[PATCH RFT net-next 0/4] via-rhine receive buffers rework

[PATCH RFT net-next 0/4] via-rhine receive buffers rework

[Francois Romieu]

This is an hopefully readable rework of my patch from two days ago.
It's different enough that it imho requires someone's Tested-by.

Patches #1..#3 are mostly there to slim patch #4. Patch #1 may make
some sense as a standalone fix though.

The series applies without error against v3.19 - what Nix uses - and
davem-next as of 5888b93b750609680735d6b8b737703083ef40ff ("Merge branch
'nf-hook-compress'").

Please test and review.

Francois Romieu (4):
  via-rhine: commit receive buffer address before descriptor status update.
  via-rhine: add allocation helpers.
  via-rhine: gotoize rhine_open error path.
  via-rhine: forbid holes in the receive descriptor ring.

 drivers/net/ethernet/via/via-rhine.c | 163 +++++++++++++++++++++--------------
 1 file changed, 100 insertions(+), 63 deletions(-)

-- 
2.1.0

[PATCH net-next 1/4] via-rhine: commit receive buffer address before descriptor status update.

[Francois Romieu]

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
---
 drivers/net/ethernet/via/via-rhine.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/net/ethernet/via/via-rhine.c b/drivers/net/ethernet/via/via-rhine.c
index a191afc..00fea3d 100644
--- a/drivers/net/ethernet/via/via-rhine.c
+++ b/drivers/net/ethernet/via/via-rhine.c
@@ -2063,6 +2063,7 @@ static int rhine_rx(struct net_device *dev, int limit)
 				break;
 			}
 			rp->rx_ring[entry].addr = cpu_to_le32(rp->rx_skbuff_dma[entry]);
+			wmb();
 		}
 		rp->rx_ring[entry].rx_status = cpu_to_le32(DescOwn);
 	}
-- 
2.1.0

[PATCH net-next 2/4] via-rhine: add allocation helpers.

[Francois Romieu]

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
---
 drivers/net/ethernet/via/via-rhine.c | 57 +++++++++++++++++++++++++++---------
 1 file changed, 43 insertions(+), 14 deletions(-)

diff --git a/drivers/net/ethernet/via/via-rhine.c b/drivers/net/ethernet/via/via-rhine.c
index 00fea3d..cf8dfcd 100644
--- a/drivers/net/ethernet/via/via-rhine.c
+++ b/drivers/net/ethernet/via/via-rhine.c
@@ -1211,12 +1211,47 @@ static void free_ring(struct net_device* dev)
 
 }
 
-static void alloc_rbufs(struct net_device *dev)
+struct rhine_skb_dma {
+	struct sk_buff *skb;
+	dma_addr_t dma;
+};
+
+static inline int rhine_skb_dma_init(struct net_device *dev,
+				     struct rhine_skb_dma *sd)
 {
 	struct rhine_private *rp = netdev_priv(dev);
 	struct device *hwdev = dev->dev.parent;
+	const int size = rp->rx_buf_sz;
+
+	sd->skb = netdev_alloc_skb(dev, size);
+	if (!sd->skb)
+		return -ENOMEM;
+
+	sd->dma = dma_map_single(hwdev, sd->skb->data, size, DMA_FROM_DEVICE);
+	if (unlikely(dma_mapping_error(hwdev, sd->dma))) {
+		netif_err(rp, drv, dev, "Rx DMA mapping failure\n");
+		dev_kfree_skb_any(sd->skb);
+		return -EIO;
+	}
+
+	return 0;
+}
+
+static inline void rhine_skb_dma_nic_store(struct rhine_private *rp,
+					   struct rhine_skb_dma *sd, int entry)
+{
+	rp->rx_skbuff_dma[entry] = sd->dma;
+	rp->rx_skbuff[entry] = sd->skb;
+
+	rp->rx_ring[entry].addr = cpu_to_le32(sd->dma);
+	wmb();
+}
+
+static void alloc_rbufs(struct net_device *dev)
+{
+	struct rhine_private *rp = netdev_priv(dev);
 	dma_addr_t next;
-	int i;
+	int rc, i;
 
 	rp->dirty_rx = rp->cur_rx = 0;
 
