From 7b3e80a957684a7cfedbb6168aa47ade918d6cc0 Mon Sep 17 00:00:00 2001
From: Oliver Schmidt <ol.sc@web.de>
Date: Sun, 26 Apr 2015 14:23:33 +0200
Subject: [PATCH] Remove received packet(s) to allow to send one.

Behave just like the CS8900A driver: Both the CS8900A and the LAN91C96 dynamically share a buffer for received packets and packets to be send. If the chip is exposed to a network with a lot of broadcasts the shared buffer might fill quicker with received packets than the 6502 reads them (via polling). So we might need to drop some received packets in order to be able to send anything at all.
---
 cpu/6502/net/lan91c96.S | 53 ++++++++++++++++++++++-------------------
 1 file changed, 29 insertions(+), 24 deletions(-)

diff --git a/cpu/6502/net/lan91c96.S b/cpu/6502/net/lan91c96.S
index 0bfb4e35b..e94fdda7c 100644
--- a/cpu/6502/net/lan91c96.S
+++ b/cpu/6502/net/lan91c96.S
@@ -89,7 +89,7 @@ fixup:	.byte	fixup02-fixup01, fixup03-fixup02, fixup04-fixup03
 	.byte	fixup29-fixup28, fixup30-fixup29, fixup31-fixup30
 	.byte	fixup32-fixup31, fixup33-fixup32, fixup34-fixup33
 	.byte	fixup35-fixup34, fixup36-fixup35, fixup37-fixup36
-	.byte	fixup38-fixup37, fixup39-fixup38, fixup40-fixup39
+	.byte	fixup38-fixup37, fixup39-fixup38
 
 fixups	= * - fixup
 
@@ -272,9 +272,7 @@ fixup22:lda ethdata
 	bcs :++
 
 	; Yes, skip packet
-	; Remove and release RX packet from the FIFO
-	lda #%10000000
-fixup23:sta ethmmucr
+	jsr releasepacket
 
 	; No packet available
 	lda #$00
@@ -285,7 +283,7 @@ fixup23:sta ethmmucr
 	; Read bytes into buffer
 :	jsr adjustptr
 :
-fixup24:lda ethdata
+fixup23:lda ethdata
 	sta (ptr),y
 	iny
 	bne :-
@@ -294,8 +292,7 @@ fixup24:lda ethdata
 	bpl :-
 
 	; Remove and release RX packet from the FIFO
-	lda #%10000000
-fixup25:sta ethmmucr
+	jsr releasepacket
 
 	; Return packet length
 	lda len
@@ -313,19 +310,19 @@ send:
 	; Allocate memory for TX
 	txa
 	ora #%00100000
-fixup26:sta ethmmucr
+fixup24:sta ethmmucr
 
 	; 8 retries
 	ldy #$08
 
 	; Wait for allocation ready
 :
-fixup27:lda ethist
+fixup25:lda ethist
 	and #%00001000		; ALLOC INT
 	bne :+
 
-	; Shouldn't we do something here to actively free memory,
-	; maybe removing and releasing an RX packet from the FIFO ???
+	; No space avaliable, skip a received frame
+	jsr releasepacket
 
 	; And try again
 	dey
@@ -335,21 +332,21 @@ fixup27:lda ethist
 
 	; Acknowledge interrupt, is it necessary ???
 :	lda #%00001000
-fixup28:sta ethack
+fixup26:sta ethack
 
 	; Set packet address
-fixup29:lda etharr
-fixup30:sta ethpnr
+fixup27:lda etharr
+fixup28:sta ethpnr
 
 	lda #$00
 	ldx #%01000000		; AUTO INCR.
-fixup31:sta ethptr
-fixup32:stx ethptr+1
+fixup29:sta ethptr
+fixup30:stx ethptr+1
 
 	; Status written by CSMA
 	lda #$00
-fixup33:sta ethdata
-fixup34:sta ethdata
+fixup31:sta ethdata
+fixup32:sta ethdata
 
 	; Check packet length parity:
 	; - Even packet length -> carry set   -> add 6 bytes
@@ -361,10 +358,10 @@ fixup34:sta ethdata
 	; The packet contains 3 extra words
 	lda len
 	adc #$05		; Actually 5 or 6 depending on carry
-fixup35:sta ethdata
+fixup33:sta ethdata
 	lda len+1
 	adc #$00
-fixup36:sta ethdata
+fixup34:sta ethdata
 
 	; Send the packet
 	; ---------------
@@ -372,7 +369,7 @@ fixup36:sta ethdata
 	; Write bytes from buffer
 	jsr adjustptr
 :	lda (ptr),y
-fixup37:sta ethdata
+fixup35:sta ethdata
 	iny
 	bne :-
 	inc ptr+1
@@ -390,13 +387,13 @@ fixup37:sta ethdata
 
 	; No
 :	lda #$00
-fixup38:sta ethdata		; Fill byte
+fixup36:sta ethdata		; Fill byte
 :	
-fixup39:sta ethdata		; Control byte
+fixup37:sta ethdata		; Control byte
 
 	; Add packet to FIFO
 	lda #%11000000		; ENQUEUE PACKET - transmit packet
-fixup40:sta ethmmucr
+fixup38:sta ethmmucr
 	clc
 	rts
 
@@ -407,6 +404,14 @@ exit:
 
 ;---------------------------------------------------------------------
 
+releasepacket:
+	; Remove and release RX packet from the FIFO
+	lda #%10000000
+fixup39:sta ethmmucr
+	rts
+
+;---------------------------------------------------------------------
+
 adjustptr:
 	lda len
 	ldx len+1