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

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Message-ID: <20220414151146.a2fncklswo6utiyd@skbuf>
Date:   Thu, 14 Apr 2022 18:11:46 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Clément Léger <clement.leger@...tlin.com>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzk+dt@...nel.org>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Magnus Damm <magnus.damm@...il.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Russell King <linux@...linux.org.uk>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        Herve Codina <herve.codina@...tlin.com>,
        Miquèl Raynal <miquel.raynal@...tlin.com>,
        Milan Stevanovic <milan.stevanovic@...com>,
        Jimmy Lalande <jimmy.lalande@...com>,
        linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
        linux-renesas-soc@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH net-next 02/12] net: dsa: add Renesas RZ/N1 switch tag
 driver

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?