@@ -1237,20 +1272,14 @@ static void alloc_rbufs(struct net_device *dev)
 
 	/* Fill in the Rx buffers.  Handle allocation failure gracefully. */
 	for (i = 0; i < RX_RING_SIZE; i++) {
-		struct sk_buff *skb = netdev_alloc_skb(dev, rp->rx_buf_sz);
-		rp->rx_skbuff[i] = skb;
-		if (skb == NULL)
-			break;
+		struct rhine_skb_dma sd;
 
-		rp->rx_skbuff_dma[i] =
-			dma_map_single(hwdev, skb->data, rp->rx_buf_sz,
-				       DMA_FROM_DEVICE);
-		if (dma_mapping_error(hwdev, rp->rx_skbuff_dma[i])) {
-			rp->rx_skbuff_dma[i] = 0;
-			dev_kfree_skb(skb);
+		rc = rhine_skb_dma_init(dev, &sd);
+		if (rc < 0)
 			break;
-		}
-		rp->rx_ring[i].addr = cpu_to_le32(rp->rx_skbuff_dma[i]);
+
+		rhine_skb_dma_nic_store(rp, &sd, i);
+
 		rp->rx_ring[i].rx_status = cpu_to_le32(DescOwn);
 	}
 	rp->dirty_rx = (unsigned int)(i - RX_RING_SIZE);
-- 
2.1.0

[PATCH net-next 3/4] via-rhine: gotoize rhine_open error path.

[Francois Romieu]

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
---
 drivers/net/ethernet/via/via-rhine.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/drivers/net/ethernet/via/via-rhine.c b/drivers/net/ethernet/via/via-rhine.c
index cf8dfcd..ae1ab6b 100644
--- a/drivers/net/ethernet/via/via-rhine.c
+++ b/drivers/net/ethernet/via/via-rhine.c
@@ -1684,15 +1684,14 @@ static int rhine_open(struct net_device *dev)
 
 	rc = request_irq(rp->irq, rhine_interrupt, IRQF_SHARED, dev->name, dev);
 	if (rc)
-		return rc;
+		goto out;
 
 	netif_dbg(rp, ifup, dev, "%s() irq %d\n", __func__, rp->irq);
 
 	rc = alloc_ring(dev);
-	if (rc) {
-		free_irq(rp->irq, dev);
-		return rc;
-	}
+	if (rc < 0)
+		goto out_free_irq;
+
 	alloc_rbufs(dev);
 	alloc_tbufs(dev);
 	rhine_chip_reset(dev);
@@ -1705,7 +1704,12 @@ static int rhine_open(struct net_device *dev)
 
 	netif_start_queue(dev);
 
-	return 0;
+out:
+	return rc;
+
+out_free_irq:
+	free_irq(rp->irq, dev);
+	goto out;
 }
 
 static void rhine_reset_task(struct work_struct *work)
-- 
2.1.0

[PATCH net-next 4/4] via-rhine: forbid holes in the receive descriptor ring.

[Francois Romieu]

Rationales:
- throttle work under memory pressure;
- lower receive descriptor recycling latency for the network adapter;
- lower the maintenance burden of uncommon paths.

The patch is twofold:
1. it fails early if the receive buffer ring can't be completely
   initialized at dev->open() time
2. it drops packets on the floor in the napi receive handler so as to
   keep the ring full.

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
---
 drivers/net/ethernet/via/via-rhine.c | 101 ++++++++++++++++++-----------------
 1 file changed, 52 insertions(+), 49 deletions(-)

diff --git a/drivers/net/ethernet/via/via-rhine.c b/drivers/net/ethernet/via/via-rhine.c
index ae1ab6b..94729b1 100644
--- a/drivers/net/ethernet/via/via-rhine.c
+++ b/drivers/net/ethernet/via/via-rhine.c
@@ -471,7 +471,7 @@ struct rhine_private {
 	/* Frequently used values: keep some adjacent for cache effect. */
 	u32 quirks;
 	struct rx_desc *rx_head_desc;
-	unsigned int cur_rx, dirty_rx;	/* Producer/consumer ring indices */
+	unsigned int cur_rx;
 	unsigned int cur_tx, dirty_tx;
 	unsigned int rx_buf_sz;		/* Based on MTU+slack. */
 	struct rhine_stats rx_stats;
@@ -1237,6 +1237,16 @@ static inline int rhine_skb_dma_init(struct net_device *dev,
 	return 0;
 }
 
