linux-kernel - Re: [PATCH net-next v7 04/11] net: ti: prueth: Adds link detection, RX and TX support.

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Message-ID: <ce36ce0e-ad16-4950-b601-ae1a555f2cfb@redhat.com>
Date: Tue, 6 May 2025 11:53:06 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Parvathi Pudi <parvathi@...thit.com>, danishanwar@...com,
 rogerq@...nel.org, andrew+netdev@...n.ch, davem@...emloft.net,
 edumazet@...gle.com, kuba@...nel.org, robh@...nel.org, krzk+dt@...nel.org,
 conor+dt@...nel.org, nm@...com, ssantosh@...nel.org, tony@...mide.com,
 richardcochran@...il.com, glaroque@...libre.com, schnelle@...ux.ibm.com,
 m-karicheri2@...com, s.hauer@...gutronix.de, rdunlap@...radead.org,
 diogo.ivo@...mens.com, basharath@...thit.com, horms@...nel.org,
 jacob.e.keller@...el.com, m-malladi@...com, javier.carrasco.cruz@...il.com,
 afd@...com, s-anna@...com
Cc: linux-arm-kernel@...ts.infradead.org, netdev@...r.kernel.org,
 devicetree@...r.kernel.org, linux-kernel@...r.kernel.org, pratheesh@...com,
 prajith@...com, vigneshr@...com, praneeth@...com, srk@...com, rogerq@...com,
 krishna@...thit.com, pmohan@...thit.com, mohan@...thit.com
Subject: Re: [PATCH net-next v7 04/11] net: ti: prueth: Adds link detection,
 RX and TX support.

On 5/3/25 3:11 PM, Parvathi Pudi wrote:
> +/**
> + * icssm_emac_rx_thread - EMAC Rx interrupt thread handler
> + * @irq: interrupt number
> + * @dev_id: pointer to net_device
> + *
> + * EMAC Rx Interrupt thread handler - function to process the rx frames in a
> + * irq thread function. There is only limited buffer at the ingress to
> + * queue the frames. As the frames are to be emptied as quickly as
> + * possible to avoid overflow, irq thread is necessary. Current implementation
> + * based on NAPI poll results in packet loss due to overflow at
> + * the ingress queues. Industrial use case requires loss free packet
> + * processing. Tests shows that with threaded irq based processing,
> + * no overflow happens when receiving at ~92Mbps for MTU sized frames and thus
> + * meet the requirement for industrial use case.

The above statement is highly suspicious. On an non idle system the
threaded irq can be delayed for an unbound amount of time. On an idle
system napi_poll should be invoked with a latency comparable - if not
less - to the threaded irq. Possibly you tripped on some H/W induced
latency to re-program the ISR?

In any case I think we need a better argumented statement to
intentionally avoid NAPI.

Cheers,

Paolo