[PATCH 1/4] ftgmac100: Fix race issue on TX descriptor[0]

On Mon, 19 Oct 2020 at 23:20, Benjamin Herrenschmidt
[off-list ref] wrote:

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.

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];
      netdev->stats.tx_packets++;
      netdev->stats.tx_bytes += skb->len;

@@ -756,6 +759,9 @@ static netdev_tx_t

ftgmac100_hard_start_xmit(struct sk_buff *skb,
      pointer = priv->tx_pointer;
      txdes = first = &priv->txdes[pointer];

+     if (le32_to_cpu(txdes->txdes0) & ~priv->txdes0_edotr_mask)
+             goto drop;
+
      /* Setup it up with the packet head. Don't write the head to
the
       * ring just yet
       */

@@ -787,6 +793,10 @@ static netdev_tx_t

ftgmac100_hard_start_xmit(struct sk_buff *skb,
              /* Setup descriptor */
              priv->tx_skbs[pointer] = skb;
              txdes = &priv->txdes[pointer];
+
+             if (le32_to_cpu(txdes->txdes0) & ~priv-

quoted

txdes0_edotr_mask)

+                     goto dma_err;
+
              ctl_stat = ftgmac100_base_tx_ctlstat(priv, pointer);
              ctl_stat |= FTGMAC100_TXDES0_TXDMA_OWN;
              ctl_stat |= FTGMAC100_TXDES0_TXBUF_SIZE(len);