Re: [PATCH v2] net/nfp: update how MAX MTU is read

[Walter Heymans <walter.heymans@corigine.com>]

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On 5/03/2022 9:05 PM, Ferruh Yigit wrote:
> On 4/20/2022 2:46 PM, Walter Heymans wrote:
> > The 'max_rx_pktlen' value was previously read from hardware, which was
> > set by the running firmware. This caused confusion due to different
> > meanings of 'MAX_MTU'. This patch updates the 'max_rx_pktlen' to the
> > maximum value that the NFP NIC can support. The 'max_mtu' value that is
> > read from hardware, is assigned to the 'dev_info->max_mtu' variable.
> >
> > If more layer 2 metadata must be used, the firmware can be updated to
> > report a smaller 'max_mtu' value.
> >
> > The constant defined for NFP_FRAME_SIZE_MAX is derived for the maximum
> > supported buffer size of 10240, minus 136 bytes that is reserved by the
> > hardware and another 56 bytes reserved for expansion in firmware. This
> > results in a usable maximum packet length of 10048 bytes.
> >
> > Signed-off-by: Walter Heymans <walter.heymans@corigine.com>
> > Signed-off-by: Niklas Söderlund <niklas.soderlund@corigine.com>
> > Reviewed-by: Louis Peens <louis.peens@corigine.com>
> > Reviewed-by: Chaoyong He <chaoyong.he@corigine.com>
> > Reviewed-by: Richard Donkin <richard.donkin@corigine.com>
> > ---
> >   drivers/net/nfp/nfp_common.c | 11 ++++++++++-
> >   drivers/net/nfp/nfp_common.h |  3 +++
> >   2 files changed, 13 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/net/nfp/nfp_common.c b/drivers/net/nfp/nfp_common.c
> > index b26770dbfb..52fbda1a79 100644
> > --- a/drivers/net/nfp/nfp_common.c
> > +++ b/drivers/net/nfp/nfp_common.c
> > @@ -692,7 +692,16 @@ nfp_net_infos_get(struct rte_eth_dev *dev, struct rte_eth_dev_info *dev_info)
> >        dev_info->max_rx_queues = (uint16_t)hw->max_rx_queues;
> >        dev_info->max_tx_queues = (uint16_t)hw->max_tx_queues;
> >        dev_info->min_rx_bufsize = RTE_ETHER_MIN_MTU;
> > -     dev_info->max_rx_pktlen = hw->max_mtu;
> > +     /*
> > +      * The maximum rx packet length (max_rx_pktlen) is set to the
> > +      * maximum supported frame size that the NFP can handle. This
> > +      * includes layer 2 headers, CRC and other metadata that can
> > +      * optionally be used.
> > +      * The maximum layer 3 MTU (max_mtu) is read from hardware,
> > +      * which was set by the firmware loaded onto the card.
> > +      */
> > +     dev_info->max_rx_pktlen = NFP_FRAME_SIZE_MAX;
> > +     dev_info->max_mtu = hw->max_mtu;
>
> Hi Walter,
>
> ethdev uses 'max_mtu' and 'max_rx_pktlen' to calculate the frame
> overhead, like:
> 'overhead_len = max_rx_pktlen - max_mtu;'
>
> In above change 'max_rx_pktlen' is hardcoded but 'max_mtu' is 'hw'
> depended, can you confirm 'hw->max_mtu' will always have the frame
> overhead as according above calculation?

Hi Ferruh,
​
The 'max_rx_pktlen' is hardcoded to the maximum packet length that
the hardware can support. The 'hw->max_mtu' is read from hardware,
but it is a variable value, depending on the firmware running on the
card. The firmware sets this value to account for the maximum overhead
required. To answer you question, yes, this value should always be
available.
________________________________
From: Ferruh Yigit <ferruh.yigit@xilinx.com>
Sent: 03 May 2022 21:04
To: Walter Heymans <walter.heymans@corigine.com>; dev@dpdk.org <dev@dpdk.org>
Cc: Niklas Soderlund <niklas.soderlund@corigine.com>; Louis Peens <louis.peens@corigine.com>; Chaoyong He <chaoyong.he@corigine.com>; Richard Donkin <richard.donkin@corigine.com>
Subject: Re: [PATCH v2] net/nfp: update how MAX MTU is read