+static void rhine_reset_rbufs(struct rhine_private *rp)
+{
+	int i;
+
+	rp->cur_rx = 0;
+
+	for (i = 0; i < RX_RING_SIZE; i++)
+		rp->rx_ring[i].rx_status = cpu_to_le32(DescOwn);
+}
+
 static inline void rhine_skb_dma_nic_store(struct rhine_private *rp,
 					   struct rhine_skb_dma *sd, int entry)
 {
@@ -1247,14 +1257,14 @@ static inline void rhine_skb_dma_nic_store(struct rhine_private *rp,
 	wmb();
 }
 
-static void alloc_rbufs(struct net_device *dev)
+static void free_rbufs(struct net_device* dev);
+
+static int alloc_rbufs(struct net_device *dev)
 {
 	struct rhine_private *rp = netdev_priv(dev);
 	dma_addr_t next;
 	int rc, i;
 
-	rp->dirty_rx = rp->cur_rx = 0;
-
 	rp->rx_buf_sz = (dev->mtu <= 1500 ? PKT_BUF_SZ : dev->mtu + 32);
 	rp->rx_head_desc = &rp->rx_ring[0];
 	next = rp->rx_ring_dma;
@@ -1275,14 +1285,17 @@ static void alloc_rbufs(struct net_device *dev)
 		struct rhine_skb_dma sd;
 
 		rc = rhine_skb_dma_init(dev, &sd);
-		if (rc < 0)
-			break;
+		if (rc < 0) {
+			free_rbufs(dev);
+			goto out;
+		}
 
 		rhine_skb_dma_nic_store(rp, &sd, i);
-
-		rp->rx_ring[i].rx_status = cpu_to_le32(DescOwn);
 	}
-	rp->dirty_rx = (unsigned int)(i - RX_RING_SIZE);
+
+	rhine_reset_rbufs(rp);
+out:
+	return rc;
 }
 
 static void free_rbufs(struct net_device* dev)
@@ -1692,7 +1705,10 @@ static int rhine_open(struct net_device *dev)
 	if (rc < 0)
 		goto out_free_irq;
 
-	alloc_rbufs(dev);
+	rc = alloc_rbufs(dev);
+	if (rc < 0)
+		goto out_free_ring;
+
 	alloc_tbufs(dev);
 	rhine_chip_reset(dev);
 	rhine_task_enable(rp);
@@ -1707,6 +1723,8 @@ static int rhine_open(struct net_device *dev)
 out:
 	return rc;
 
+out_free_ring:
+	free_ring(dev);
 out_free_irq:
 	free_irq(rp->irq, dev);
 	goto out;
@@ -1729,9 +1747,9 @@ static void rhine_reset_task(struct work_struct *work)
 
 	/* clear all descriptors */
 	free_tbufs(dev);
-	free_rbufs(dev);
 	alloc_tbufs(dev);
-	alloc_rbufs(dev);
+
+	rhine_reset_rbufs(rp);
 
 	/* Reinitialize the hardware. */
 	rhine_chip_reset(dev);
@@ -2021,16 +2039,18 @@ static int rhine_rx(struct net_device *dev, int limit)
 				}
 			}
 		} else {
-			struct sk_buff *skb = NULL;
 			/* Length should omit the CRC */
 			int pkt_len = data_size - 4;
+			struct sk_buff *skb;
 			u16 vlan_tci = 0;
 
