Re: [PATCH] Bluetooth: btusb: Lower passive lescan interval on Marvell 88W8897

Jonas Dreßler <verdre@xxxxxxx> · Fri, 17 Sep 2021 14:36:50 +0200

On 9/17/21 2:30 PM, Pali Rohár wrote:
On Friday 17 September 2021 14:27:18 Jonas Dreßler wrote:
The Marvell 88W8897 combined wifi and bluetooth card (pcie+usb version)
is used in a lot of Microsoft Surface devices, and all those devices
suffer from very low 2.4GHz wifi connection speeds while bluetooth is
enabled.

Hello! Do you know if this issue is specific only to this one Marvell
88W8897 chip or if this issue affects also other Marvell wifi+bt combo
chips?

Hi! No idea, good question, but I don't own any other devices with 
Marvell chips. Maybe it's even safe to apply it to all Bluetooth 
devices, I guess less scanning load is good for everyone (as long as it 
doesn't impact the scanning results of course, so that would need testing).


The reason for that is that the default passive scanning
interval for Bluetooth Low Energy devices is quite high on Linux
(interval of 60 msec and scan window of 30 msec, see le_scan_interval
and le_scan_window in hci_core.c), and the Marvell chip is known for its
bad bt+wifi coexisting performance.

So decrease that passive scan interval and make the scan window shorter
on this particular device to allow for spending more time transmitting
wifi signals: The new scan interval is 250 msec (0x190 * 0.625 msec) and
the new scan window is 6.25 msec (0xa * 0.625 msec).

This change has a very large impact on the 2.4GHz wifi speeds and gets
it up to performance comparable with the Windows driver, which seems to
apply a similar quirk.

The scan interval and scan window length were tested and found to work
very well with a bunch of Bluetooth Low Energy devices, including the
Surface Pen, a Bluetooth Speaker and two modern Bluetooth headphones.
All devices were discovered immediately after turning them on. Even
lower values were also tested, but these introduced longer delays until
devices get discovered.

Signed-off-by: Jonas Dreßler <verdre@xxxxxxx>
---
  drivers/bluetooth/btusb.c | 15 +++++++++++++++
  1 file changed, 15 insertions(+)

diff --git a/drivers/bluetooth/btusb.c b/drivers/bluetooth/btusb.c
index 60d2fce59a71..05b11179c839 100644
--- a/drivers/bluetooth/btusb.c
+++ b/drivers/bluetooth/btusb.c
@@ -59,6 +59,7 @@ static struct usb_driver btusb_driver;
  #define BTUSB_WIDEBAND_SPEECH	0x400000
  #define BTUSB_VALID_LE_STATES   0x800000
  #define BTUSB_QCA_WCN6855	0x1000000
+#define BTUSB_LOWER_LESCAN_INTERVAL	0x2000000
  #define BTUSB_INTEL_BROKEN_INITIAL_NCMD 0x4000000
  
  static const struct usb_device_id btusb_table[] = {
@@ -356,6 +357,7 @@ static const struct usb_device_id blacklist_table[] = {
  	{ USB_DEVICE(0x1286, 0x2044), .driver_info = BTUSB_MARVELL },
  	{ USB_DEVICE(0x1286, 0x2046), .driver_info = BTUSB_MARVELL },
  	{ USB_DEVICE(0x1286, 0x204e), .driver_info = BTUSB_MARVELL },
+	{ USB_DEVICE(0x1286, 0x204c), .driver_info = BTUSB_LOWER_LESCAN_INTERVAL },
  
  	/* Intel Bluetooth devices */
  	{ USB_DEVICE(0x8087, 0x0025), .driver_info = BTUSB_INTEL_COMBINED },
@@ -3813,6 +3815,19 @@ static int btusb_probe(struct usb_interface *intf,
  	if (id->driver_info & BTUSB_MARVELL)
  		hdev->set_bdaddr = btusb_set_bdaddr_marvell;
  
+	/* The Marvell 88W8897 combined wifi and bluetooth card is known for
+	 * very bad bt+wifi coexisting performance.
+	 *
+	 * Decrease the passive BT Low Energy scan interval a bit
+	 * (0x0190 * 0.625 msec = 250 msec) and make the scan window shorter
+	 * (0x000a * 0,625 msec = 6.25 msec). This allows for significantly
+	 * higher wifi throughput while passively scanning for BT LE devices.
+	 */
+	if (id->driver_info & BTUSB_LOWER_LESCAN_INTERVAL) {
+		hdev->le_scan_interval = 0x0190;
+		hdev->le_scan_window = 0x000a;
+	}
+
  	if (IS_ENABLED(CONFIG_BT_HCIBTUSB_MTK) &&
  	    (id->driver_info & BTUSB_MEDIATEK)) {
  		hdev->setup = btusb_mtk_setup;
--
2.31.1








