Re: [PATCH net-next 02/12] net: dsa: add Renesas RZ/N1 switch tag driver

[Vladimir Oltean]

On Thu, Apr 14, 2022 at 04:35:46PM +0200, Clément Léger wrote:
> > Please keep variable declarations sorted in decreasing order of line
> > length (applies throughout the patch series, I won't repeat this comment).
> 
> Acked, both PCS and DSA driver are ok with that rule. Missed that one
> though.

Are you sure? Because a5psw_port_stp_state_set() says otherwise.

> > sizeof(tag), to be consistent with the other use of sizeof() above?
> > Although, hmm, I think you could get away with editing "ptag" in place.
> 
> I was not sure of the alignment guarantee I would have here. If the
> VLAN header is guaranteed to be aligned on 2 bytes, then I guess it's
> ok to do that in-place.

If I look at Documentation/core-api/unaligned-memory-access.rst

| Alignment vs. Networking
| ========================
| 
| On architectures that require aligned loads, networking requires that the IP
| header is aligned on a four-byte boundary to optimise the IP stack. For
| regular ethernet hardware, the constant NET_IP_ALIGN is used. On most
| architectures this constant has the value 2 because the normal ethernet
| header is 14 bytes long, so in order to get proper alignment one needs to
| DMA to an address which can be expressed as 4*n + 2. One notable exception
| here is powerpc which defines NET_IP_ALIGN to 0 because DMA to unaligned
| addresses can be very expensive and dwarf the cost of unaligned loads.

Your struct a5psw_tag *ptag starts at 10 bytes (8 for tag, 2 for Ethertype)
before the IP header, so I'm thinking it is aligned at a 2 byte boundary
as well. A VLAN header between the DSA header and the IP header should
also not affect that alignment, since its length is 4 bytes.

If "ctrl_tag" is aligned at a 4 byte boundary - 10, it means "ctrl_data"
is aligned at a 4 byte boundary - 8, so also a 4 byte boundary.

But "ctrl_data2" is aligned at a 4 byte boundary + 2, so you might want
to break up the u32 access into 2 u16 accesses, just to be on the safe
side?