netdev - Re: [net-next 04/10] net/mlx5e: Unify constants for WQE_EMPTY_DS

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Date:   Tue, 8 Sep 2020 11:06:25 +0200
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Maxim Mikityanskiy <maximmi@...dia.com>
Cc:     Saeed Mahameed <saeedm@...dia.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Network Development <netdev@...r.kernel.org>,
        Maxim Mikityanskiy <maximmi@...lanox.com>
Subject: Re: [net-next 04/10] net/mlx5e: Unify constants for WQE_EMPTY_DS_COUNT

On Tue, Sep 8, 2020 at 10:59 AM Maxim Mikityanskiy <maximmi@...dia.com> wrote:
>
> On 2020-09-04 18:05, Willem de Bruijn wrote:
> > On Thu, Sep 3, 2020 at 11:01 PM Saeed Mahameed <saeedm@...dia.com> wrote:
> >>
> >> From: Maxim Mikityanskiy <maximmi@...lanox.com>
> >>
> >> A constant for the number of DS in an empty WQE (i.e. a WQE without data
> >> segments) is needed in multiple places (normal TX data path, MPWQE in
> >> XDP), but currently we have a constant for XDP and an inline formula in
> >> normal TX. This patch introduces a common constant.
> >>
> >> Additionally, mlx5e_xdp_mpwqe_session_start is converted to use struct
> >> assignment, because the code nearby is touched.
> >>
> >> Signed-off-by: Maxim Mikityanskiy <maximmi@...lanox.com>
> >> Signed-off-by: Saeed Mahameed <saeedm@...dia.com>
> >> ---
> >>   .../net/ethernet/mellanox/mlx5/core/en/txrx.h |  2 ++
> >>   .../net/ethernet/mellanox/mlx5/core/en/xdp.c  | 13 +++++++-----
> >>   .../net/ethernet/mellanox/mlx5/core/en/xdp.h  | 21 +++++++------------
> >>   .../net/ethernet/mellanox/mlx5/core/en_tx.c   |  2 +-
> >>   4 files changed, 19 insertions(+), 19 deletions(-)
> >>
> >> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en/txrx.h b/drivers/net/ethernet/mellanox/mlx5/core/en/txrx.h
> >> index d4ee22789ab0..155b89998891 100644
> >> --- a/drivers/net/ethernet/mellanox/mlx5/core/en/txrx.h
> >> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en/txrx.h
> >> @@ -7,6 +7,8 @@
> >>   #include "en.h"
> >>   #include <linux/indirect_call_wrapper.h>
> >>
> >> +#define MLX5E_TX_WQE_EMPTY_DS_COUNT (sizeof(struct mlx5e_tx_wqe) / MLX5_SEND_WQE_DS)
> >> +
> >
> > Out of curiosity, what is the logic for dividing this struct by 16?
>
> The hardware needs the size of a WQE in DS units (16 bytes). An empty
> WQE takes 2 DS (for the ctrl and eth segments), and this macro is this
> initial size of an empty WQE (2 DS). As data segments are added to the
> WQE, it grows, and its size in DS also grows.
>
> > struct mlx5e_tx_wqe {
> >          struct mlx5_wqe_ctrl_seg ctrl;
> >          struct mlx5_wqe_eth_seg  eth;
> >          struct mlx5_wqe_data_seg data[0];
> > };

Thanks. It was not obvious to me that the first two are the size as
data_segs. But that actually is pretty logical. Ack.