 			/* Check if the packet is long enough to accept without
 			   copying to a minimally-sized skbuff. */
-			if (pkt_len < rx_copybreak)
+			if (pkt_len < rx_copybreak) {
 				skb = netdev_alloc_skb_ip_align(dev, pkt_len);
-			if (skb) {
+				if (unlikely(!skb))
+					goto drop;
+
 				dma_sync_single_for_cpu(hwdev,
 							rp->rx_skbuff_dma[entry],
 							rp->rx_buf_sz,
@@ -2039,25 +2059,28 @@ static int rhine_rx(struct net_device *dev, int limit)
 				skb_copy_to_linear_data(skb,
 						 rp->rx_skbuff[entry]->data,
 						 pkt_len);
-				skb_put(skb, pkt_len);
+
 				dma_sync_single_for_device(hwdev,
 							   rp->rx_skbuff_dma[entry],
 							   rp->rx_buf_sz,
 							   DMA_FROM_DEVICE);
 			} else {
+				struct rhine_skb_dma sd;
+
+				if (unlikely(rhine_skb_dma_init(dev, &sd) < 0))
+					goto drop;
+
 				skb = rp->rx_skbuff[entry];
-				if (skb == NULL) {
-					netdev_err(dev, "Inconsistent Rx descriptor chain\n");
-					break;
-				}
-				rp->rx_skbuff[entry] = NULL;
-				skb_put(skb, pkt_len);
+
 				dma_unmap_single(hwdev,
 						 rp->rx_skbuff_dma[entry],
 						 rp->rx_buf_sz,
 						 DMA_FROM_DEVICE);
+				rhine_skb_dma_nic_store(rp, &sd, entry);
 			}
 
+			skb_put(skb, pkt_len);
+
 			if (unlikely(desc_length & DescTag))
 				vlan_tci = rhine_get_vlan_tci(skb, data_size);
 
@@ -2072,36 +2095,17 @@ static int rhine_rx(struct net_device *dev, int limit)
 			rp->rx_stats.packets++;
 			u64_stats_update_end(&rp->rx_stats.syncp);
 		}
+give_descriptor_to_nic:
+		rp->rx_ring[entry].rx_status = cpu_to_le32(DescOwn);
 		entry = (++rp->cur_rx) % RX_RING_SIZE;
 		rp->rx_head_desc = &rp->rx_ring[entry];
 	}
 
-	/* Refill the Rx ring buffers. */
-	for (; rp->cur_rx - rp->dirty_rx > 0; rp->dirty_rx++) {
-		struct sk_buff *skb;
-		entry = rp->dirty_rx % RX_RING_SIZE;
-		if (rp->rx_skbuff[entry] == NULL) {
-			skb = netdev_alloc_skb(dev, rp->rx_buf_sz);
-			rp->rx_skbuff[entry] = skb;
-			if (skb == NULL)
-				break;	/* Better luck next round. */
-			rp->rx_skbuff_dma[entry] =
-				dma_map_single(hwdev, skb->data,
-					       rp->rx_buf_sz,
-					       DMA_FROM_DEVICE);
-			if (dma_mapping_error(hwdev,
-					      rp->rx_skbuff_dma[entry])) {
-				dev_kfree_skb(skb);
-				rp->rx_skbuff_dma[entry] = 0;
-				break;
-			}
-			rp->rx_ring[entry].addr = cpu_to_le32(rp->rx_skbuff_dma[entry]);
-			wmb();
-		}
-		rp->rx_ring[entry].rx_status = cpu_to_le32(DescOwn);
-	}
-
 	return count;
+
+drop:
+	dev->stats.rx_dropped++;
+	goto give_descriptor_to_nic;
 }
 
 static void rhine_restart_tx(struct net_device *dev) {
@@ -2506,9 +2510,8 @@ static int rhine_resume(struct device *device)
 	enable_mmio(rp->pioaddr, rp->quirks);
 	rhine_power_init(dev);
 	free_tbufs(dev);
-	free_rbufs(dev);
 	alloc_tbufs(dev);
-	alloc_rbufs(dev);
+	rhine_reset_rbufs(rp);
 	rhine_task_enable(rp);
 	spin_lock_bh(&rp->lock);
 	init_registers(dev);
-- 
2.1.0

Re: [PATCH RFT net-next 0/4] via-rhine receive buffers rework

[Nix]

On 6 Apr 2015, Francois Romieu outgrape:

> This is an hopefully readable rework of my patch from two days ago.
> It's different enough that it imho requires someone's Tested-by.

I'll give it a torturing tomorrow.

