netdev - Re: [PATCH net-next v9 2/2] net: ti: icssg_prueth: add TAPRIO offload support

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Date: Mon, 3 Jun 2024 17:05:59 +0300
From: Vladimir Oltean <vladimir.oltean@....com>
To: MD Danish Anwar <danishanwar@...com>
Cc: Jan Kiszka <jan.kiszka@...mens.com>,
	Dan Carpenter <dan.carpenter@...aro.org>,
	Andrew Lunn <andrew@...n.ch>, Simon Horman <horms@...nel.org>,
	Diogo Ivo <diogo.ivo@...mens.com>,
	Wolfram Sang <wsa+renesas@...g-engineering.com>,
	Randy Dunlap <rdunlap@...radead.org>,
	Niklas Schnelle <schnelle@...ux.ibm.com>,
	Vignesh Raghavendra <vigneshr@...com>,
	Richard Cochran <richardcochran@...il.com>,
	Roger Quadros <rogerq@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Eric Dumazet <edumazet@...gle.com>,
	"David S. Miller" <davem@...emloft.net>,
	linux-arm-kernel@...ts.infradead.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, srk@...com,
	Jacob Keller <jacob.e.keller@...el.com>,
	Roger Quadros <rogerq@...com>
Subject: Re: [PATCH net-next v9 2/2] net: ti: icssg_prueth: add TAPRIO
 offload support

On Mon, Jun 03, 2024 at 04:51:00PM +0300, Vladimir Oltean wrote:
> > >> +static void tas_reset(struct prueth_emac *emac)
> > >> +{
> > >> +	struct tas_config *tas = &emac->qos.tas.config;
> > >> +	int i;
> > >> +
> > >> +	for (i = 0; i < TAS_MAX_NUM_QUEUES; i++)
> > >> +		tas->max_sdu_table.max_sdu[i] = 2048;
> > > 
> > > Macro + short comment for the magic number, please.
> > > 
> > 
> > Sure I will add it. Each elements in this array is a 2 byte value
> > showing the maximum length of frame to be allowed through each gate.
> 
> Is the queueMaxSDU[] array active even with the TAS being in the reset
> state? Does this configuration have any impact upon the device MTU?
> I don't know why 2048 was chosen.

Another comment here is: in the tc-taprio UAPI, a max-sdu value of 0
is special and means "no maxSDU limit for this TX queue". You are
programming the values from taprio straight away to hardware, so,
assuming there's no bug there, it means that the hardware also
understands 0 to mean "no maxSDU limit".

If so, then during tas_reset(), after which the TAS should be disabled,
why aren't you also using 0 as a default value, but 2048?