Fix F7 ADC #854 (#855)

* invalidate before reading ADC

* fix degree signs

* comment

* remove unecessary alignments, add comments

firmware/hw_layer/AdcConfiguration.h

@@ -22,8 +22,14 @@ public:
 	int conversionCount;
 	int errorsCount;
 	int getAdcValueByIndex(int internalIndex) const;
+	void invalidateSamplesCache();
 
-	adcsample_t samples[ADC_MAX_CHANNELS_COUNT * MAX_ADC_GRP_BUF_DEPTH];
+	// This must be aligned on a 32-byte boundary, and be a multiple of 32 bytes long.
+	// When we invalidate the cache line(s) for ADC samples, we don't want to nuke any
+	// adjacent data
+	__ALIGNED(32) adcsample_t samples[ADC_MAX_CHANNELS_COUNT * MAX_ADC_GRP_BUF_DEPTH];
+	// Assert multiple of 32 bytes long so we don't stomp on the data after the buffer
+	static_assert(sizeof(samples) % 32 == 0);
 
 	int getAdcValueByHwChannel(int hwChannel) const;
 

firmware/hw_layer/adc_inputs.cpp

@@ -256,10 +256,10 @@ static void pwmpcb_fast(PWMDriver *pwmp) {
 float getMCUInternalTemperature(void) {
 #if defined(ADC_CHANNEL_SENSOR)
 	float TemperatureValue = adcToVolts(slowAdc.getAdcValueByHwChannel(ADC_CHANNEL_SENSOR));
-	TemperatureValue -= 0.760; // Subtract the reference voltage at 25°C
+	TemperatureValue -= 0.760; // Subtract the reference voltage at 25 deg C
 	TemperatureValue /= .0025; // Divide by slope 2.5mV
 
-	TemperatureValue += 25.0; // Add the 25°C
+	TemperatureValue += 25.0; // Add the 25 deg C
 	return TemperatureValue;
 #else
 	return 0;
@@ -342,6 +342,17 @@ int AdcDevice::getAdcValueByIndex(int internalIndex) const {
 	return values.adc_data[internalIndex];
 }
 
+void AdcDevice::invalidateSamplesCache() {
+#if PROJECT_CPU == ARCH_STM32F7
+	// The STM32F7xx has a data cache
+	// DMA operations DO NOT invalidate cache lines, since the ARM m7 doesn't have 
+	// anything like a CCI that maintains coherency across multiple bus masters.
+	// As a result, we have to manually invalidate the D-cache any time we (the CPU)
+	// would like to read something that somebody else wrote (ADC via DMA, in this case)
+	SCB_InvalidateDCache_by_Addr(reinterpret_cast<uint32_t*>(samples), sizeof(samples));
+#endif
+}
+
 void AdcDevice::init(void) {
 	hwConfig->num_channels = size();
 	hwConfig->sqr1 += ADC_SQR1_NUM_CH(size());
@@ -432,6 +443,9 @@ int getSlowAdcCounter() {
 static void adc_callback_slow(ADCDriver *adcp, adcsample_t *buffer, size_t n) {
 	(void) buffer;
 	(void) n;
+
+	slowAdc.invalidateSamplesCache();
+
 	efiAssertVoid(CUSTOM_ERR_6671, getCurrentRemainingStack() > 128, "lowstck#9c");
 	/* Note, only in the ADC_COMPLETE state because the ADC driver fires
 	 * an intermediate callback when the buffer is half full. */

firmware/hw_layer/hardware.cpp

@@ -196,6 +196,7 @@ extern AdcDevice fastAdc;
  * This method is not in the adc* lower-level file because it is more business logic then hardware.
  */
 void adc_callback_fast(ADCDriver *adcp, adcsample_t *buffer, size_t n) {
+	fastAdc.invalidateSamplesCache();
 
 	(void) buffer;
 	(void) n;