ec/ite: Move e-flash signature to 0x80

According to the datasheet, the e-flash signature can be at any 16-byte
aligned address in 0x40-0xF0. Because of an address conflict with SDCC
4.5.0, move the signature to 0x80. This matches the address for Zephyr
RISC-V SoCs, like 82302.

Ref: zephyrproject-rtos/zephyr#36379
Signed-off-by: Tim Crawford <tcrawford@system76.com>

Author: crawfxrd

src/ec/ite/signature.c

@@ -1,17 +1,34 @@
 // SPDX-License-Identifier: GPL-3.0-only
 
+// ITE SoCs have a hardware mechanism to check if e-flash is programmed by a
+// 16-byte signature check at power-on. It must be at a 16-byte aligned address
+// at 0x40-0xF0.
+
 #include <ec/espi.h>
 
+// NOTE: SDCC 4.5.0 causes a conflict with the default address of 0x40, so the
+// signature is moved to 0x80, which matches the address used by RISC-V SoCs
+// in Zephyr.
+#define EFLASH_SIG_ADDR 0x80
+
+// Byte 7: Undocumented
+// - Bit 0: Host bus type (0=eSPI, 1=LPC)
 #if CONFIG_BUS_ESPI
-// eSPI signature (byte 7 = 0xA4)
-static __code const uint8_t __at(0x40) SIGNATURE[16] = {
-    0xA5, 0xA5, 0xA5, 0xA5, 0xA5, 0xA5, 0xA4, 0x95,
-    0x85, 0x12, 0x5A, 0x5A, 0xAA, 0x00, 0x55, 0x55,
-};
-#else // CONFIG_BUS_ESPI
-// LPC signature (byte 7 = 0xA5)
-static __code const uint8_t __at(0x40) SIGNATURE[16] = {
-    0xA5, 0xA5, 0xA5, 0xA5, 0xA5, 0xA5, 0xA5, 0x94,
+#define HBUS 0xA4
+#else
+#define HBUS 0xA5
+#endif
+
+// Byte 8: "flag" (mostly undocumented)
+// - Bit 6: Suspend internal-to-external clock switching (0=Disable)
+// - Bit 4: 32 kHz clock selection (1=Use internal clock generator)
+#define FLAG 0x95
+
+// Byte 14: "Undocumented"; Using the documented value (0xAA) results in
+// firmware not running at power-on. Proprietary firmware programs the byte
+// with values such as 0x7F and 0xFF.
+
+static __code const uint8_t __at(EFLASH_SIG_ADDR) SIGNATURE[16] = {
+    0xA5, 0xA5, 0xA5, 0xA5, 0xA5, 0xA5, HBUS, FLAG,
     0x85, 0x12, 0x5A, 0x5A, 0xAA, 0x00, 0x55, 0x55,
 };
-#endif // CONFIG_BUS_ESPI