From d96b9b36a2d88798e3617c6f9ec4ce4f3fcd57a9 Mon Sep 17 00:00:00 2001
From: Sameer Puri <sameer@users.noreply.github.com>
Date: Thu, 2 Apr 2020 13:40:50 -0400
Subject: [PATCH] Set USB SRAM QoS to sensitive latency

---
 cores/arduino/USB/SAMD21_USBDevice.h | 4 ++++
 cores/arduino/USB/USBCore.cpp        | 2 ++
 2 files changed, 6 insertions(+)

diff --git a/cores/arduino/USB/SAMD21_USBDevice.h b/cores/arduino/USB/SAMD21_USBDevice.h
index 2bb71a408..d1ef4f161 100644
--- a/cores/arduino/USB/SAMD21_USBDevice.h
+++ b/cores/arduino/USB/SAMD21_USBDevice.h
@@ -50,6 +50,10 @@ class USBDevice_SAMD21G18x {
 	inline void noRunInStandby() { usb.CTRLA.bit.RUNSTDBY = 0; }
 	inline void wakeupHost()     { usb.CTRLB.bit.UPRSM = 1; }
 
+	// USB QoS
+	inline void setDataSensitiveQoS() { usb.QOSCTRL.bit.DQOS = 2; }
+	inline void setConfigSensitiveQoS() { usb.QOSCTRL.bit.CQOS = 2; }
+
 	// USB speed
 	inline void setFullSpeed()       { usb.CTRLB.bit.SPDCONF = USB_DEVICE_CTRLB_SPDCONF_FS_Val;   }
 	inline void setLowSpeed()        { usb.CTRLB.bit.SPDCONF = USB_DEVICE_CTRLB_SPDCONF_LS_Val;   }
diff --git a/cores/arduino/USB/USBCore.cpp b/cores/arduino/USB/USBCore.cpp
index c49cc077d..bf7888acb 100644
--- a/cores/arduino/USB/USBCore.cpp
+++ b/cores/arduino/USB/USBCore.cpp
@@ -295,6 +295,8 @@ void USBDeviceClass::init()
 	usbd.reset();
 
 	usbd.calibrate();
+	usbd.setDataSensitiveQoS();
+	usbd.setConfigSensitiveQoS();
 	usbd.setUSBDeviceMode();
 	usbd.runInStandby();
 	usbd.setFullSpeed();