Re: What is the motivation for conn->power_save

[Marcel Holtmann <marcel@holtmann.org>]

Hi Nick,

> >> If I understand correctly, conn->power_save prevents the host stack
> >> from requesting active mode if it was not the host stack that
> >> requested sniff mode.
> >>
> >> I don't understand the motivation for this. If we have ACL data to
> >> send, then it seems like a good idea for the host stack to explicitly
> >> request active mode, regardless of the reason that we entered sniff
> >> mode.
> >>
> >> We want to enter active mode more aggressively when setting up SCO
> >> connections, to avoid a 5 second delay with certain sniff modes. But
> >> the conn->power_save code is getting in the way and doesn't appear to
> >> be useful in the first place.
> >
> > we have discussed this a few times. And if you lock through the code
> > history then you see that initially we just took devices out of sniff
> > mode if we had to send data. However with HID devices this falls flat on
> > its face. They need to stay in control of sniff mode if they initiated
> > it. Some of them crash and others just drain the battery. With sniff
> > mode you can send small amounts of data even while in sniff and for HID
> > that is sometimes used. So the remote side better not interfere.
> >
> > What we really need is a socket option where we can control this on a
> > per socket basis if we take devices out of sniff mode. And one extra
> > case might be when we try to establish a SCO channel, because then it is
> > clearly not an HID device. However even A2DP has this sort of problems
> > sometimes where the stream setup takes time.
> 
> Makes sense. Thanks for the explanation.

this means you will be working on a patch for this :)

> > Not sure if we have to make SCO setup special. The only reason would be
> > if there is a case where we don't get an AT command before SCO needs to
> > be established.
> 
> If you are in-call, and transfer audio from handset to BT headset,
> then there is SCO setup without any AT command.

Fair enough.

> I think for the SCO setup case we would always want to enter active
> mode. I could modify enter_active_mode() to take a parameter like 'int
> force' that would force us to enter active mode regardless of the
> state of power_save, and use this when setting up SCO. What do you
> think?

Actually when you leave sniff mode, then all bets for the power_save
value are off again. So you better set power_save and just call
enter_active_mode. Something like this:

diff --git a/net/bluetooth/hci_conn.c b/net/bluetooth/hci_conn.c
index a975098..e4591e0 100644
--- a/net/bluetooth/hci_conn.c
+++ b/net/bluetooth/hci_conn.c
@@ -376,6 +376,9 @@ struct hci_conn *hci_connect(struct hci_dev *hdev, int type,
 
        if (acl->state == BT_CONNECTED &&
                        (sco->state == BT_OPEN || sco->state == BT_CLOSED)) {
+               acl->power_save = 1;
+               hci_conn_enter_active_mode(acl);
+
                if (lmp_esco_capable(hdev))
                        hci_setup_sync(sco, acl->handle);
                else

Alternatively we could create hci_conn_force_active_mode() or just
implement a proper per socket sniff/active policy.

Regards

Marcel