain key
characteristics & services.
A specific event or property to signal the completion of service
discovery could be useful to think about for the future IMHO. Since
all the services, characteristics, etc. are discovered asynchronously
I worry there could be subtle race conditions with a simple approach
people might take that just looks for the existence of a service.
For example if I connect to a BLE UART device and wait for the UART
service to be discovered and then start interacting with the service's
RX & TX characteristics, could there be a small window between being
notified of the service existing but the characteristics not yet being
discovered (and attempts to use them failing)? If so that's why I was
thinking a single event / property that tells you everything is done
would be a little easier for most application code to deal with. That
way you don't need to try to reconstruct your expected service &
characteristic tree from all the async events--just look for the
service discovery complete event and move on from there.
For now though I think I'm good to just look for the few services and
characteristics I care about being available before using them.
Thanks again for the help!
-Tony
Hi Tony,
> On Fri, Apr 17, 2015 at 9:00 PM, Tony DiCola <[email protected]> wrote:
> On Fri, Apr 17, 2015 at 12:04 PM, Tony DiCola <[email protected]> wrote:
>> Hi all, I'm using bluez 5.3's dbus API (also running bluetoothd with
>> the --experimental option) and wanted to check is it possible to
>> connect to a BLE device and discover its GATT services without pairing
>> to the device?
>>
>> I'm using the dbus APIs and see that the device Pair() function docs
>> call out that it will perform GATT service discovery. This matches
>> what I see in practice--when I turn on an adapter's discovery with
>> StartDiscovery() I start to see nearby BLE devices added to the bluez
>> dbus root (with the org.bluez.Device1 interface). I can connect to
>> those devices, but only when I call the device Pair() function do I
>> see all the GATT services, characteristics, descriptors, etc. added to
>> the bluez dbus root (using the org.bluez.GattService1,
>> org.bluez.GattCharacteristic1, etc. interfaces).
>>
>> However using just the dbus interface is it possible to trigger the
>> discovery of all those GATT service, characteristic, etc. objects
>> without pairing with a device? I.e. is there an explicit GATT service
>> discovery command for a connected device?
>>
>> Let me know if the question isn't clear, and thanks for any help foks
>> can provide.
>>
>> -Tony
>
> Just to follow up and close the loop, I went back and realized I
> actually am getting all the GATT services, characteristics, etc.
> without pairing. I must have had an issue with what I was trying
> earlier and see now after just a device connect call the GATT
> information shows up a short while after connecting.
>
> One quick question, is there any easy way to check if a device has
> finished its service discovery? Right now I can watch the bluez
> ObjectManager's InterfacesAdded signal to see exactly when GATT
> objects are created, or even just poll the bluez dbus object hiearchy
> for their existance. Just curious if there's a more explicit way like
> a signal or changed property to know when a device's service discovery
> completes. Thanks!
>
It's pretty straightforward to know when all the attributes of an
individual service have been discovered; basically, once all the
GattCharacteristic1 objects under a service have been exported, the
"Characteristics" property of the service (GattService1) will be
updated and you'll receive a PropertiesChanged signal for it.
Finding out that the complete list of services have been discovered
and the objects have been exported is a bit trickier. Usually, the
"UUIDs" property on org.bluez.Device1 will be populated before a
connection with services obtained via advertising/inquiry. Upon
connection and when GATT/SDP service discovery has completed (and
after the service objects have been exported), the UUIDs property will
update with the full list of services. Here you could do something
like tracking InterfacesAdded signals and comparing the newly added
services to the initial UUID list.
I would be open to adding a signal/property to notify applications
when service discovery completes though this hasn't really been an
issue so far. Usually applications can be built in a very event based
manner and update their state on a per-service basis as service
hierarchies get added and removed.
I hope this helps!
Arman
On Fri, Apr 17, 2015 at 12:04 PM, Tony DiCola <[email protected]> wrote:
> Hi all, I'm using bluez 5.3's dbus API (also running bluetoothd with
> the --experimental option) and wanted to check is it possible to
> connect to a BLE device and discover its GATT services without pairing
> to the device?
>
> I'm using the dbus APIs and see that the device Pair() function docs
> call out that it will perform GATT service discovery. This matches
> what I see in practice--when I turn on an adapter's discovery with
> StartDiscovery() I start to see nearby BLE devices added to the bluez
> dbus root (with the org.bluez.Device1 interface). I can connect to
> those devices, but only when I call the device Pair() function do I
> see all the GATT services, characteristics, descriptors, etc. added to
> the bluez dbus root (using the org.bluez.GattService1,
> org.bluez.GattCharacteristic1, etc. interfaces).
>
> However using just the dbus interface is it possible to trigger the
> discovery of all those GATT service, characteristic, etc. objects
> without pairing with a device? I.e. is there an explicit GATT service
> discovery command for a connected device?
>
> Let me know if the question isn't clear, and thanks for any help foks
> can provide.
>
> -Tony
Just to follow up and close the loop, I went back and realized I
actually am getting all the GATT services, characteristics, etc.
without pairing. I must have had an issue with what I was trying
earlier and see now after just a device connect call the GATT
information shows up a short while after connecting.
One quick question, is there any easy way to check if a device has
finished its service discovery? Right now I can watch the bluez
ObjectManager's InterfacesAdded signal to see exactly when GATT
objects are created, or even just poll the bluez dbus object hiearchy
for their existance. Just curious if there's a more explicit way like
a signal or changed property to know when a device's service discovery
completes. Thanks!
-Tony