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From:   Kenny Bian <kennybian@gmail.com>
Date:   Thu, 15 Apr 2021 17:20:41 -0700
Message-ID: <CAC9s0NZtc_8fXQdZn_1USxixSzbaj2d1OcOn=EoJGFHhX75LjQ@mail.gmail.com>
Subject: Re: Disabled bluetooth cache. But the app still getting wrong data?
To:     Luiz Augusto von Dentz <luiz.dentz@gmail.com>
Cc:     Jamie Mccrae <Jamie.Mccrae@lairdconnect.com>,
        "linux-bluetooth@vger.kernel.org" <linux-bluetooth@vger.kernel.org>
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Hi Luiz,

Thank you so much. I appreciate it.

That was my bad. I just checked. Our system is Ubuntu 18.04. The BlueZ
version is "5.48-0ubuntu3.4" which is different from the versions in
http://www.bluez.org/. I'm trying to figure out if this version
includes the version 5.50. Do you know the corresponding BlueZ version
for "5.48-0ubuntu3.4"?

Thanks!
Kenny

On Thu, Apr 15, 2021 at 1:31 PM Luiz Augusto von Dentz
<luiz.dentz@gmail.com> wrote:
>
> Hi Kenny,
>
> On Wed, Apr 14, 2021 at 10:44 PM Kenny Bian <kennybian@gmail.com> wrote:
> >
> > Hi Luiz,
> >
> > There is a UI example of nRF Connect which shows the "Service Changed
> > Indication" is under "Generic
> > Attribute"(http://forum.espruino.com/conversations/357737/). We're
> > using BlueZ 5.48. I tried to add it by modifying example-gatt-server:
>
>
> You should have mentioned that you are using 5.48, you need the following=
 patch:
>
> commit a0b886e26c83ad4dfbf0b2b2e024c8ce9ead9a7d
> Author: Szymon Janc <szymon.janc@codecoup.pl>
> Date:   Wed Mar 28 12:10:55 2018 +0200
>
>     gatt: Add support for storing Service Changed CCC value
>
>     This adds support for storing CCC value of Service Changed
>     characteristic. Once bluetoothd is restart stored values are read
>     and any device subscribed to indications will receive Service Changed
>     indication with 0x00010-0xffff value. This is to invalidate any
>     non-core services since there is no way to verify if applications
>     will register their services in same order (or at all).
>
>     This fix accessing invalid handles by stacks that rely only on Servic=
e
>     Changed indication for rediscovery ie. Apple iOS.
>
> Or upgrade to a BlueZ release that has the above patch, which should be 5=
.50:
>
> +ver 5.50:
> +       Fix issue with GATT and reading long values.
> +       Fix issue with GATT and reading multiple includes.
> +       Fix issue with GATT and service changes when offline.
> +       Fix issue with handling secondary service discovery.
> +       Fix issue with handling persistency of CCC values.
>
> >
> > class ServiceChangedChrc(Characteristic):
> >     SCI_UUID =3D '00002a05-0000-1000-8000-00805f9b34fb'
> >
> >     def __init__(self, bus, index, service):
> >         Characteristic.__init__(
> >                 self, bus, index,
> >                 self.SCI_UUID,
> >                 ['indicate'],
> >                 service)
> >
> >     def IndicateValue(self, options):
> >         # This function doesn't exist. Just list it here.
> >         pass
> >
> >
> > class ServiceChangedService(Service):
> >     MY_UUID =3D '00001801-0000-1000-8000-00805f9b34fb'
> >
> >     def __init__(self, bus, index):
> >         Service.__init__(self, bus, index, self.MY_UUID, True)
> >         self.add_characteristic(ServiceChangedChrc(bus, 0, self))
> >
> >
> > class Application(dbus.service.Object):
> >     """
> >     org.bluez.GattApplication1 interface implementation
> >     """
> >     def __init__(self, bus):
> >         self.path =3D '/'
> >         self.services =3D []
> >         dbus.service.Object.__init__(self, bus, self.path)
> >         self.add_service(HeartRateService(bus, 0))
> >         self.add_service(BatteryService(bus, 1))
> >         self.add_service(TestService(bus, 2))
> >         self.add_service(ServiceChangedService(bus, 3))
> >
> >
> > When I run it, there is "Failed to register application:
> > org.bluez.Error.Failed: Failed to create entry in database". That is
> > in the function client_ready_cb() in gatt-database.c in BlueZ code. Do
> > you know how to fix it?
>
> You don't need to register Service Changed, the daemon already takes
> care of registering it and there could only be one Service Changed in
> the database.
>
> > Thanks!
> > Kenny
> >
> > On Tue, Apr 13, 2021 at 9:46 PM Kenny Bian <kennybian@gmail.com> wrote:
> > >
> > > Hi Jamie, Luiz,
> > >
> > > Thanks for your info.
> > >
> > > Luiz, to answer your questions:
> > > BlueZ doesn't seem to emit "Service Changed Indication". Or it does
> > > emit it, but we don't know how to verify it.
> > > Yes, BlueZ acts as a GATT server which runs on Linux.
> > >
> > > So my questions:
> > > How can I tell if "Service Changed Indication" is working? I need to
> > > show it to our app developers that the GATT server does send the
> > > "Service Changed Indication" when there is a service change.
> > > By disabling the GATT's cache in "/etc/bluetooth/main.conf", the
> > > "Service Changed Indication" is disabled?
> > > Can "Service Changed Indication" be enabled in a conf file? Or should
> > > it be done in the Python code? Could you please help find out that
> > > info?
> > > Our GATT server is based on the Python code
> > > examples(https://git.kernel.org/pub/scm/bluetooth/bluez.git/tree/test=
).
> > > I don't know how to enable "Service Changed Indication" or "Robust
