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SubjectRe: [PATCH v7 net-next 5/5] net: ethernet: ti: cpsw: add XDP support
On Fri, Jul 05, 2019 at 01:13:54PM +0200, Jesper Dangaard Brouer wrote:
>On Fri, 5 Jul 2019 02:14:06 +0300
>Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org> wrote:
>
>> +static int cpsw_xdp_tx_frame(struct cpsw_priv *priv, struct xdp_frame *xdpf,
>> + struct page *page)
>> +{
>> + struct cpsw_common *cpsw = priv->cpsw;
>> + struct cpsw_meta_xdp *xmeta;
>> + struct cpdma_chan *txch;
>> + dma_addr_t dma;
>> + int ret, port;
>> +
>> + xmeta = (void *)xdpf + CPSW_XMETA_OFFSET;
>> + xmeta->ndev = priv->ndev;
>> + xmeta->ch = 0;
>> + txch = cpsw->txv[0].ch;
>> +
>> + port = priv->emac_port + cpsw->data.dual_emac;
>> + if (page) {
>> + dma = page_pool_get_dma_addr(page);
>> + dma += xdpf->data - (void *)xdpf;
>
>This code is only okay because this only happens for XDP_TX, where you
>know this head-room calculation will be true. The "correct"
>calculation of the head-room would be:
>
> dma += xdpf->headroom + sizeof(struct xdp_frame);
>
>The reason behind not using xdpf pointer itself as "data_hard_start",
>is to allow struct xdp_frame to be located in another memory area.

My assumption was based on:

struct xdp_frame *convert_to_xdp_frame(struct xdp_buff *xdp)
{
...
xdp_frame = xdp->data_hard_start;
...

xdp_frame->headroom = headroom - sizeof(*xdp_frame);
...
}

But agree, it doesn't contradict the reason in question.
So, better use proposed variant. Will check and do this in v8 a little later:

dma += xdpf->headroom + sizeof(struct xdp_frame);

>This will be useful for e.g. AF_XDP transmit, or other zero-copy
>transmit to go through ndo_xdp_xmit() (as we don't want userspace to
>be-able to e.g. "race" change xdpf->len during transmit/DMA-completion).
>
>
>> + ret = cpdma_chan_submit_mapped(txch, cpsw_xdpf_to_handle(xdpf),
>> + dma, xdpf->len, port);
>> + } else {
>> + if (sizeof(*xmeta) > xdpf->headroom) {
>> + xdp_return_frame_rx_napi(xdpf);
>> + return -EINVAL;
>> + }
>> +
>> + ret = cpdma_chan_submit(txch, cpsw_xdpf_to_handle(xdpf),
>> + xdpf->data, xdpf->len, port);
>> + }
>
>
>
>--
>Best regards,
> Jesper Dangaard Brouer
> MSc.CS, Principal Kernel Engineer at Red Hat
> LinkedIn: http://www.linkedin.com/in/brouer

--
Regards,
Ivan Khoronzhuk

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