On Tue, 20 Oct 2020 at 06:23, Benjamin Herrenschmidt
[off-list ref] wrote:
On Tue, 2020-10-20 at 04:13 +0000, Joel Stanley wrote:
quoted
On Mon, 19 Oct 2020 at 23:20, Benjamin Herrenschmidt
[off-list ref] wrote:
quoted
On Mon, 2020-10-19 at 16:57 +0800, Dylan Hung wrote:
quoted
These rules must be followed when accessing the TX descriptor:
1. A TX descriptor is "cleanable" only when its value is non-zero
and the owner bit is set to "software"
Can you elaborate ? What is the point of that change ? The owner
bit
should be sufficient, why do we need to check other fields ?
I would like Dylan to clarify too. The datasheet has a footnote below
the descriptor layout:
- TXDES#0: Bits 27 ~ 14 are valid only when FTS = 1
- TXDES#1: Bits 31 ~ 0 are valid only when FTS = 1
So the ownership bit (31) is not valid unless FTS is set. However,
this isn't what his patch does. It adds checks for EDOTR.
No I think it adds a check for everything except EDOTR which just marks
the end of ring and needs to be ignored in the comparison.
Of course. I missed the invert.
I did some testing with just this patch (and "[4/4] ftgmac100: Restart
MAC HW once") from Dylan. It seemed to resolve the hang, but there
were occasional retries. Putting in some tracing I only hit the
condition in ftgmac100_tx_complete_packet, never in
ftgmac100_hard_start_xmit.
That said, we do need a better explanation.
One potential bug I did find by looking at my code however is:
static bool ftgmac100_tx_complete_packet(struct ftgmac100 *priv)
{
struct net_device *netdev = priv->netdev;
struct ftgmac100_txdes *txdes;
struct sk_buff *skb;
unsigned int pointer;
u32 ctl_stat;
pointer = priv->tx_clean_pointer;
txdes = &priv->txdes[pointer];
ctl_stat = le32_to_cpu(txdes->txdes0);
if (ctl_stat & FTGMAC100_TXDES0_TXDMA_OWN)
return false;
skb = priv->tx_skbs[pointer];
netdev->stats.tx_packets++;
netdev->stats.tx_bytes += skb->len;
ftgmac100_free_tx_packet(priv, pointer, skb, txdes, ctl_stat);
txdes->txdes0 = cpu_to_le32(ctl_stat & priv->txdes0_edotr_mask);
^^^^ There should probably be an smp_wmb() here to ensure that all the above
stores are visible before the tx clean pointer is updated.
priv->tx_clean_pointer = ftgmac100_next_tx_pointer(priv, pointer);
return true;
}
Similarly we probablu should have one before setting tx_pointer in start_xmit().
I added the two smp_wmb you suggested (with only 4/4 applied). This
did the trick; iperf on a gigabit link is running well with no
retries.
diff --git a/drivers/net/ethernet/faraday/ftgmac100.c
b/drivers/net/ethernet/faraday/ftgmac100.c
index 331d4bdd4a67..15cdfeb135b0 100644
--- a/drivers/net/ethernet/faraday/ftgmac100.c
+++ b/drivers/net/ethernet/faraday/ftgmac100.c
@@ -653,6 +653,11 @@ static bool ftgmac100_tx_complete_packet(struct
ftgmac100 *priv)
ftgmac100_free_tx_packet(priv, pointer, skb, txdes, ctl_stat);
txdes->txdes0 = cpu_to_le32(ctl_stat & priv->txdes0_edotr_mask);
+ /* Ensure the descriptor config is visible before setting the tx
+ * pointer.
+ */
+ smp_wmb();
+
priv->tx_clean_pointer = ftgmac100_next_tx_pointer(priv, pointer);
return true;@@ -806,6 +811,11 @@ static netdev_tx_t
ftgmac100_hard_start_xmit(struct sk_buff *skb,
dma_wmb();
first->txdes0 = cpu_to_le32(f_ctl_stat);
+ /* Ensure the descriptor config is visible before setting the tx
+ * pointer.
+ */
+ smp_wmb();
+
/* Update next TX pointer */
priv->tx_pointer = pointer;
I left the test running while writing this email and I did start to
see some retries. I'm not sure if that's because my laptop is one of
the test machines, or if we have another issue.
I will do some further testing over night.
Cheers,
Joel
As for the read side of this, I'm not 100% sure, I'll have to think more about
it, it *think* the existing barriers are sufficient at first sight.
Cheers,
Ben.
quoted
quoted
quoted
2. A TX descriptor is "writable" only when its value is zero
regardless the edotr mask.
Again, why is that ? Can you elaborate ? What race are you trying
to
address here ?
Cheers,
Ben.
quoted
Fixes: 52c0cae87465 ("ftgmac100: Remove tx descriptor accessors")
Signed-off-by: Dylan Hung <redacted>
Signed-off-by: Joel Stanley <joel@jms.id.au>
---
drivers/net/ethernet/faraday/ftgmac100.c | 10 ++++++++++
1 file changed, 10 insertions(+)
diff --git a/drivers/net/ethernet/faraday/ftgmac100.c
b/drivers/net/ethernet/faraday/ftgmac100.c
index 00024dd41147..7cacbe4aecb7 100644
--- a/drivers/net/ethernet/faraday/ftgmac100.c
+++ b/drivers/net/ethernet/faraday/ftgmac100.c
@@ -647,6 +647,9 @@ static bool
ftgmac100_tx_complete_packet(struct
ftgmac100 *priv)
if (ctl_stat & FTGMAC100_TXDES0_TXDMA_OWN)
return false;
+ if ((ctl_stat & ~(priv->txdes0_edotr_mask)) == 0)
+ return false;
+
skb = priv->tx_skbs[pointer];
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