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Darwin 0.2 Driver Kit

		      m88k ethernet buffer notes	
		      

/*
 * This is exactly 2k bytes, aligned to a 2k byte boundary. The actual
 * DMA data is inside of here, aligned for proper enet DMA.
 *
 * An enetBuf as associated with a netbuf_t at getbuf time via the 
 * freefunc and freefuncarg mechanism, and at RX DMA enqueue time by 
 * setting dma_id = netbuf.
 *
 * We keep separate pools of TX and RX ebetBufs. For now, a fixed number 
 * of TX and RX enetBufs are allocated at init time; no more are 
 * allocated after that.
 */
typedef struct {
	enetBuf_t *next;
	id owner;
	boolean_t Rx;
	char filler[to align dmaData for DMA];
	char dmaData[max packet size];
	char filler[to fill out to 2048 bytes];
} enetBuf_t;

boot {
  	create a list of empty enetBuf_t's, enqueue on RxBufList and 
		TxBufList;
	rxDmaQCount = 0;
	do {
		get an enetBuf from RxBufList;
		netbuf = nb_alloc_wrapper(&enetBuf->dmaData,
			size of DMA Data,
			&enetBufFree(),
			&enetBuf);
		/*
		 * Our record of netbuf is via dma_id!
		 */
		dma_enqueue(data = &enetBuf.dmaData,
			dma_id = netbuf, 
			length = sizeof DMA data);
		rxDmaQCount++;
	} for each buf in RxBufList;
}
  
getbuf {
  	get a free enetBuf from TxBufList;
	return NULL if list empty;
	return(nb_alloc_wrapper(&enetBuf->dmaData,
		size of DMA Data,
		&enetBufFree(),
		&enetBuf));
}
  
output {
	dma_enqueue(data = nb_map(netbuf), 
		dma_id = netbuf, 
		length = nb_size(netbuf));
}

TX DMA complete {
	do {
		dma_id = chan_dma_dequeue(COMPLETED);
		nb_free((netbuf_t)dma_id);
	} for all frames;
}

RX DMA complete {
	do {
		dma_id = chan_dma_dequeue(COMPLETED);
		if_handle_input(netif, (netbuf_t)dma_id, NULL);
	} for all frames;
	do {
		get an enetBuf from RxBufList;
		netbuf = nb_alloc_wrapper(&enetBuf->dmaData,
			size of DMA Data,
			&enetBufFree(),
			&enetBuf);
		dma_enqueue(data = &enetBuf.dmaData,
			dma_id = netbuf, 
			length = sizeof DMA data);
		rxDmaQCount++;
	
	} for each buf in RxBufList; 
}

/*
 * Called out from nb_free(), as freefunc(freefunc_arg).
 */
enetBufFree(enetBuf_t *enetBuf) {
  	enqueue this on TxBufList or RxBufList;
}

Notes
-----

* Memory throttle is via getbuf returning NULL. Joel has a limit of 10 RX and
  10 TX buffers in the Token Ring driver. Current 68k enet driver has no 
  limit. Brad and Joel say that it should not cause problems for the
  driver to return NULL on getbuf()...Brad will help track down problems
  in this area.
  

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