-- 
NULL && (void)

Re: [PATCH RFT net-next 0/4] via-rhine receive buffers rework

[Nix]

On 6 Apr 2015, nix@esperi.org.uk verbalised:

> On 6 Apr 2015, Francois Romieu outgrape:
>
>> This is an hopefully readable rework of my patch from two days ago.
>> It's different enough that it imho requires someone's Tested-by.
>
> I'll give it a torturing tomorrow.

Tormented by routing a simultaneous iperf and make-over-nfs through it
(with the make using ccache, and the ccache cache directory *also*
accessed via nfs, to hit it really hard :) ).

No crashes in several hours of running: looks good!

-- 
NULL && (void)

Re: [PATCH RFT net-next 0/4] via-rhine receive buffers rework

[Francois Romieu]

Nix <nix@esperi.org.uk> :
[...]
> No crashes in several hours of running: looks good!

Mildly. :o/

You may never notice if you don't resume and the reset work queue doesn't
run but patch #4 lacks the bits below.

(rx_head_desc should go away)

diff --git a/drivers/net/ethernet/via/via-rhine.c b/drivers/net/ethernet/via/via-rhine.c
index a54a9f4..7f31c3a 100644
--- a/drivers/net/ethernet/via/via-rhine.c
+++ b/drivers/net/ethernet/via/via-rhine.c
@@ -1244,6 +1244,7 @@ static void rhine_reset_rbufs(struct rhine_private *rp)
 	int i;
 
 	rp->cur_rx = 0;
+	rp->rx_head_desc = rp->rx_ring;
 
 	for (i = 0; i < RX_RING_SIZE; i++)
 		rp->rx_ring[i].rx_status = cpu_to_le32(DescOwn);
@@ -1268,7 +1269,6 @@ static int alloc_rbufs(struct net_device *dev)
 	int rc, i;
 
 	rp->rx_buf_sz = (dev->mtu <= 1500 ? PKT_BUF_SZ : dev->mtu + 32);
-	rp->rx_head_desc = &rp->rx_ring[0];
 	next = rp->rx_ring_dma;
 
 	/* Init the ring entries */

Re: [PATCH RFT net-next 0/4] via-rhine receive buffers rework

[Nix]

On 7 Apr 2015, Francois Romieu stated:

> Nix <nix@esperi.org.uk> :
> [...]
>> No crashes in several hours of running: looks good!
>
> Mildly. :o/
>
> You may never notice if you don't resume and the reset work queue doesn't
> run but patch #4 lacks the bits below.

I don't suspend this box, no, it uses only 5W of power and it's my link
to the internet so suspending it would be a bad move :)

> (rx_head_desc should go away)

I'll test this out on top of the other stuff tomorrow...

-- 
NULL && (void)

Re: [PATCH net-next 1/4] via-rhine: commit receive buffer address before descriptor status update.

[David Miller]

From: Francois Romieu <romieu@fr.zoreil.com>
Date: Mon,  6 Apr 2015 20:01:49 +0200

> Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
> ---
>  drivers/net/ethernet/via/via-rhine.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/net/ethernet/via/via-rhine.c b/drivers/net/ethernet/via/via-rhine.c
> index a191afc..00fea3d 100644
> --- a/drivers/net/ethernet/via/via-rhine.c
> +++ b/drivers/net/ethernet/via/via-rhine.c
> @@ -2063,6 +2063,7 @@ static int rhine_rx(struct net_device *dev, int limit)
>  				break;
>  			}
>  			rp->rx_ring[entry].addr = cpu_to_le32(rp->rx_skbuff_dma[entry]);
> +			wmb();

dma_wmb() perhaps?  I think this is exactly the situation that interface was
added for.

Re: [PATCH net-next 1/4] via-rhine: commit receive buffer address before descriptor status update.

