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

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Date:   Tue, 24 Mar 2020 09:13:11 +0100
From:   Lukas Wunner <lukas@...ner.de>
To:     Marek Vasut <marex@...x.de>
Cc:     netdev@...r.kernel.org, "David S . Miller" <davem@...emloft.net>,
        Petr Stetiar <ynezz@...e.cz>,
        YueHaibing <yuehaibing@...wei.com>, Andrew Lunn <andrew@...n.ch>
Subject: Re: [PATCH 07/14] net: ks8851: Use 16-bit writes to program MAC
 address

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().


>  static void ks8851_read_mac_addr(struct net_device *dev)
>  {
>  	struct ks8851_net *ks = netdev_priv(dev);
> +	u16 reg;
>  	int i;
>  
>  	mutex_lock(&ks->lock);
>  
> -	for (i = 0; i < ETH_ALEN; i++)
> -		dev->dev_addr[i] = ks8851_rdreg8(ks, KS_MAR(i));
> +	for (i = 0; i < ETH_ALEN; i += 2) {
> +		reg = ks8851_rdreg16(ks, KS_MAR(i + 1));
> +		dev->dev_addr[i] = reg & 0xff;
> +		dev->dev_addr[i + 1] = reg >> 8;
> +	}

Same here.

These seem to be the only two places where KS_MAR() is used.
You may want to adjust that macro so that you don't have to add 1
in each of the two places.

Thanks,

Lukas