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

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Message-ID: <be4c96dc-87ab-27a9-cf51-c1e54853b528@denx.de>
Date:   Tue, 24 Mar 2020 14:09:18 +0100
From:   Marek Vasut <marex@...x.de>
To:     Lukas Wunner <lukas@...ner.de>, Andrew Lunn <andrew@...n.ch>
Cc:     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 3/24/20 1:36 PM, Lukas Wunner wrote:
> 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().

I have a feeling this whole thing might be more messed up then we
thought. At least the KS8851-16MLL has an "endian mode" bit in the CCR
register, the SPI variant does not.

So what I think you need to do here is write exactly the registers
0x14/0x12/0x10 and let the accessors swap the endianness as needed.