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

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Date:   Fri, 17 Sep 2021 14:35:52 +0200
From:   Marcel Holtmann <marcel@...tmann.org>
To:     Jonas Dreßler <verdre@...d.nl>
Cc:     Amitkumar Karwar <amitkarwar@...il.com>,
        Ganapathi Bhat <ganapathi017@...il.com>,
        Xinming Hu <huxinming820@...il.com>,
        Kalle Valo <kvalo@...eaurora.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Johan Hedberg <johan.hedberg@...il.com>,
        Luiz Augusto von Dentz <luiz.dentz@...il.com>,
        Tsuchiya Yuto <kitakar@...il.com>,
        linux-wireless@...r.kernel.org,
        linux-bluetooth <linux-bluetooth@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Maximilian Luz <luzmaximilian@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Pali Rohár <pali@...nel.org>
Subject: Re: [PATCH] Bluetooth: btusb: Lower passive lescan interval on
 Marvell 88W8897

Hi Jonas,

> 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. 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@...d.nl>
> ---
> 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;
> +	}
> +

you can not do it this way. Modifying hci_dev internals from within the driver is not acceptable.

Regards

Marcel