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Date:	Thu, 27 Dec 2012 00:17:21 +0100
From:	Francois Romieu <romieu@...zoreil.com>
To:	Rafał Miłecki <zajec5@...il.com>
Cc:	netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH] bgmac: driver for GBit MAC core on BCMA bus

Rafał Miłecki <zajec5@...il.com> :
[...]
> diff --git a/drivers/net/ethernet/broadcom/bgmac.c b/drivers/net/ethernet/broadcom/bgmac.c
[...]
> +static void bgmac_dma_tx_reset(struct bgmac *bgmac, struct bgmac_dma_ring *ring)
> +{
> +	u32 val;
> +	int i;
> +
> +	if (!ring->mmio_base)
> +		return;

static int bgmac_probe(struct bcma_device *core)
[...]
	bgmac_chip_reset(bgmac);

	err = bgmac_dma_alloc(bgmac);

mmio_base is only set in bgmac_dma_alloc.

-> remove the useless bgmac_chip_reset() in bgmac_probe() and the driver
won't need to test for !ring->mmio_base.

> +
> +	/* Susend DMA TX ring first.

	/* Suspend DMA TX ring first.

[...]
> +static netdev_tx_t bgmac_dma_tx_add(struct bgmac *bgmac,
> +				    struct bgmac_dma_ring *ring,
> +				    struct sk_buff *skb)
> +{
> +	struct device *dma_dev = bgmac->core->dma_dev;
> +	struct bgmac_dma_desc *dma_desc;
> +	struct bgmac_slot_info *slot;
> +	u32 ctl0, ctl1;
> +	int free_slots;
> +
> +	if (skb->len > BGMAC_DESC_CTL1_LEN) {
> +		bgmac_err(bgmac, "Too long skb (%d)\n", skb->len);
> +		return NETDEV_TX_BUSY;
> +	}

The driver will loop if you don't stop queuing. It won't help.

> +
> +	if (ring->start <= ring->end)
> +		free_slots = ring->start - ring->end + BGMAC_TX_RING_SLOTS;
> +	else
> +		free_slots = ring->start - ring->end;
> +	if (free_slots <= 1) {
> +		bgmac_err(bgmac, "No free slots on ring 0x%X!\n", ring->mmio_base);
> +		netif_stop_queue(bgmac->net_dev);
> +		return NETDEV_TX_BUSY;
> +	}

The driver should stop queueing after its processing if it detects that
it can't handle more requests.

> +
> +	slot = &ring->slots[ring->end];
> +	slot->skb = skb;
> +	slot->dma_addr = dma_map_single(dma_dev, skb->data, skb->len, DMA_TO_DEVICE);
> +	if (dma_mapping_error(dma_dev, slot->dma_addr)) {
> +		bgmac_err(bgmac, "Mapping error of skb on ring 0x%X\n", ring->mmio_base);
> +		return NETDEV_TX_BUSY;
> +	}

Why don't you drop the packet and return TX_OK ?

> +
> +	ctl0 = BGMAC_DESC_CTL0_IOC | BGMAC_DESC_CTL0_SOF | BGMAC_DESC_CTL0_EOF;
> +	if (ring->end == ring->num_slots - 1)
> +		ctl0 |= BGMAC_DESC_CTL0_EOT;
> +	ctl1 = skb->len & BGMAC_DESC_CTL1_LEN;
> +
> +	dma_desc = ring->cpu_base;
> +	dma_desc += ring->end;

	struct bgmac_dma_desc *dma_desc = ring->cpu_base + ring->end;

> +	dma_desc->addr_low = cpu_to_le32(lower_32_bits(slot->dma_addr));
> +	dma_desc->addr_high = cpu_to_le32(upper_32_bits(slot->dma_addr));
> +	dma_desc->ctl0 = cpu_to_le32(ctl0);
> +	dma_desc->ctl1 = cpu_to_le32(ctl1);
> +
> +	wmb();

Are we sure that the hardware will never read a descriptor where ctl0 is
set and ctl1 is not (start_xmit, dma starts, competing start_xmit, oops) ?

> +
> +	/* Increase ring->end to point empty slot. We tell hardware the first
> +	 * slot it shold *not* read.

	 * slot it should *not* read.

[...]
> +static void bgmac_dma_tx_free(struct bgmac *bgmac, struct bgmac_dma_ring *ring)
> +{
> +	struct device *dma_dev = bgmac->core->dma_dev;
> +	struct bgmac_dma_desc *dma_desc;

The scope of 'slot' and 'dma_desc' can be reduced.

> +	struct bgmac_slot_info *slot;
> +	int empty_slot;
> +
> +	/* The last slot that hardware didn't consume yet */
> +	empty_slot = bgmac_read(bgmac, ring->mmio_base + BGMAC_DMA_TX_STATUS);
> +	empty_slot &= BGMAC_DMA_TX_STATDPTR;
> +	empty_slot /= sizeof(struct bgmac_dma_desc);
> +
> +	while (ring->start != empty_slot) {
> +		/* Set pointers */
> +		dma_desc = ring->cpu_base;
> +		dma_desc += ring->start;
> +		slot = &ring->slots[ring->start];
> +
> +		if (slot->skb) {
> +			/* Unmap no longer used buffer */
> +			dma_unmap_single(dma_dev, slot->dma_addr,
> +					 slot->skb->len, DMA_TO_DEVICE);
> +			slot->dma_addr = 0;
> +
> +			/* Free memory! :) */
> +			dev_kfree_skb_any(slot->skb);

This is irq enabled NAPI poll context so you can dev_kfree_skb.

> +			slot->skb = NULL;
> +		} else {
> +			bgmac_err(bgmac, "Hardware reported transmission for empty TX ring slot %d! End of ring: %d", ring->start, ring->end);
> +		}
> +
> +		if (++ring->start >= BGMAC_TX_RING_SLOTS)
> +			ring->start = 0;
> +	}
> +}
> +
> +static void bgmac_dma_rx_reset(struct bgmac *bgmac, struct bgmac_dma_ring *ring)
> +{
> +	if (!ring->mmio_base)
> +		return;

See remark about bgmac_dma_tx_reset.

[...]
> +static int bgmac_dma_rx_read(struct bgmac *bgmac, struct bgmac_dma_ring *ring,
> +			     int weight)
> +{
> +	struct device *dma_dev = bgmac->core->dma_dev;
> +	struct bgmac_dma_desc *dma_desc;
> +	struct bgmac_slot_info *slot;
> +	struct sk_buff *skb, *new_skb;
> +	struct bgmac_rx_header *rx;
> +	u32 end_slot;
> +	u16 len, flags;
> +	int handled = 0;

The scope of several variables can be reduced as well.

-- 
Ueimor
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