RE: [PATCH net-next v2 10/10] net: ethernet: adi: Add a driver for the ADIN1140 MACPHY

[Regus, Ciprian]

> > +static netdev_tx_t adin1140_start_xmit(struct sk_buff *skb,
> > +				       struct net_device *netdev)
> > +{
> > +	struct adin1140_priv *priv = netdev_priv(netdev);
> > +
> > +	/* Pad frames to minimum Ethernet frame size (60 bytes without
> FCS).
> > +	 * The MAC will append the FCS, but we need to ensure the frame is
> > +	 * at least ETH_ZLEN bytes.
> > +	 */
> > +	if (skb_put_padto(skb, ETH_ZLEN))
> > +		return NETDEV_TX_OK;
> 
> Does the standard say anything about this? It seems like something
> which could be moved into the core.

The comment is a mistake on my part, as it doesn't clearly describe why we
have to pad the skb. It's a case of a MAC device which doesn't pad a frame to
a minimum size when the host wants to transmit less than 64 bytes. I'll update
it in v3 to say:

/* The MAC doesn't automatically pad the frame to a 64 byte minimum size in
 * case the host sent a shorter skb, so we have to do it in the driver. The FCS
 * will be added by the MAC.
 */

As for what the OA TC6 standard says, this is the relevant section
(7.3 - Data Transaction Protocol for Ethernet Frames)

"Ethernet frames are typically transferred from the SPI host to the MAC-PHY without
any padding or frame check sequence (FCS). The MAC will automatically pad the Ethernet
frame to the minimum frame size of 64 bytes and append a computed FCS. However, the
Ethernet specification allows for the SPI host to optionally perform the frame padding and
FCS computation prior to transfer to the MAC-PHY. Similarly, the MAC-PHY will typically
strip the FCS from received Ethernet frames prior to transfer to the SPI host. However,
the Ethernet specification allows the option for the Ethernet frame to be transferred to the
MAC client with the FCS.

The IEEE Ethernet standard [2] defines the behavior of the MAC and therefore is beyond
the scope of this specification. As a result, support for allowing the SPI host to perform frame
padding and FCS computation, or passing the FCS to the SPI host is optional. When supported,
the method for configuring the MAC-PHY to enable these modes of frame transfer is
implementation specific."

As I understand, from the TC6 standard point of view, the padding and FCS offload is optional.
So, I think this shouldn't go into the core. 

> 
> 	Andrew