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

[Ferruh Yigit <ferruh.yigit@xilinx.com>]

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>
>