[PATCH] net: lwip: add TFTPSERVERIP Kconfig option

[Quentin Schulz]

Hi Jonas,

On 1/14/26 7:29 PM, Jonas Karlman wrote:
> Hi Quentin,
> 
> On 1/14/2026 6:08 PM, Quentin Schulz wrote:
>> Hi Jonas,
>>
>> On 1/14/26 4:37 PM, Jonas Karlman wrote:
>>> Hi Quentin,
>>>
>>> On 1/14/2026 4:01 PM, Quentin Schulz wrote:
>>>> Hi Jonas,
>>>>
>>>> On 1/6/26 9:34 PM, Jonas Karlman wrote:
>>>>> With the legacy networking stack, it is possible to use USE_SERVERIP,
>>>>> SERVERIP and BOOTP_PREFER_SERVERIP Kconfg options to force use of a
>>>>> specific TFTP server ip.
>>>>>
>>>>> Using the lwIP networking stack use of the 'tftpserverip' environment
>>>>> variable provide the closest equivalent functionality.
>>>>>
>>>>
>>>> It does read serverip environment variable if it's set, so this should
>>>> match the behavior with the legacy network stack no?
>>>
>>> This is what I first thought, however it seem that the lwip stack differ
>>> slightly from the legacy stack, in that the dhcp lookup always seem to
>>> reset the serverip env var.
>>>
>>>> As for BOOTP_PREFER_SERVERIP, this would need to be implemented in lwip
>>>> and isn't covered by this new variable is it?
>>>
>>> For the lwip stack it looks like the BOOTP_PREFER_SERVERIP Kconfig
>>> option possible was replaced with use of a 'tftpserverip' env var.
>>>
>>> For legacy stack I had something like following in a .config fragment:
>>>
>>>     CONFIG_BOOTP_PREFER_SERVERIP=y
>>>     CONFIG_SERVERIP="192.168.2.50"
>>>     CONFIG_USE_SERVERIP=y
>>>
>>
>> And this meant the serverip would not be overwritten by the dhcp command
>> for example (in the legacy stack)? When doing PXE/TFTP, I always do
>>
>> setenv autoload no; dhcp; setenv serverip X.Y.Z.A; pxe get; pxe boot
> 
> I tried to keep it as simple as possible, as I have not used network
> boot before, and used something like following:
> 
>    env set boot_targets pxe; bootflow scan -lbG
> 
> and when I need to use a USB Ethernet dongle I use something like:
> 
>    env delete -f ethaddr; env set boot_targets usb pxe; bootflow scan -lbG
> 
>> When I control the DHCP server, I can actually set the TFTP server
>> directly and can skip setting the serverip as it'll be set by the dhcp
>> command correctly it seems?
> 
> For my homelab I use UniFi and should be able to control DHCP options,
> however looking at U-Boot it looks like option 66 or 150 is just ignored,
> see [1].
> 
>    case 66:	/* Ignore TFTP server name */
> 
> [1] https://source.denx.de/u-boot/u-boot/-/blob/master/net/bootp.c?ref_type=heads#L952
> 
>  From my very limited testing it looks like I am forced to set serverip
> in env and cannot use a DHCP option to set the TFTP server to use.
> 

Still on my RK3588 Tiger (on a different network, legacy stack):

=> env set boot_targets pxe; bootflow scan -lbG
Scanning for bootflows in all bootdevs
Seq  Method       State   Uclass    Part  Name                      Filename
---  -----------  ------  --------  ----  ------------------------ 
----------------
pcie_dw_rockchip pcie at fe150000: PCIe-0 Link Fail
Scanning bootdev 'ethernet at fe1b0000.bootdev':
ethernet at fe1b0000 Waiting for PHY auto negotiation to complete...... done
BOOTP broadcast 1
DHCP client bound to address 192.168.1.144 (5 ms)
missing environment variable: pxeuuid
Retrieving file: pxelinux.cfg/01-aa-38-40-51-be-af
Using ethernet at fe1b0000 device
TFTP from server 192.168.1.211; our IP address is 192.168.1.144

My DHCP server is on OpenWRT at 192.168.1.1, and my TFTP server is on 
192.168.1.211, with the setup steps explained in my first answer. If I 
don't boot (remove -b), then I can see serverip is set to 192.168.1.211.

So *something* is working here.

> And when I then also tried lwIP, and that seem to result in new or
> different issues, e.g.:
> 
> - lwIP seem to be configured to allow very few packet fragments, trying
>    to use a reasonoble tftp blocksize, e.g. TFTP_BLOCKSIZE=4096 or more
>    seem to cause drop of packets.
> 
> - lwIP is very very slow to timeout compared to legacy when serverip
>    point to an invalid TFTP server and pxelinux.cfg files is looked up.
> 
> - sometimes got hit with random SError issues when second file was
>    loaded from TFTP, this issue was thankfully fixed with [2].
>    

Rough first experience with lwIP :)

Are you planning on new Rockchip board/SoC support to use lwIP?

> [2] https://patchwork.ozlabs.org/project/uboot/patch/20260114-tftp_fix-v2-1-8de8e6ecf4e8@linaro.org/
> 
>>
>>   From my notes, for an OpenWRT DHCP server:
>>
>> Network → DHCP and DNS → PXE/TFTP tab → Add button
>>       Filename: default
>>
>>       Server name: fileserver
>>
>>       Server address: <TFTP server IP address>
>>
>>       Force: checked (may not be necessary)
>>
>> Save & Apply button
>>
>> I'm assuming you need this for your labgrid farm so hopefully you have a
>> way to control the DHCP server :)
> 
> Correct, I have been playing around with network boot in my labgrid farm
> and hit a few mines along the way :-)
> 
> Should result in a few network related patches in next few days.
> 
>>
>>> Adding NET_LWIP=y made pxe booting stop working and 'serverip' always
>>> ended up as the dhpc server regardless if it was set to something else.
>>>
>>
>> Interesting. With tiger-rk3588_defconfig (legacy stack), I get:
>>
>> => setenv serverip 192.192.192.192
>> => setenv autoload no
>> => dhcp
>> ethernet at fe1b0000 Waiting for PHY auto negotiation to complete........ done
>> BOOTP broadcast 1
>> BOOTP broadcast 2
>> BOOTP broadcast 3
>> DHCP client bound to address 10.2.162.193 (1394 ms)
>> => printenv serverip
>> serverip=10.2.0.122
>>
>>> Since lwip instead prioritize the 'tftpserverip' env over the 'serverip'
>>> env for tftp use, adding a similar env var Kconfig option was easy but
>>> may be wrong path?
>>>
>>
>> I don't mind the addition but I'm wondering if it's not a band-aid patch
>> for something else? Maybe lwip missing setting the BOOTP server from a
>> DHCP lease for example?
> 
> Possible, setting tftp from dhcp also seem to be missing from legacy
> stack, if I am not mistaken. I do think the separation of tftpserverip
> and serverip for lwIP stack at least is a step in right direction, I am
> new to network booting from U-Boot so what do I know :-)
> 

I'm not sure the point of lwip is to keep all the quirks from the legacy 
stack, but it doesn't hurt to bring this up :)

Cheers,
Quentin