On 4/20/2022 2:46 PM, Walter Heymans wrote:
> The 'max_rx_pktlen' value was previously read from hardware, which was
> set by the running firmware. This caused confusion due to different
> meanings of 'MAX_MTU'. This patch updates the 'max_rx_pktlen' to the
> maximum value that the NFP NIC can support. The 'max_mtu' value that is
> read from hardware, is assigned to the 'dev_info->max_mtu' variable.
>
> If more layer 2 metadata must be used, the firmware can be updated to
> report a smaller 'max_mtu' value.
>
> The constant defined for NFP_FRAME_SIZE_MAX is derived for the maximum
> supported buffer size of 10240, minus 136 bytes that is reserved by the
> hardware and another 56 bytes reserved for expansion in firmware. This
> results in a usable maximum packet length of 10048 bytes.
>
> Signed-off-by: Walter Heymans <walter.heymans@corigine.com>
> Signed-off-by: Niklas Söderlund <niklas.soderlund@corigine.com>
> Reviewed-by: Louis Peens <louis.peens@corigine.com>
> Reviewed-by: Chaoyong He <chaoyong.he@corigine.com>
> Reviewed-by: Richard Donkin <richard.donkin@corigine.com>
> ---
>   drivers/net/nfp/nfp_common.c | 11 ++++++++++-
>   drivers/net/nfp/nfp_common.h |  3 +++
>   2 files changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/net/nfp/nfp_common.c b/drivers/net/nfp/nfp_common.c
> index b26770dbfb..52fbda1a79 100644
> --- a/drivers/net/nfp/nfp_common.c
> +++ b/drivers/net/nfp/nfp_common.c
> @@ -692,7 +692,16 @@ nfp_net_infos_get(struct rte_eth_dev *dev, struct rte_eth_dev_info *dev_info)
>        dev_info->max_rx_queues = (uint16_t)hw->max_rx_queues;
>        dev_info->max_tx_queues = (uint16_t)hw->max_tx_queues;
>        dev_info->min_rx_bufsize = RTE_ETHER_MIN_MTU;
> -     dev_info->max_rx_pktlen = hw->max_mtu;
> +     /*
> +      * The maximum rx packet length (max_rx_pktlen) is set to the
> +      * maximum supported frame size that the NFP can handle. This
> +      * includes layer 2 headers, CRC and other metadata that can
> +      * optionally be used.
> +      * The maximum layer 3 MTU (max_mtu) is read from hardware,
> +      * which was set by the firmware loaded onto the card.
> +      */
> +     dev_info->max_rx_pktlen = NFP_FRAME_SIZE_MAX;
> +     dev_info->max_mtu = hw->max_mtu;

Hi Walter,

ethdev uses 'max_mtu' and 'max_rx_pktlen' to calculate the frame
overhead, like:
'overhead_len = max_rx_pktlen - max_mtu;'

In above change 'max_rx_pktlen' is hardcoded but 'max_mtu' is 'hw'
depended, can you confirm 'hw->max_mtu' will always have the frame
overhead as according above calculation?


>        /* Next should change when PF support is implemented */
>        dev_info->max_mac_addrs = 1;
>
> diff --git a/drivers/net/nfp/nfp_common.h b/drivers/net/nfp/nfp_common.h
> index 8b35fa119c..8db5ec23f8 100644
> --- a/drivers/net/nfp/nfp_common.h
> +++ b/drivers/net/nfp/nfp_common.h
> @@ -98,6 +98,9 @@ struct nfp_net_adapter;
>   /* Number of supported physical ports */
>   #define NFP_MAX_PHYPORTS    12
>
> +/* Maximum supported NFP frame size (MTU + layer 2 headers) */
> +#define NFP_FRAME_SIZE_MAX   10048
> +
>   #include <linux/types.h>
>   #include <rte_io.h>
>


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