netdev - Re: net-shapers plan

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Message-ID: <20250410161611.5321eb9f@kernel.org>
Date: Thu, 10 Apr 2025 16:16:11 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Carolina Jubran <cjubran@...dia.com>
Cc: Cosmin Ratiu <cratiu@...dia.com>, "netdev@...r.kernel.org"
 <netdev@...r.kernel.org>, "horms@...nel.org" <horms@...nel.org>,
 "andrew+netdev@...n.ch" <andrew+netdev@...n.ch>, "davem@...emloft.net"
 <davem@...emloft.net>, Tariq Toukan <tariqt@...dia.com>, Gal Pressman
 <gal@...dia.com>, "jiri@...nulli.us" <jiri@...nulli.us>,
 "edumazet@...gle.com" <edumazet@...gle.com>, Saeed Mahameed
 <saeedm@...dia.com>, "pabeni@...hat.com" <pabeni@...hat.com>
Subject: Re: net-shapers plan

On Thu, 10 Apr 2025 18:23:56 +0300 Carolina Jubran wrote:
> We do configure the correct priority-to-queue mapping in the driver when 
> mqprio is used in DCB mode. In this setup, each traffic class has its 
> own dedicated Tx queue(s), and the driver programs the mapping 
> accordingly. The hardware performs its default priority check, sees that 
> the packet matches the configured queue, and proceeds to transmit 
> without taking any further action — everything behaves as expected.
> 
> When DCB mode is not enabled, there is no fixed mapping between traffic 
> classes and Tx queues. In this case, the hardware still performs the 
> check, and if it detects a mismatch, it moves the send queue to the 
> appropriate scheduling queue to maintain proper traffic class behavior.
> The priority check is always active by default, but when the mapping is 
> configured properly, it’s followed by a noop.

I hope you understand my concern, tho. Since you're providing the first
implementation, if the users can grow dependent on such behavior we'd
be in no position to explain later that it's just a quirk of mlx5 and
not how the API is intended to operate.