[Alexander Duyck]

On 04/06/2015 11:01 AM, Francois Romieu wrote:
> Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
> ---
>   drivers/net/ethernet/via/via-rhine.c | 1 +
>   1 file changed, 1 insertion(+)
>
> diff --git a/drivers/net/ethernet/via/via-rhine.c b/drivers/net/ethernet/via/via-rhine.c
> index a191afc..00fea3d 100644
> --- a/drivers/net/ethernet/via/via-rhine.c
> +++ b/drivers/net/ethernet/via/via-rhine.c
> @@ -2063,6 +2063,7 @@ static int rhine_rx(struct net_device *dev, int limit)
>   				break;
>   			}
>   			rp->rx_ring[entry].addr = cpu_to_le32(rp->rx_skbuff_dma[entry]);
> +			wmb();
>   		}
>   		rp->rx_ring[entry].rx_status = cpu_to_le32(DescOwn);
>   	}

Do you need a full wmb() here or would a dma_wmb() be enough?  It could 
make a difference on some processor architectures.

- Alex

Re: [PATCH net-next 1/4] via-rhine: commit receive buffer address before descriptor status update.

[Francois Romieu]

David Miller <davem@davemloft.net> :
> From: Francois Romieu <romieu@fr.zoreil.com>
[...]
> > @@ -2063,6 +2063,7 @@ static int rhine_rx(struct net_device *dev, int limit)
> >  				break;
> >  			}
> >  			rp->rx_ring[entry].addr = cpu_to_le32(rp->rx_skbuff_dma[entry]);
> > +			wmb();
> 
> dma_wmb() perhaps?  I think this is exactly the situation that interface was
> added for.

I need the buffer address to be written in the receive descriptor before
the descriptor status is. The cpu does W1, W2 and the nic mustn't see W2, W1.

-- 
Ueimor

Re: [PATCH net-next 1/4] via-rhine: commit receive buffer address before descriptor status update.

[Alexander Duyck]

On 04/07/2015 02:02 PM, Francois Romieu wrote:
> David Miller <davem@davemloft.net> :
>> From: Francois Romieu <romieu@fr.zoreil.com>
> [...]
>>> @@ -2063,6 +2063,7 @@ static int rhine_rx(struct net_device *dev, int limit)
>>>   				break;
>>>   			}
>>>   			rp->rx_ring[entry].addr = cpu_to_le32(rp->rx_skbuff_dma[entry]);
>>> +			wmb();
>> dma_wmb() perhaps?  I think this is exactly the situation that interface was
>> added for.
> I need the buffer address to be written in the receive descriptor before
> the descriptor status is. The cpu does W1, W2 and the nic mustn't see W2, W1.

That is the point of the dma_wmb().  If you are writing both W1 and W2 
to system memory then dma_wmb should be enough, if W1 is system memory 
and W2 is device memory (MMIO) then you need wmb().

You can think of dma_wmb as being something similar to smp_wmb w/o the 
SMP processor requirement.  On architectures that are strong ordered 
such as most x86 the wmb() translates to a barrier.  On other 
architectures it is something usually a little lighter than a full wmb() 
so for example on PowerPC wmb is sync() while dma_wmb() is lwsync().

- Alex

Re: [PATCH net-next 1/4] via-rhine: commit receive buffer address before descriptor status update.

[David Miller]

From: Francois Romieu <romieu@fr.zoreil.com>
Date: Tue, 7 Apr 2015 23:02:48 +0200

> David Miller <davem@davemloft.net> :
>> From: Francois Romieu <romieu@fr.zoreil.com>
> [...]
>> > @@ -2063,6 +2063,7 @@ static int rhine_rx(struct net_device *dev, int limit)
>> >  				break;
>> >  			}
>> >  			rp->rx_ring[entry].addr = cpu_to_le32(rp->rx_skbuff_dma[entry]);
>> > +			wmb();
>> 
>> dma_wmb() perhaps?  I think this is exactly the situation that interface was
>> added for.
> 
> I need the buffer address to be written in the receive descriptor before
> the descriptor status is. The cpu does W1, W2 and the nic mustn't see W2, W1.

That's exactly dma_wmb().

It barriers cpu writes so that the device sees things in a certain
order.

It's what all the most common ethernet chip drivers use in their
descriptor handling routines now.

Re: [PATCH net-next 1/4] via-rhine: commit receive buffer address before descriptor status update.

[Eric Dumazet]

On Tue, 2015-04-07 at 17:27 -0400, David Miller wrote:

> That's exactly dma_wmb().
> 
> It barriers cpu writes so that the device sees things in a certain
> order.
> 
> It's what all the most common ethernet chip drivers use in their
> descriptor handling routines now.

