[U-Boot] ethernet: ROM MAC address vs env variable MAC address

[U-Boot] ethernet: ROM MAC address vs env variable MAC address

[Prabhakar Kushwaha]

Hi All, 

I am seeing following type of code for MAC address write for a network device.

net/eth-uclass.c  eth_post_probe()   or net/eth_legacy.c eth_write_hwaddr()

	eth_env_get_enetaddr_by_index("eth", dev->seq, env_enetaddr);
	if (!is_zero_ethaddr(env_enetaddr)) {
		if (!is_zero_ethaddr(pdata->enetaddr) &&
		    memcmp(pdata->enetaddr, env_enetaddr, ARP_HLEN)) {
			printf("\nWarning: %s MAC addresses don't match:\n",
			       dev->name);
			printf("Address in ROM is          %pM\n",
			       pdata->enetaddr);
			printf("Address in environment is  %pM\n",
			       env_enetaddr);
		}

		/* Override the ROM MAC address */
		memcpy(pdata->enetaddr, env_enetaddr, ARP_HLEN);

Why ROM MAC address getting overwritten by environment env MAC address.  
MAC address is something unique and assigned to a particular device. So one should never change its MAC address.

Please suggest. 

Regards,
Prabhakar

[U-Boot] ethernet: ROM MAC address vs env variable MAC address

[Lukasz Majewski]

Hi Prabhakar,

> Hi All, 
> 
> I am seeing following type of code for MAC address write for a
> network device.
> 
> net/eth-uclass.c  eth_post_probe()   or net/eth_legacy.c
> eth_write_hwaddr()
> 
> 	eth_env_get_enetaddr_by_index("eth", dev->seq, env_enetaddr);
> 	if (!is_zero_ethaddr(env_enetaddr)) {
> 		if (!is_zero_ethaddr(pdata->enetaddr) &&
> 		    memcmp(pdata->enetaddr, env_enetaddr, ARP_HLEN)) {
> 			printf("\nWarning: %s MAC addresses don't
> match:\n", dev->name);
> 			printf("Address in ROM is          %pM\n",
> 			       pdata->enetaddr);
> 			printf("Address in environment is  %pM\n",
> 			       env_enetaddr);
> 		}
> 
> 		/* Override the ROM MAC address */
> 		memcpy(pdata->enetaddr, env_enetaddr, ARP_HLEN);
> 
> Why ROM MAC address getting overwritten by environment env MAC
> address. MAC address is something unique and assigned to a particular
> device. So one should never change its MAC address.
> 

It may happen that company which uses NXP processor wants to use their
own set of MAC addresses.

> Please suggest. 
> 
> Regards,
> Prabhakar
> 
> _______________________________________________
> U-Boot mailing list
> U-Boot at lists.denx.de
> https://lists.denx.de/listinfo/u-boot



Best regards,

Lukasz Majewski

--

DENX Software Engineering GmbH,      Managing Director: Wolfgang Denk
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany
Phone: (+49)-8142-66989-10 Fax: (+49)-8142-66989-80 Email: wd at denx.de
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[U-Boot] ethernet: ROM MAC address vs env variable MAC address

[Wolfgang Denk]

Dear Prabhakar,

In message <HE1PR04MB12410876E4BA09FC9CD7800C97560@HE1PR04MB1241.eurprd04.prod.outlook.com> you wrote:
> 
> Why ROM MAC address getting overwritten by environment env MAC address.  

Because in U-Boot we give the user the freedom to do what he
needs/wants to do.  Usually the environment value gets initialized
from the value in the ROM, so there is no difference anyway. But if
the user wants a specific setting, he can change it.

> MAC address is something unique and assigned to a particular device. So one should never change its MAC address.

U-Boot follows good old UNIX style here:

"UNIX was not designed to stop you from doing stupid things,  because
that would also stop you from doing clever things."       - Doug Gwyn




Best regards,

Wolfgang Denk

-- 
DENX Software Engineering GmbH,      Managing Director: Wolfgang Denk
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany
Phone: (+49)-8142-66989-10 Fax: (+49)-8142-66989-80 Email: wd at denx.de
If in any problem you find yourself doing an immense amount of  work,
the answer can be obtained by simple inspection.

[U-Boot] ethernet: ROM MAC address vs env variable MAC address

[Mike Looijmans]

[-- Warning: decoded text below may be mangled, UTF-8 assumed --]
[-- Attachment #1: Type: text/plain; charset="utf-8-sig", Size: 1594 bytes --]

On 13-11-17 20:49, Wolfgang Denk wrote:
> Dear Prabhakar,
> 
> In message <HE1PR04MB12410876E4BA09FC9CD7800C97560@HE1PR04MB1241.eurprd04.prod.outlook.com> you wrote:
>>
>> Why ROM MAC address getting overwritten by environment env MAC address.
> 
> Because in U-Boot we give the user the freedom to do what he
> needs/wants to do.  Usually the environment value gets initialized
> from the value in the ROM, so there is no difference anyway. But if
> the user wants a specific setting, he can change it.
> 
>> MAC address is something unique and assigned to a particular device. So one should never change its MAC address.
> 
> U-Boot follows good old UNIX style here:
> 
> "UNIX was not designed to stop you from doing stupid things,  because
> that would also stop you from doing clever things."       - Doug Gwyn

Many board manufacturers "assign" this unique MAC address by printing it on a 
sticker and sticking that on the board somewhere. It's pretty darn hard to 
read a printed sticker in software, so we have to revert to solutions that 
actually work.

In the "clever things" department, protocols like IPv6 merrily broadcast your 
MAC address across gateways on the big bad internet, so if you value your 
privacy, you'll appreciate the possibility to change your MAC address at will.



Kind regards,

Mike Looijmans
System Expert

TOPIC Products
Materiaalweg 4, NL-5681 RJ Best
Postbus 440, NL-5680 AK Best
Telefoon: +31 (0) 499 33 69 79
E-mail: mike.looijmans at topicproducts.com
Website: www.topicproducts.com

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