OSTC over BLE: take care of credits

Handle credits. Do not just ask for maximum credits all the time as this
will stop the download. Also do not let the credits go back to 0 (while
this might work, this is not tested). Getting back the 0 credits stops
the download, and even when it can be restarted, it is less efficient
(and not needed). Notice also that it takes some time before a grant
request is honoured. During testing I saw reception of up to 25 packets
between request and grant. So a lower bound for the request of
32 packets seems resonable.

One aspect the Telit/Stollmann TIO puzzeled me. Sections 4.1 and 4.2
both talk about credits, but my hyphothesis is that there are two
credits counters in play. One for traffic either way. This commit
only deals with credits granted by Subsurface to the OSTC to send
data. Credits granted by the OSTC to allow Subsurface to send new
commands is NOT part of this commit, and is seemingly not needed
in our scenario. As we only send new commands to the OSTC when
a previous one is finished (per HW's interface spec), the OSTC
does not run out of credits to receive commands.

Signed-off-by: Jan Mulder <jlmulder@xs4all.nl>

1 files changed, 2 insertions, 0 deletions

diff --git a/core/qt-ble.h b/core/qt-ble.h
index 7368b71cd..ad5dfda2b 100644
--- a/core/qt-ble.h
+++ b/core/qt-ble.h
@@ -32,6 +32,7 @@ public slots:
 	void characteristicWritten(const QLowEnergyCharacteristic &c, const QByteArray &value);
 	void writeCompleted(const QLowEnergyDescriptor &d, const QByteArray &value);
 	dc_status_t setupHwTerminalIo(QList<QLowEnergyCharacteristic>);
+	dc_status_t setHwCredit(unsigned int c);
 private:
 	QVector<QLowEnergyService *> services;
 
@@ -39,6 +40,7 @@ private:
 	QList<QByteArray> receivedPackets;
 	bool isCharacteristicWritten;
 	dc_user_device_t *device;
+	unsigned int hw_credit = 0;
 
 	QList<QUuid> hwAllCharacteristics = {
 		"{00000001-0000-1000-8000-008025000000}", // HW_OSTC_BLE_DATA_RX