To be fair, only 2 drivers currently use dma_wmb()

Re: [PATCH net-next 1/4] via-rhine: commit receive buffer address before descriptor status update.

[Alexander Duyck]

On 04/07/2015 02:54 PM, Eric Dumazet wrote:
> On Tue, 2015-04-07 at 17:27 -0400, David Miller wrote:
>
>> That's exactly dma_wmb().
>>
>> It barriers cpu writes so that the device sees things in a certain
>> order.
>>
>> It's what all the most common ethernet chip drivers use in their
>> descriptor handling routines now.
> To be fair, only 2 drivers currently use dma_wmb()

Hey, I got at least 4..  :-)  I only got around to patching 3 Intel 
drivers and one RealTek since that is what I had to test with.  I was 
honestly hoping there would be more interest from other developers to 
pick this up and update their drivers to avoid unnecessary barriers but 
it doesn't look like I have had much luck on that front.

Maybe what I can do is submit a set of patches over the next couple of 
weeks to try and update all the spots that either need to have a barrier 
added, such as what is being addressed here, or can have a barrier 
weakened as I have already done for a few other drivers.

- Alex

Re: [PATCH net-next 1/4] via-rhine: commit receive buffer address before descriptor status update.

[Eric Dumazet]

On Tue, 2015-04-07 at 15:12 -0700, Alexander Duyck wrote:
> On 04/07/2015 02:54 PM, Eric Dumazet wrote:

> > To be fair, only 2 drivers currently use dma_wmb()
> 
> Hey, I got at least 4..  :-)  I only got around to patching 3 Intel 
> drivers and one RealTek since that is what I had to test with.  I was 
> honestly hoping there would be more interest from other developers to 
> pick this up and update their drivers to avoid unnecessary barriers but 
> it doesn't look like I have had much luck on that front.

Strange I see only 2 drivers here, and no Intel ones :

# git grep -l dma_wmb drivers/net
drivers/net/ethernet/amd/xgbe/xgbe-dev.c
drivers/net/ethernet/realtek/r8169.c

I probably missed something ;)

Re: [PATCH net-next 1/4] via-rhine: commit receive buffer address before descriptor status update.

[Alexander Duyck]

On 04/07/2015 03:22 PM, Eric Dumazet wrote:
> On Tue, 2015-04-07 at 15:12 -0700, Alexander Duyck wrote:
>> On 04/07/2015 02:54 PM, Eric Dumazet wrote:
>>> To be fair, only 2 drivers currently use dma_wmb()
>> Hey, I got at least 4..  :-)  I only got around to patching 3 Intel
>> drivers and one RealTek since that is what I had to test with.  I was
>> honestly hoping there would be more interest from other developers to
>> pick this up and update their drivers to avoid unnecessary barriers but
>> it doesn't look like I have had much luck on that front.
> Strange I see only 2 drivers here, and no Intel ones :
>
> # git grep -l dma_wmb drivers/net
> drivers/net/ethernet/amd/xgbe/xgbe-dev.c
> drivers/net/ethernet/realtek/r8169.c
>
> I probably missed something ;)

Yeah, I was thinking of the dma_rmb more than the dma_wmb, they are 
essentially just two sides to the same coin.

dma_rmb - read descriptor status for ownership, then dma_rmb, then if we 
own it process rest of descriptor
dma_wmb - write descriptor fields, then dma_wmb, then update status bit 
to release ownership

A wmb is still needed if a PIO doorbell must be used to notify the 
device of additional descriptors.

- Alex

RE: [PATCH net-next 1/4] via-rhine: commit receive buffer address before descriptor status update.

[David Laight]

From: Alexander Duyck
> Sent: 08 April 2015 00:21
...
> dma_wmb - write descriptor fields, then dma_wmb, then update status bit
> to release ownership
> 
> A wmb is still needed if a PIO doorbell must be used to notify the
> device of additional descriptors.

That is needed after the write that releases ownership.
So you are likely to need a dma_wmb() before the status bit update
and a full wmb() just after it.

If you are adding a lot of frames then you should be able to update
the status for the first fragment last - and only need the barriers
on the final update.

	David