USB/usb-mic: Fix buffer handling

- RingBuffer would store a size of zero if you wrote the entire
   the entire buffer in one call.

 - ResetBuffers() should be called before starting the stream,
   otherwise you risk a race where the callback happens before
   the buffer is allocated.

 - Fix incorrect latency being passed into Cubeb.

pcsx2/USB/shared/ringbuffer.cpp

@@ -162,7 +162,7 @@ void RingBuffer::write(size_t bytes)
 
 	// push m_begin forward if m_end overlaps it
 	if ((m_end < m_begin && m_end + bytes > m_begin) ||
-		m_end + bytes > m_begin + m_capacity)
+		m_end + bytes >= m_begin + m_capacity)
 	{
 		m_overrun = true;
 		m_begin = (m_end + bytes) % m_capacity;

pcsx2/USB/usb-mic/audiodev-cubeb.cpp

@@ -150,14 +150,13 @@ namespace usb_mic
 			params.prefs = CUBEB_STREAM_PREF_NONE;
 
 			// Prefer minimum latency, reduces the chance of dropped samples due to the extra buffer.
-			u32 streamLatency;
+			if (cubeb_get_min_latency(mContext, &params, &mStreamLatency) != CUBEB_OK)
-			if (cubeb_get_min_latency(mContext, &params, &streamLatency) != CUBEB_OK)
+				mStreamLatency = (mLatency * mSampleRate) / 1000u;
-				streamLatency = mLatency;
 
 			const bool input = (mAudioDir == AUDIODIR_SOURCE);
 			int res = cubeb_stream_init(mContext, &mStream, fmt::format("{}", (void*)this).c_str(),
 				input ? mDeviceId : nullptr, input ? &params : nullptr, input ? nullptr : mDeviceId,
-				input ? nullptr : &params, (streamLatency * mSampleRate) / 1000u,
+				input ? nullptr : &params, mStreamLatency,
 				&CubebAudioDevice::DataCallback, &CubebStateCallback, this);
 			if (res != CUBEB_OK)
 			{
@@ -165,6 +164,8 @@ namespace usb_mic
 				return false;
 			}
 
+			ResetBuffers();
+
 			res = cubeb_stream_start(mStream);
 			if (res != CUBEB_OK)
 			{
@@ -174,7 +175,6 @@ namespace usb_mic
 				return false;
 			}
 
-			ResetBuffers();
 			return true;
 		}
 
@@ -252,9 +252,8 @@ namespace usb_mic
 
 		void CubebAudioDevice::ResetBuffers()
 		{
-			// TODO: Do we want to make the buffer size adjustable? Currently 100ms max.
 			std::lock_guard<std::mutex> lk(mMutex);
-			const u32 samples = ((mSampleRate * mChannels) * mLatency) / 1000u;
+			const u32 samples = std::max(((mSampleRate * mChannels) * mLatency) / 1000u, mStreamLatency * mChannels);
 			mBuffer.reserve(sizeof(u16) * samples);
 		}
 

pcsx2/USB/usb-mic/audiodev-cubeb.h

@@ -51,6 +51,7 @@ namespace usb_mic
 
 			u32 mSampleRate = 48000;
 			u32 mLatency = 50;
+			u32 mStreamLatency = 0;
 			cubeb* mContext;
 			cubeb_stream* mStream = nullptr;
 			std::string mDeviceName;