linux-kernel - Re: [PATCH net] net: ethernet: ti: am65-cpsw: default to round-robin for host port receive

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Message-ID: <eb573f51-4363-4cd7-ab1b-0d7304820d85@intel.com>
Date: Fri, 20 Dec 2024 11:41:26 +0100
From: Przemek Kitszel <przemyslaw.kitszel@...el.com>
To: Siddharth Vadapalli <s-vadapalli@...com>
CC: <stable@...r.kernel.org>, <netdev@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <andrew+netdev@...n.ch>,
	<davem@...emloft.net>, <edumazet@...gle.com>, <kuba@...nel.org>,
	<pabeni@...hat.com>, <rogerq@...nel.org>, <horms@...nel.org>,
	<dan.carpenter@...aro.org>, <c-vankar@...com>, <jpanis@...libre.com>,
	<npitre@...libre.com>, <linux-arm-kernel@...ts.infradead.org>,
	<vigneshr@...com>, <srk@...com>
Subject: Re: [PATCH net] net: ethernet: ti: am65-cpsw: default to round-robin
 for host port receive

On 12/20/24 08:56, Siddharth Vadapalli wrote:
> The Host Port (i.e. CPU facing port) of CPSW receives traffic from Linux
> via TX DMA Channels which are Hardware Queues consisting of traffic
> categorized according to their priority. The Host Port is configured to
> dequeue traffic from these Hardware Queues on the basis of priority i.e.
> as long as traffic exists on a Hardware Queue of a higher priority, the
> traffic on Hardware Queues of lower priority isn't dequeued. An alternate
> operation is also supported wherein traffic can be dequeued by the Host
> Port in a Round-Robin manner.
> 
> Until [0], the am65-cpsw driver enabled a single TX DMA Channel, due to
> which, unless modified by user via "ethtool", all traffic from Linux is
> transmitted on DMA Channel 0. Therefore, configuring the Host Port for
> priority based dequeuing or Round-Robin operation is identical since
> there is a single DMA Channel.
> 
> Since [0], all 8 TX DMA Channels are enabled by default. Additionally,
> the default "tc mapping" doesn't take into account the possibility of
> different traffic profiles which various users might have. This results
> in traffic starvation at the Host Port due to the priority based dequeuing
> which has been enabled by default since the inception of the driver. The
> traffic starvation triggers NETDEV WATCHDOG timeout for all TX DMA Channels
> that haven't been serviced due to the presence of traffic on the higher
> priority TX DMA Channels.

I get it right that the starving is caused by HW/DMA (not SW)

> 
> Fix this by defaulting to Round-Robin dequeuing at the Host Port, which
> shall ensure that traffic is dequeued from all TX DMA Channels irrespective
> of the traffic profile. This will address the NETDEV WATCHDOG timeouts.
> At the same time, users can still switch from Round-Robin to Priority
> based dequeuing at the Host Port with the help of the "p0-rx-ptype-rrobin"

why the flag has rx in the name?

> private flag of "ethtool". Users are expected to setup an appropriate
> "tc mapping" that suits their traffic profile when switching to priority
> based dequeuing at the Host Port.
> 
> [0] commit be397ea3473d ("net: ethernet: am65-cpsw: Set default TX channels to maximum")
> Fixes: be397ea3473d ("net: ethernet: am65-cpsw: Set default TX channels to maximum")
> Cc: <stable@...r.kernel.org>
> Signed-off-by: Siddharth Vadapalli <s-vadapalli@...com>
> ---
> 
> Hello,
> 
> This patch is based on commit
> 8faabc041a00 Merge tag 'net-6.13-rc4' of git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net
> of Mainline Linux.
> 
> Regards,
> Siddharth.
> 
>   drivers/net/ethernet/ti/am65-cpsw-nuss.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/ti/am65-cpsw-nuss.c b/drivers/net/ethernet/ti/am65-cpsw-nuss.c
> index 14e1df721f2e..5465bf872734 100644
> --- a/drivers/net/ethernet/ti/am65-cpsw-nuss.c
> +++ b/drivers/net/ethernet/ti/am65-cpsw-nuss.c
> @@ -3551,7 +3551,7 @@ static int am65_cpsw_nuss_probe(struct platform_device *pdev)
>   	init_completion(&common->tdown_complete);
>   	common->tx_ch_num = AM65_CPSW_DEFAULT_TX_CHNS;
>   	common->rx_ch_num_flows = AM65_CPSW_DEFAULT_RX_CHN_FLOWS;
> -	common->pf_p0_rx_ptype_rrobin = false;
> +	common->pf_p0_rx_ptype_rrobin = true;
>   	common->default_vlan = 1;
>   
>   	common->ports = devm_kcalloc(dev, common->port_num,