netdev - Re: [PATCH 07/14] net: ks8851: Use 16-bit writes to program MAC address

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Date:   Tue, 24 Mar 2020 13:36:23 +0100
From:   Lukas Wunner <lukas@...ner.de>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Marek Vasut <marex@...x.de>, netdev@...r.kernel.org,
        "David S . Miller" <davem@...emloft.net>,
        Petr Stetiar <ynezz@...e.cz>,
        YueHaibing <yuehaibing@...wei.com>
Subject: Re: [PATCH 07/14] net: ks8851: Use 16-bit writes to program MAC
 address

On Tue, Mar 24, 2020 at 01:25:53PM +0100, Andrew Lunn wrote:
> On Tue, Mar 24, 2020 at 09:13:11AM +0100, Lukas Wunner wrote:
> > On Tue, Mar 24, 2020 at 12:42:56AM +0100, Marek Vasut wrote:
> > > On the SPI variant of KS8851, the MAC address can be programmed with
> > > either 8/16/32-bit writes. To make it easier to support the 16-bit
> > > parallel option of KS8851 too, switch both the MAC address programming
> > > and readout to 16-bit operations.
> > [...]
> > >  static int ks8851_write_mac_addr(struct net_device *dev)
> > >  {
> > >  	struct ks8851_net *ks = netdev_priv(dev);
> > > +	u16 val;
> > >  	int i;
> > >  
> > >  	mutex_lock(&ks->lock);
> > > @@ -358,8 +329,12 @@ static int ks8851_write_mac_addr(struct net_device *dev)
> > >  	 * the first write to the MAC address does not take effect.
> > >  	 */
> > >  	ks8851_set_powermode(ks, PMECR_PM_NORMAL);
> > > -	for (i = 0; i < ETH_ALEN; i++)
> > > -		ks8851_wrreg8(ks, KS_MAR(i), dev->dev_addr[i]);
> > > +
> > > +	for (i = 0; i < ETH_ALEN; i += 2) {
> > > +		val = (dev->dev_addr[i] << 8) | dev->dev_addr[i + 1];
> > > +		ks8851_wrreg16(ks, KS_MAR(i + 1), val);
> > > +	}
> > > +
> > 
> > This looks like it won't work on little-endian machines:  The MAC bytes
> > are stored in dev->dev_addr as 012345, but in the EEPROM they're stored
> > as 543210.  The first 16-bit value that you write is 10 on big-endian
> > and 01 on little-endian if I'm not mistaken.
> > 
> > By only writing 8-bit values, the original author elegantly sidestepped
> > this issue.
> > 
> > Maybe the simplest and most readable solution is something like:
> > 
> >       u8 val[2];
> >       ...
> >       val[0] = dev->dev_addr[i+1];
> >       val[1] = dev->dev_addr;
> > 
> > Then cast val to a u16 when passing it to ks8851_wrreg16().
> > 
> > Alternatively, use cpu_to_be16().
> 
> There is a cpu_to_be16() inside ks8851_wrreg16(). Something i already
> checked, because i wondered about endianess issues as well.

There's a cpu_to_le16() in ks8851_wrreg16(), not a cpu_to_be16().

Thanks,

Lukas