Re: [v6,net-next,15/15] net: fec: add AF_XDP zero-copy support

[Jakub Kicinski <kuba@kernel.org>]

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---
net: fec: add AF_XDP zero-copy support

This commit adds AF_XDP zero-copy support for both TX and RX on the FEC
Ethernet driver. It introduces new functions for XSK buffer allocation,
RX/TX queue processing in zero-copy mode, and XSK pool setup/teardown.

> For TX, there are two cases, one is the frames from the AF_XDP socket,
> so fec_enet_xsk_xmit() is added to directly transmit the frames from
> the socket and the buffer type is marked as FEC_TXBUF_T_XSK_XMIT. The
> other one is the frams from the RX queue (XDP_TX action), the buffer

This isn't a bug, but "frams" should be "frames".

> diff --git a/drivers/net/ethernet/freescale/fec_main.c b/drivers/net/ethernet/freescale/fec_main.c
> --- a/drivers/net/ethernet/freescale/fec_main.c
> +++ b/drivers/net/ethernet/freescale/fec_main.c

[ ... ]

> +static int fec_enet_xsk_tx_xmit(struct fec_enet_private *fep,
> +				struct xdp_buff *xsk, int cpu,
> +				int queue)
> +{

[ ... ]

> +	bdp->cbd_bufaddr = cpu_to_fec32(dma);
> +	bdp->cbd_datlen = cpu_to_fec16(len);
> +
> +	if (fep->bufdesc_ex) {
> +		struct bufdesc_ex *ebdp = (struct bufdesc_ex *)bdp;
> +
> +		if (fep->quirks & FEC_QUIRK_HAS_AVB)
> +			estatus |= FEC_TX_BD_FTYPE(txq->bd.qid);
> +
> +		ebdp->cbd_bdu = 0;
> +		ebdp->cbd_esc = cpu_to_fec32(estatus);
> +	}
> +
> +	status |= (BD_ENET_TX_READY | BD_ENET_TX_TC);
> +	bdp->cbd_sc = cpu_to_fec16(status);
> +	dma_wmb();

Is there a missing dma_wmb() before setting BD_ENET_TX_READY? The
descriptor fields (cbd_bufaddr, cbd_datlen, cbd_esc) are written before
the status, but without a memory barrier, the hardware could see the
READY bit before those fields are visible on weakly-ordered architectures.

Compare to fec_enet_xsk_xmit() in the same file which does:

	dma_wmb();
	status |= BD_ENET_TX_READY | BD_ENET_TX_TC;
	bdp->cbd_sc = cpu_to_fec16(status);
	dma_wmb();

The dma_wmb() after setting status only ensures ordering with subsequent
writes (bd.cur update), not with the preceding descriptor field writes.