linux-kernel - Re: [PATCH net-next 7/8] net: ethernet: ti: am65-cpsw: prepare xmit/rx path for multi-port devices in mac-only mode

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Message-Id: <20201002.190931.2160541172091214230.davem@davemloft.net>
Date:   Fri, 02 Oct 2020 19:09:31 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     grygorii.strashko@...com
Cc:     netdev@...r.kernel.org, kuba@...nel.org, vigneshr@...com,
        nsekhar@...com, linux-kernel@...r.kernel.org,
        linux-omap@...r.kernel.org, m-karicheri2@...com
Subject: Re: [PATCH net-next 7/8] net: ethernet: ti: am65-cpsw: prepare
 xmit/rx path for multi-port devices in mac-only mode

From: Grygorii Strashko <grygorii.strashko@...com>
Date: Thu, 1 Oct 2020 13:52:57 +0300

> This patch adds multi-port support to TI AM65x CPSW driver xmit/rx path in
> preparation for adding support for multi-port devices, like Main CPSW0 on
> K3 J721E SoC or future CPSW3g on K3 AM64x SoC.
> Hence DMA channels are common/shared for all ext Ports and the RX/TX NAPI
> and DMA processing going to be assigned to first netdev this patch:
>  - ensures all RX descriptors fields are initialized;
>  - adds synchronization for TX DMA push/pop operation (locking) as
> Networking core is not enough any more;
>  - updates TX bql processing for every packet in
> am65_cpsw_nuss_tx_compl_packets() as every completed TX skb can have
> different ndev assigned (come from different netdevs).
> 
> Signed-off-by: Grygorii Strashko <grygorii.strashko@...com>

This locking is unnecessary in single-port non-shared DMA situations
and therefore will impose unnecessary performance loss for basically
all existing supported setups.

Please do this another way.

In fact, I would encourage you to find a way to avoid the new atomic
operations even in multi-port configurations.

Thank you.