Andreas,
Considering that this patch has already made it's way into stable trees,
and the only options are to back it out or to patch it again, I would
obviously prefer to keep it in and patch it again. I think that it
would be fairly quick and painless to just set the default min/max to
6/3200, or add an additional flag as you suggested, which should resolve
any problems in the near term.
Longer term, I think that there are several ways to allow the system
admin to configure the allowable min/max, but it only matters if the
system will check requests against the configured min/max and respond
appropriately, which is what the patch in question does. The best
current way that I'm aware of to control the system-wide connection
interval is through the existing debugfs mechanism, and in my opinion it
didn't work correctly without this patch.
Regarding power consumption, I have not done any power measurement
testing to determine the effect of connection interval on power
consumption, so perhaps that was not the best use case. Here is the
exact real world use case that caused me to write the patch in the first
place:
I was writing a Linux-based test system for an embedded
BLE-communication based product. The product embedded code was written
to attempt to renegotiate the connection interval to a fairly low number
after the initial connection was established. My test system had a
requirement to be able to perform the tests using various pre-defined
connection intervals so that I could gather data throughput and product
performance metrics. Every time my test system attempted to perform a
test a the desired connection interval, the device would immediately
request to renegiate to a lower value. Linux would accept that request
and return a successful response, and I had no way to stop it. Let's
not go down the "Why didn't you just change the embedded code?" / "Why
doesn't Linux reject values outside of the configured bounds?" rabbit
hole... :)
Carey
On 8/19/19 10:08 AM, Andreas Kemnade wrote:
Hi Carey,
On Mon, 19 Aug 2019 07:58:19 -0600
Carey Sonsino <csonsino@xxxxxxxxx> wrote:
This seems like the exact "downside" situation that I described in the
patch writeup.
I would still claim that as a Linux system administrator, I should have
control over my system. If I am operating in a low power environment, I
do not want to allow a remote device to apply a setting which causes me
to use more power without any say in the matter.
In principle I agree here. High connection interval has its downsides,
low connection interval also. Just curios: What are the numbers about
power consumption here? A few mA? I have only compared these values on
peripherals running on low-power SoCs like e.g. the nrf stuff from nordic.
I see around 1 mA difference there with a power consumption besides of that
usually measured in the µA range. Never tested these things on a linux machine.
The point here is that with this patch there is insufficient control
about this. Yes, there is the debugfs interface.
But if you want to provide a driver to a gatt service living on top of
bluetoothd dbus api? Ask people to not use distribution kernels?
What options do you have?
using the monitor interface to sniff the connection handle and then
call hci_le_conn_update() to set things?
The connection min/max interval settings are configuration options that
control how my bluetooth device operates. Why are these down in debugfs
anyway? I think that a much more appropriate fix would be to migrate
some of the BLE configuration options to sysconfdir (some place like
/etc/bluetooth/ble.conf). That would also help in the persistence of
these configuration settings, which is kind of a pain with the debugfs
mechanism that gets wiped out and recreated on bootup.
I think that these things should be part of the dbus api provided
by bluetoothd so that a driver could decide and having defaults outside
of such a dark corner like the debug fs.
A quicker fix would be to simply set the default connection min interval
and connection max interval values to the full range (6, 3200).
Or just maybe a flag allowing such behavior?
*think* that this could be done by simply updating the values in
hci_core.c, the hci_alloc_dev() function:
hdev->le_conn_min_interval = 0x0018;
hdev->le_conn_max_interval = 0x0028;
would become:
hdev->le_conn_min_interval = 0x0006;
hdev->le_conn_max_interval = 0x0c80;
This should allow all devices to negotiate whatever connection interval
they want by default. If I'm running on a device with debugfs (which I
happen to be most of the time), then I can still override these defaults
to control my system.
Please let me know if you would like me to do any further work towards
resolving this issue. I'd be happy to test and submit a patch that
changes the default connection min/max interval values- I could probably
get that done in the next day or two. If you would like me to
investigate migrating configuration settings to /etc then I'd be happy
to do that as well, but it might take a bit more effort and time.
Well, all these things are important, but are new features but there is a
problem:
The kernel patch has gone into the stable trees and from there into distributions,
so how can these new features flow down the same path.
Regards,
Andreas
