From: Alan Cox <alan@linux.intel.com>
Date: Mon, 26 Nov 2018 12:13:22 +0000 (+0000)
Subject: zxdiv,tty: optimize interrupt path
X-Git-Url: https://git.ndcode.org/public/gitweb.cgi?a=commitdiff_plain;h=f15ec518037c2f4fa5631a648684f1e49884a6c4;p=FUZIX.git

zxdiv,tty: optimize interrupt path

This saves us a lot of clocks of pointless processing if no keyboard line
changed state.
---

diff --git a/Kernel/platform-zxdiv/devtty.c b/Kernel/platform-zxdiv/devtty.c
index f1f2dbf6..77b2b01c 100644
--- a/Kernel/platform-zxdiv/devtty.c
+++ b/Kernel/platform-zxdiv/devtty.c
@@ -113,24 +113,34 @@ void tty_data_consumed(uint8_t minor)
 {
 }
 
-void update_keyboard(void)
+static uint8_t update_keyboard(void) __naked
 {
-	/* We need this assembler code because it is much faster, of course  */
-	/* TODO: make it naked? */
+	/*
+	 *	This is run 50 time a second so we do it in asm and also return
+	 *	0 if nothing changed. That allows us to avoid the main tty
+	 *	processing on most interrupt events which saves us a lot of
+	 *	clocks.
+	 */
 	__asm
 		ld hl,#_keybuf
 		ld c, #0xFE
 		ld b, #0x7f
-		ld e, #8        ; 8 keyboard ports, 7FFE, BFFE, DFFE and so on
+		ld de, #8        ; 8 keyboard ports, 7FFE, BFFE, DFFE and so on
+				 ; D to 0 for no change found
 	read_halfrow:
 		in a, (c)
-;		and #0
 		cpl
-		ld(hl), a
+		cp (hl)
+		jr z,nochange
+		inc d		; there 8 ports so we cannot overflow
+		ld (hl), a
+	nochange:
 		rrc b
 		inc hl
 		dec e
 		jr nz, read_halfrow
+		ld l,d
+		ret
 	__endasm;
 }
 
@@ -138,10 +148,12 @@ void tty_pollirq(void)
 {
 	int i;
 
-	update_keyboard();
-
 	newkey = 0;
 
+	/* Nothing changed - no processing required */
+	if (!update_keyboard())
+		return;
+
 	for (i = 0; i < 8; i++) {
 		int n;
 		uint8_t key = keybuf[i] ^ keymap[i];