> > > Caching" in Python code on the GATT server side. There is no document
> > > or code samples about that. Please let me know if there is any
> > > document or Python code example.
> > >
> > > Thanks!
> > > Kenny
> > >
> > > On Tue, Apr 13, 2021 at 2:57 PM Luiz Augusto von Dentz
> > > <luiz.dentz@gmail.com> wrote:
> > > >
> > > > Hi Jamie, Brian,
> > > >
> > > > On Tue, Apr 13, 2021 at 2:03 AM Jamie Mccrae
> > > > <Jamie.Mccrae@lairdconnect.com> wrote:
> > > > >
> > > > > Hi Kenny,
> > > > > Why not just add the service changed indication as you refer to b=
elow? It was purposely designed for this specific purpose, you're trying to=
 work around an issue created because you don't want to use the feature tha=
t prevents this issue. Any workaround is just that, a workaround, and might=
 not work as intended.
> > > >
> > > > Yep, and while at it implement the so called Robust Caching feature=
 so
> > > > we can detect if anything has changed by reading the DB Hash.
> > > >
> > > > > Thanks,
> > > > > Jamie
> > > > >
> > > > > -----Original Message-----
> > > > > From: Kenny Bian <kennybian@gmail.com>
> > > > > Sent: 13 April 2021 06:59
> > > > > To: linux-bluetooth@vger.kernel.org
> > > > > Subject: Disabled bluetooth cache. But the app still getting wron=
g data?
> > > > >
> > > > > EXTERNAL EMAIL: Be careful with attachments and links.
> > > > >
> > > > > Previously we had an issue: if there is a change of characteristi=
cs in the new build of our firmware, then the app will get the wrong data.
> > > > > By saying changed characteristics, it can be an added or removed =
characteristic, or adding notification to an existing characteristic.
> > > > > In order to keep the pairing information, the "/var/lib/bluetooth=
"
> > > > > folder is copied over to the new build's partition. We realized t=
hat there is no "service changed indication". The app can't handle the chan=
ged services. So we disabled the bluetooth cache by set this in
> > > > > "/etc/bluetooth/main.conf":
> > > > > [GATT]
> > > > > Cache =3D no
> > > >
> > > > When you say the app can't handle changed service do you mean BlueZ
> > > > doesn't emit changes to the attributes (via Service Changed) or is =
it
> > > > really the application not being able to handle the changes?
> > > >
> > > > > But recently, we saw the problem again even if the bluetooth cach=
e is
> > > > > disabled: in the build number 101, a characteristic is removed. B=
ut when we upgrade the build from 100 to 101, the app gets the wrong data. =
We looked at the log. When the app tries to read temperature by using the t=
emperature UUID, somehow the bluetooth service we created received the requ=
est to read the "device name"(device name UUID). So the "device name" is re=
turned to the app as the temperature. This looks like the same behavior as =
the bluetooth cache is not disabled. I looked at the "/var/lib/bluetooth/[B=
T_MAC]/cache" folder. There is no "[Attributes]" section in the files in th=
e folder. That means the disabled cache seems working.
> > > >
> > > > So BlueZ is acting as the server, right? The Cache only applies to =
the
> > > > client portion, there is no such thing as disabling the remote cach=
e.
> > > > I don't see any incoming Read By Group Request from the remote so i=
t
> > > > is very likely that it has cached the values, there is no Read By T=
ype
> > > > for the DB Hash either which is quite surprising to me since that i=
s
> > > > required for stacks supporting Robust Caching which I believe is th=
e
> > > > case of iOS.
> > > >
> > > > Anyway, I would check that the following lines are being triggered:
> > > >
> > > > When starting:
> > > >
> > > > https://git.kernel.org/pub/scm/bluetooth/bluez.git/tree/src/gatt-da=
tabase.c#n3798
> > > >
> > > > On connect:
> > > >
> > > > https://git.kernel.org/pub/scm/bluetooth/bluez.git/tree/src/gatt-da=
tabase.c#n3741
> > > >
> > > > If those lines are not being triggered it is likely a client proble=
m
> > > > which for some reason had not subscribed to received service change=
s
> > > > for some reason, if it doesn't subscribe to service changes then it
> > > > shall not cache any attribute and attempt to rediscover on every
> > > > connection.
> > > >
> > > > > The only way to fix this issue is to force exit the mobile app on=
 the phone and "Forget This Device" in iOS or "Unpair" in Android.
> > > > >
> > > > > I looked at the btmon(see attached). For the working btmon log, t=
here is "Attribute group list: XX entries" under "ACL Data TX". But there i=
s no "Attribute group list: XX entries" under "ACL Data TX" in the attached=
 problematic btmon log.
> > > > >
> > > > > Questions:
> > > > > 1. How is it possible that this still happens even if the bluetoo=
th cache is disabled?
> > > > > 2. Is this the problem on the Linux side which runs the GATT serv=
er or on the mobile side?
> > > > > 3. Is there anything else we should look into?
> > > > >
> > > > > We're going to release our product soon. This is a critical issue=
 for us. Please help if you have any suggestions.
> > > > >
> > > > > Thanks!
> > > > > THIS MESSAGE, ANY ATTACHMENT(S), AND THE INFORMATION CONTAINED HE=
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> > > >
> > > >
> > > >
> > > > --
> > > > Luiz Augusto von Dentz
>
>
>
> --
> Luiz Augusto von Dentz