netdev - Re: [EXTERNAL] [PATCH v7 net-next 6/8] net: ethernet: ti: am65-cpsw-qos: Add Frame Preemption MAC Merge support

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Message-ID: <20231204110249.nwayybk5q4uhsgsa@skbuf>
Date: Mon, 4 Dec 2023 13:02:49 +0200
From: Vladimir Oltean <vladimir.oltean@....com>
To: Roger Quadros <rogerq@...nel.org>
Cc: "Varis, Pekka" <p-varis@...com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"edumazet@...gle.com" <edumazet@...gle.com>,
	"kuba@...nel.org" <kuba@...nel.org>,
	"pabeni@...hat.com" <pabeni@...hat.com>,
	"Vadapalli, Siddharth" <s-vadapalli@...com>,
	"Gunasekaran, Ravi" <r-gunasekaran@...com>,
	"Raghavendra, Vignesh" <vigneshr@...com>,
	"Govindarajan, Sriramakrishnan" <srk@...com>,
	"horms@...nel.org" <horms@...nel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [EXTERNAL] [PATCH v7 net-next 6/8] net: ethernet: ti:
 am65-cpsw-qos: Add Frame Preemption MAC Merge support

On Mon, Dec 04, 2023 at 11:30:53AM +0200, Roger Quadros wrote:
> >> Due to Silicon Errata i2208 [2] we set limit min IET fragment size to 124.
> > 
> > Should be 128 not 124
> 
> User space setting is without FCS.

Technically it's called mCRC for preemptible frames, but yes.

> >> +	/* Errata i2208: RX min fragment size cannot be less than 124 */
> >> +	state->rx_min_frag_size = 124;
> > 
> > /* Errata i2208: RX min fragment size cannot be less than 128 */
> > state->rx_min_frag_size = 128;
> 
> ethtool man page says
> "           tx-min-frag-size
>                   Shows the minimum size (in octets) of transmitted non-
>                   final fragments which can be received by the link
>                   partner. Corresponds to the standard addFragSize
>                   variable using the formula:
> 
>                   tx-min-frag-size = 64 * (1 + addFragSize) - 4"
> 
> Which means user needs to put a -4 offset i.e. drop FCS size.
> 
> Drivers show rx-min-frag-size also without the FCS.
> 
> e.g.
> https://elixir.bootlin.com/linux/latest/source/drivers/net/ethernet/mscc/ocelot_mm.c#L260

Ack.