MEGA65: attempt to let hdmi output master control co-efficients map to xemu master volume #272

git cherry-pick from gurcei@47b105e
from @gurcei
thanks!

Author: gurcei

targets/mega65/io_mapper.c

@@ -48,6 +48,8 @@ int    core_age_in_days;
 static const Uint8 fpga_firmware_version[] = { 'X','e','m','u' };
 static const Uint8 cpld_firmware_version[] = { 'N','o','w','!' };
 
+static Uint8 coeff_addr = 0x00;
+static Uint8 mixer_coeffs[0x100];
 
 #define RETURN_ON_IO_READ_NOT_IMPLEMENTED(func, fb) \
 	do { DEBUG("IO: NOT IMPLEMENTED read (emulator lacks feature), %s $%04X fallback to answer $%02X" NL, func, addr, fb); \
@@ -342,6 +344,22 @@ Uint8 io_read ( unsigned int addr )
 	}
 }
 
+void update_mixer_coeffs(Uint8 data)
+{
+	mixer_coeffs[coeff_addr] = data;
+
+	if (coeff_addr == 0x1E || coeff_addr == 0x1F) {	// HDMI-LEFT?
+		int vol = (mixer_coeffs[0x1E] << 8) + mixer_coeffs[0x1F];
+		vol = vol * 100 / 65536;
+		audio_set_stereo_parameters(vol, AUDIO_UNCHANGED_SEPARATION);
+	}
+
+	if (coeff_addr == 0x3E || coeff_addr == 0x3F)	{ // HDMI-RIGHT?
+		int vol = (mixer_coeffs[0x3E] << 8) + mixer_coeffs[0x3F];
+		vol = vol * 100 / 65536;
+		audio_set_stereo_parameters(vol, AUDIO_UNCHANGED_SEPARATION);
+	}
+}
 
 
 /* Please read comments at io_read() above, those apply here too.
@@ -463,6 +481,14 @@ void io_write ( unsigned int addr, Uint8 data )
 				eth65_write_reg(addr, data);
 				return;
 			}
+
+			if (addr == 0xF4) {		// audio mixer co-efficient address
+				coeff_addr = data;
+			}
+			if (addr == 0XF5) {		// audio mixer co-efficient value
+				update_mixer_coeffs(data);
+			}
+
 			static int d6cf_exit_status = 0x42;
 			switch (addr) {
 				case 0x0A:	// write bit 7 is zero -> flush the queue