On Tue, Oct 12, 2021 at 2:37 PM Alvin Šipraga [off-list ref] wrote:

From: Alvin Šipraga <alsi@bang-olufsen.dk>

This commit implements a basic version of the 8 byte tag protocol used
in the Realtek RTL8365MB-VC unmanaged switch, which carries with it a
protocol version of 0x04.

The implementation itself only handles the parsing of the EtherType
value and Realtek protocol version, together with the source or
destination port fields. The rest is left unimplemented for now.

The tag format is described in a confidential document provided to my
company by Realtek Semiconductor Corp. Permission has been granted by
the vendor to publish this driver based on that material, together with
an extract from the document describing the tag format and its fields.
It is hoped that this will help future implementors who do not have
access to the material but who wish to extend the functionality of
drivers for chips which use this protocol.

In addition, two possible values of the REASON field are specified,
based on experiments on my end. Realtek does not specify what value this
field can take.

Signed-off-by: Alvin Šipraga <alsi@bang-olufsen.dk>

The code definately looks good:
Reviewed-by: Linus Walleij <redacted>

Some nitpicky personal preferences below:

+#define RTL8_4_TAG_LEN                 8
+/* 0x04 = RTL8365MB DSA protocol
+ */
+#define RTL8_4_PROTOCOL_RTL8365MB      0x04

This is #defined

+       /* Zero FID_EN, FID, PRI_EN, PRI, KEEP; set LEARN_DIS */
+       tag[2] = htons(1 << 5);

I would create defines for the flags like this:
#define RTL8365MB_LEARN_DIS BIT(5)

+       /* Parse Protocol */
+       proto = ntohs(tag[1]) >> 8;

In the 4byte header code we have something like this:
#define RTL8_4_PROTOCOL_SHIFT 8

+       /* Parse TX (switch->CPU) */
+       port = ntohs(tag[3]) & 0xf; /* Port number is the LSB 4 bits */

This I think is fair enough. No need to define that mask.

Yours,
Linus Walleij