netdev - Re: [PATCH repost net-next] dsa: mv88e6xxx: Optimise atu

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Date:   Wed, 4 Jan 2017 22:19:57 +0100
From:   Andrew Lunn <andrew@...n.ch>
To:     David Miller <davem@...emloft.net>
Cc:     volodymyr.bendiuga@...il.com, vivien.didelot@...oirfairelinux.com,
        f.fainelli@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH repost net-next] dsa: mv88e6xxx: Optimise atu_get

On Wed, Jan 04, 2017 at 04:11:03PM -0500, David Miller wrote:
> From: Andrew Lunn <andrew@...n.ch>
> Date: Wed,  4 Jan 2017 19:56:24 +0100
> 
> > +static inline u64 ether_addr_to_u64(const u8 *addr)
> > +{
> > +	u64 u = 0;
> > +	int i;
> > +
> > +	for (i = 0; i < ETH_ALEN; i++)
> > +		u = u << 8 | addr[i];
> > +
> > +	return u;
> > +}
>  ...
> > +static inline void u64_to_ether_addr(u64 u, u8 *addr)
> > +{
> > +	int i;
> > +
> > +	for (i = ETH_ALEN - 1; i >= 0; i--) {
> > +		addr[i] = u & 0xff;
> > +		u = u >> 8;
> > +	}
> > +}
> 
> I think these two routines behave differently on big vs little
> endian.  And I doubt this was your intention.

I don't have a big endian system to test on.

I tried to avoid the usual pitfalls. I don't cast a collection of
bytes to a u64, which i know has no chance of working. Accessing a MAC
address as a byte array should be endian safe. The shift operation
should also be endian safe.

What exactly do you think will behave differently?

     Andrew