[xdp-hints] Re: [PATCH net-next 3/4] net: stmmac: add Rx HWTS metadata to XDP receive pkt

["Song, Yoong Siang" <yoong.siang.song@intel.com>]

On Tuesday, April 11, 2023 12:25 AM, Stanislav Fomichev <sdf@google.com> wrote:
>On 04/10, Song Yoong Siang wrote:
>> Add receive hardware timestamp metadata support via kfunc to XDP
>> receive packets.
>>
>> Signed-off-by: Song Yoong Siang <yoong.siang.song@intel.com>
>> ---
>>  drivers/net/ethernet/stmicro/stmmac/stmmac.h  |  1 +
>> .../net/ethernet/stmicro/stmmac/stmmac_main.c | 24 +++++++++++++++++--
>>  2 files changed, 23 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac.h
>> b/drivers/net/ethernet/stmicro/stmmac/stmmac.h
>> index ac8ccf851708..760445275da8 100644
>> --- a/drivers/net/ethernet/stmicro/stmmac/stmmac.h
>> +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac.h
>> @@ -94,6 +94,7 @@ struct stmmac_rx_buffer {
>>
>>  struct stmmac_xdp_buff {
>>  	struct xdp_buff xdp;
>> +	ktime_t rx_hwts;
>>  };
>>
>>  struct stmmac_rx_queue {
>> diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
>> b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
>> index f7bbdf04d20c..ca183fbfde85 100644
>> --- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
>> +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
>> @@ -5307,6 +5307,8 @@ static int stmmac_rx(struct stmmac_priv *priv, int
>limit, u32 queue)
>>  			}
>>  		}
>>
>
>[..]
>
>> +		stmmac_get_rx_hwtstamp(priv, p, np, &ctx.rx_hwts);
>
>Do we want to pay this cost for every packet?
>
>The preferred alternative is to store enough state in the stmmac_xdp_buff so we
>can get to this data from stmmac_xdp_rx_timestamp.
>
>I haven't read this code, but tentatively:
>- move priv, p, np into stmmac_xdp_buff, assign them here instead of
>  calling stmmac_get_rx_hwtstamp
>- call stmmac_get_rx_hwtstamp from stmmac_xdp_rx_timestamp with the
>  stored priv, p, np
>
>That would ensure that we won't waste the cycles pulling out the rx timestamp
>for every packet if the higher levels / users don't care.
>
>Would something like this work?

Hi Stanislav Fomichev,

Thanks for your comments.

Original stmmac_rx() function is already calling stmmac_get_rx_hwtstamp() for
every packet. This patch move the calling of stmmac_get_rx_hwtstamp() earlier
so that rx timestamp is available before running bpf_prog_run_xdp(). So, i
think no additional cost introduced here. Any other thoughts?

Furthermore, stmmac_get_rx_hwtstamp() will check whether hw timestamp is
enabled in driver and  available in the descriptor before getting the hw timestamp.

>
>> +
>>  		if (!skb) {
>>  			unsigned int pre_len, sync_len;
>>
>> @@ -5315,7 +5317,7 @@ static int stmmac_rx(struct stmmac_priv *priv,
>> int limit, u32 queue)
>>
>>  			xdp_init_buff(&ctx.xdp, buf_sz, &rx_q->xdp_rxq);
>>  			xdp_prepare_buff(&ctx.xdp, page_address(buf->page),
>> -					 buf->page_offset, buf1_len, false);
>> +					 buf->page_offset, buf1_len, true);
>>
>>  			pre_len = ctx.xdp.data_end - ctx.xdp.data_hard_start -
>>  				  buf->page_offset;
>> @@ -5411,7 +5413,7 @@ static int stmmac_rx(struct stmmac_priv *priv,
>> int limit, u32 queue)
>>
>>  		shhwtstamp = skb_hwtstamps(skb);
>>  		memset(shhwtstamp, 0, sizeof(struct skb_shared_hwtstamps));
>> -		stmmac_get_rx_hwtstamp(priv, p, np, &shhwtstamp->hwtstamp);

Original stmmac_get_rx_hwtstamp() function is called here.

Thanks & Regards
Siang

>> +		shhwtstamp->hwtstamp = ctx.rx_hwts;
>>
>>  		stmmac_rx_vlan(priv->dev, skb);
>>  		skb->protocol = eth_type_trans(skb, priv->dev); @@ -7071,6
>+7073,22
>> @@ void stmmac_fpe_handshake(struct stmmac_priv *priv, bool enable)
>>  	}
>>  }
>>
>> +static int stmmac_xdp_rx_timestamp(const struct xdp_md *_ctx, u64
>> +*timestamp) {
>> +	const struct stmmac_xdp_buff *ctx = (void *)_ctx;
>> +
>> +	if (ctx->rx_hwts) {
>> +		*timestamp = ctx->rx_hwts;
>> +		return 0;
>> +	}
>> +
>> +	return -ENODATA;
>> +}
>> +
>> +const struct xdp_metadata_ops stmmac_xdp_metadata_ops = {
>> +	.xmo_rx_timestamp		= stmmac_xdp_rx_timestamp,
>> +};
>> +
>>  /**
>>   * stmmac_dvr_probe
>>   * @device: device pointer
>> @@ -7178,6 +7196,8 @@ int stmmac_dvr_probe(struct device *device,
>>
>>  	ndev->netdev_ops = &stmmac_netdev_ops;
>>
>> +	ndev->xdp_metadata_ops = &stmmac_xdp_metadata_ops;
>> +
>>  	ndev->hw_features = NETIF_F_SG | NETIF_F_IP_CSUM |
>NETIF_F_IPV6_CSUM |
>>  			    NETIF_F_RXCSUM;
>>  	ndev->xdp_features = NETDEV_XDP_ACT_BASIC |
>NETDEV_XDP_ACT_REDIRECT
>> |
>> --
>> 2.34.1
>>