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>> +       struct pcc_header pcc_header; >> +       struct mctp_skb_cb *cb; >> +       struct sk_buff *skb; >> + >> +       mctp_pcc_ndev = container_of(c, struct mctp_pcc_ndev, inbox.client); >> +       if (!buffer) { >> +               dev_dstats_rx_dropped(mctp_pcc_ndev->mdev.dev); >> +               return; >> +       } > Mainly out of curiosity: how does this happen? How do we get a > completion where there is no original buffer? If the sk_buff allocation fails, the logic falls back to the old code, which passes on a null buffer. There is logic there with notifying the sender that I don't want to skip or modify. See the other patch, in the change to the irq handler. > I figure we're restricted to what the mailbox API provides, but is there > any way we can access the skb through a pointer, rather than having to > dig through these lists? > > I think the issue is that the mbox API is using the void * buffer as > both the data to transfer, and the callback context, so we can't stash > useful context across the completion? Correct, the SK_buff is a structure  that points to a buffer, and what gets to the send_data function is the buffer itself. That buffer has no pointer back to the sk_buff. >> + >> +       rc = mbox_send_message(mpnd->outbox.chan->mchan, skb->data); >> + >> +       if (rc < 0) { >> +               pr_info("%s fail, rc = %d", __func__, rc); >> +               return NETDEV_TX_BUSY; >> +       } > What happens on mbox_send_message failure? The skb will still be present > in the outbox.packets queue - I assume we don't see a completion > callback in that case, and so the skb will be in the outbox.packets > queue forever? > > Are you sure you want to return NETDEV_TX_BUSY here? > > Is there any situation where the mbox_send_message will continually > fail? Should we ratelimit the pr_info() message there (and regardless, > better to use one of netdev_info / netdev_warn / etc functions, since we > are dealing with netdevs here). The fail will happen if the ring buffer is full, so, yes, it makes sense not to queue the packet. I can move that to after the mbox_send_message. The NETDEV_TX_BUSY is correct, as it means resend the packet, and we don't have any reference to it. The only other failure path on the mbox_send_message code is due to timeouts, which we do not use from this driver. That is a recent change, and that was the case I was handling before.