sfp: do not block the main loop while a module wakes up

handle_sfp() called sfp_module_read() as soon as it saw a module appear, and
that path opened with delay(100). delay() spins on PCON until the tick counter
runs down, so the main loop stood still for 100 ticks - half a second at
SYS_TICK_HZ 200. Nothing else ran in that window: no STP, no LACP, no packet
handling. A diagnostic build measured the sfp phase at up to 107 ticks, the
longest stall of any phase in the loop.

Remember when the module showed up and let the loop continue. The I2C read
happens on a later pass, once the wake-up period has elapsed. Pulling a module
cancels a pending read, and a failed read still restores the detect bit, so the
existing retry-on-the-next-poll behaviour is unchanged.

The comparison is written out rather than using a helper so that this does not
depend on anything outside the current tree. The pending flag is an array
indexed by slot, like the other per-slot state in this file.

Cost on SWTGW218AS, measured against the branch point: BANK3 11173 -> 11264,
CSEG, BANK1 and BANK2 unchanged, plus 4 bytes of xdata.

sfp.c

@@ -20,6 +20,8 @@ extern __code const struct machine machine;
 
 // SFP1 b0 = 1 => module missing, b1 = 1 => LOS;
 // SFP2 b4 = 1 => module missing, b5 = 1 => LOS;
+extern volatile __xdata uint32_t ticks;
+
 __xdata uint8_t sfp_pins_last;
 __xdata char sfp_module_vendor[2][17];
 __xdata char sfp_module_model[2][17];
@@ -28,6 +30,8 @@ __xdata uint8_t sfp_options[2];
 __xdata uint8_t sfp_buf[16];	/* scratch for one I2C transaction, the controller reads at most 16 bytes */
 __xdata uint8_t sfp_speed[2];
 __xdata uint8_t sfp_quirks[2];
+__xdata uint8_t sfp_wake_at[2];
+__xdata uint8_t sfp_wake_pending[2];
 
 
 __code enum sfp_quirk {
@@ -157,7 +161,6 @@ static bool sfp_module_read(uint8_t sfp)
 
 	// Read Reg 11: Encoding, see SFF-8472 and SFF-8024
 	// Read Reg 12: Signalling rate (including overhead) in 100Mbit: 0xd: 1Gbit, 0x67:10Gbit
-	delay(100); // Delay, because some modules need time to wake up
 	if (!sfp_read_block(sfp, 11, 2))
 		return false;
 
@@ -199,6 +202,11 @@ void handle_sfp(void) __banked
 			if (sfp_pins_last & (0x1 << (sfp << 2))) {
 				sfp_pins_last &= ~(0x01 << (sfp << 2));
 				print_string("\n<MODULE INSERTED>  Slot: "); write_char('1' + sfp);
+				sfp_wake_at[sfp] = ticks;
+				sfp_wake_pending[sfp] = 1;
+			} else if (sfp_wake_pending[sfp]
+				   && (uint8_t)((uint8_t)ticks - sfp_wake_at[sfp]) >= SFP_WAKE_TICKS) {
+				sfp_wake_pending[sfp] = 0;
 				if (!sfp_module_read(sfp)) {
 					print_string("SFP: an I2C read failed, retrying on the next poll\n");
 					sfp_pins_last |= 0x01 << (sfp << 2);
@@ -207,6 +215,7 @@ void handle_sfp(void) __banked
 		} else {
 			if (!(sfp_pins_last & (0x1 << (sfp << 2)))) {
 				sfp_pins_last |= 0x01 << (sfp << 2);
+				sfp_wake_pending[sfp] = 0;
 				print_string("\n<MODULE REMOVED>  Slot: "); write_char('1' + sfp); write_char('\n');
 			}
 		}

sfp.h

@@ -1,6 +1,8 @@
 #ifndef SFP_H
 #define SFP_H
 
+#define SFP_WAKE_TICKS	100
+
 extern __xdata uint8_t sfp_pins_last;
 extern __xdata char sfp_module_vendor[2][17];
 extern __xdata char sfp_module_model[2][17];