video/pc_vga_matrox: add sketchy PLL signatures

* allows BeOS 4 to not hang during card detection bootstrap

src/devices/video/pc_vga_matrox.cpp

@@ -42,6 +42,9 @@ void matrox_vga_device::device_start()
 	save_item(NAME(m_msc));
 	save_item(NAME(m_truecolor_ctrl));
 	save_item(NAME(m_multiplex_ctrl));
+
+	save_item(NAME(m_pll_par));
+	save_pointer(NAME(m_pll_data), 12);
 }
 
 void matrox_vga_device::device_reset()
@@ -360,6 +363,68 @@ void matrox_vga_device::cursor_data_w(offs_t offset, u8 data)
 	}
 }
 
+// map(0x2d, 0x2d) PPD pixel clock PLL
+// map(0x2e, 0x2e) MPD memory clock PLL
+// map(0x2f, 0x2f) LPD loop clock PLL
+u8 matrox_vga_device::pll_data_r(offs_t offset)
+{
+	assert(offset < 3);
+	const std::string source_pll[] = { "Pixel", "MCLK", "Loop" };
+	const std::string value_pll[] = { "N-value", "M-value", "P-value", "Status" };
+
+	const u8 par = (m_pll_par >> (offset * 2)) & 3;
+
+	logerror("PLL %s %s R\n", source_pll[offset], value_pll[par]);
+	u8 res = m_pll_data[(offset << 2) | par];
+	// each of these registers wants specific signatures, beos 4 cares
+	switch(par)
+	{
+		case 0: res |= 0xc0; break;
+		case 1: res &= 0x3f; break;
+		case 2:
+			switch(offset)
+			{
+				case 0:
+					res |= 0x30;
+					// TODO: why beos 4 also expects bit 6 to be on specifically for Pixel clock?
+					// TVP documentation claims to be PCLKEN
+					res |= 0x40;
+					break;
+				case 1:
+					res &= 0x83;
+					res |= 0x30;
+					break;
+				case 2:
+					res &= 0x8b;
+					res |= 0x70;
+					break;
+			}
+
+			break;
+		case 3:
+			// HACK: always lock for now
+			res = 0x40;
+			break;
+	}
+
+	return res;
+}
+
+void matrox_vga_device::pll_data_w(offs_t offset, u8 data)
+{
+	assert(offset < 3);
+	const std::string source_pll[] = { "Pixel", "MCLK", "Loop" };
+	const std::string value_pll[] = { "N-value", "M-value", "P-value", "<Status?>" };
+
+	const u8 par = (m_pll_par >> (offset * 2)) & 3;
+
+	logerror("PLL %s %s %02x W\n", source_pll[offset], value_pll[par], data);
+	// status is read-only
+	if (par == 3)
+		return;
+	m_pll_data[(offset << 2) | par] = data;
+}
+
 void matrox_vga_device::ramdac_indexed_map(address_map &map)
 {
 	// silicon revision
@@ -397,9 +462,19 @@ void matrox_vga_device::ramdac_indexed_map(address_map &map)
 	);
 //  map(0x2a, 0x2a) IOC GPIO control (bits 4-0, 1 = data bit as output, 0 = data bit as input)
 //  map(0x2b, 0x2b) GPIO data (bits 4-0)
-//  map(0x2d, 0x2d) pixel clock PLL
-//  map(0x2e, 0x2e) memory clock PLL
-//  map(0x2f, 0x2f) loop clock PLL
+	// PLL Address Register
+	// bits 5-4 Loop clock PLL, 3-2 MCLK PLL, 1-0 Pixel clock PLL
+	map(0x2c, 0x2c).lrw8(
+		NAME([this] (offs_t offset) {
+			logerror("$2c PLL PAR R\n");
+			return m_pll_par;
+		}),
+		NAME([this] (offs_t offset, u8 data) {
+			logerror("$2c PLL PAR W %02x\n", data);
+			m_pll_par = data;
+		})
+	);
+	map(0x2d, 0x2f).rw(FUNC(matrox_vga_device::pll_data_r), FUNC(matrox_vga_device::pll_data_w));
 //  map(0x30, 0x31) color key overlay
 //  map(0x32, 0x37) color key r/g/b
 //  map(0x38, 0x38) CKC color key control

src/devices/video/pc_vga_matrox.h

@@ -103,6 +103,13 @@ private:
 	u8 m_truecolor_ctrl = 0;
 
 	u8 m_msc = 0;
+
+	// PLL
+	u8 pll_data_r(offs_t offset);
+	void pll_data_w(offs_t offset, u8 data);
+
+	u8 m_pll_par = 0;
+	u8 m_pll_data[12]{};
 };
 
 DECLARE_DEVICE_TYPE(MATROX_VGA, matrox_vga_device)