From 5ecefde9911ffea913f184c596a59690f3e12e43 Mon Sep 17 00:00:00 2001
From: Ayke van Laethem <aykevanlaethem@gmail.com>
Date: Sun, 30 May 2021 18:06:47 +0200
Subject: [PATCH] bmi160: avoid heap allocations

The nrf52 series chips use DMA for writing and reading SPI data, which
means that the transmit and receive buffer slices escape. This can
easily be solved by pre-allocating the buffer in the BMI160 driver and
reusing this buffer every time.
---
 bmi160/bmi160.go | 29 ++++++++++++++++++++++++-----
 1 file changed, 24 insertions(+), 5 deletions(-)

diff --git a/bmi160/bmi160.go b/bmi160/bmi160.go
index b5bad10..cd3e882 100644
--- a/bmi160/bmi160.go
+++ b/bmi160/bmi160.go
@@ -13,6 +13,8 @@ type DeviceSPI struct {
 	// Chip select pin
 	CSB machine.Pin
 
+	buf [7]byte
+
 	// SPI bus (requires chip select to be usable).
 	Bus drivers.SPI
 }
@@ -80,7 +82,10 @@ func (d *DeviceSPI) Reset() error {
 
 // ReadTemperature returns the temperature in celsius milli degrees (°C/1000).
 func (d *DeviceSPI) ReadTemperature() (temperature int32, err error) {
-	data := []byte{0x80 | reg_TEMPERATURE_0, 0, 0}
+	data := d.buf[:3]
+	data[0] = 0x80 | reg_TEMPERATURE_0
+	data[1] = 0
+	data[2] = 0
 	d.CSB.Low()
 	err = d.Bus.Tx(data, data)
 	d.CSB.High()
@@ -113,7 +118,11 @@ func (d *DeviceSPI) ReadTemperature() (temperature int32, err error) {
 // and the sensor is not moving the returned value will be around 1000000 or
 // -1000000.
 func (d *DeviceSPI) ReadAcceleration() (x int32, y int32, z int32, err error) {
-	data := []byte{0x80 | reg_ACC_XL, 0, 0, 0, 0, 0, 0}
+	data := d.buf[:7]
+	data[0] = 0x80 | reg_ACC_XL
+	for i := 1; i < len(data); i++ {
+		data[i] = 0
+	}
 	d.CSB.Low()
 	err = d.Bus.Tx(data, data)
 	d.CSB.High()
@@ -139,7 +148,11 @@ func (d *DeviceSPI) ReadAcceleration() (x int32, y int32, z int32, err error) {
 // rotation along one axis and while doing so integrate all values over time,
 // you would get a value close to 360000000.
 func (d *DeviceSPI) ReadRotation() (x int32, y int32, z int32, err error) {
-	data := []byte{0x80 | reg_GYR_XL, 0, 0, 0, 0, 0, 0}
+	data := d.buf[:7]
+	data[0] = 0x80 | reg_GYR_XL
+	for i := 1; i < len(data); i++ {
+		data[i] = 0
+	}
 	d.CSB.Low()
 	err = d.Bus.Tx(data, data)
 	d.CSB.High()
@@ -185,7 +198,9 @@ func (d *DeviceSPI) readRegister(address uint8) uint8 {
 	// I don't know why but it appears necessary to sleep for a bit here.
 	time.Sleep(time.Millisecond)
 
-	data := []byte{0x80 | address, 0}
+	data := d.buf[:2]
+	data[0] = 0x80 | address
+	data[1] = 0
 	d.CSB.Low()
 	d.Bus.Tx(data, data)
 	d.CSB.High()
@@ -198,7 +213,11 @@ func (d *DeviceSPI) writeRegister(address, data uint8) {
 	// I don't know why but it appears necessary to sleep for a bit here.
 	time.Sleep(time.Millisecond)
 
+	buf := d.buf[:2]
+	buf[0] = address
+	buf[1] = data
+
 	d.CSB.Low()
-	d.Bus.Tx([]byte{address, data}, []byte{0, 0})
+	d.Bus.Tx(buf, buf)
 	d.CSB.High()
 }