[U-Boot] [PATCH] net: BOOTP retry timeout improvements

[Stephen Warren]

On 08/15/2014 06:49 AM, Thierry Reding wrote:
> On Fri, Aug 15, 2014 at 02:39:45PM +0200, Thierry Reding wrote:
>> On Fri, Jul 25, 2014 at 05:30:48PM -0600, Stephen Warren wrote:
>>> From: Stephen Warren <swarren at nvidia.com>
>>>
>>> Currently, the BOOTP code sends out its initial request as soon as the
>>> Ethernet driver indicates "link up". If this packet is lost or not
>>> replied to for some reason, the code waits for a 1s timeout before
>>> retrying. For some reason, such early packets are often lost on my
>>> system, so this causes an annoying delay.
>>>
>>> To optimize this, modify the BOOTP code to have very short timeouts for
>>> the first packet transmitted, but gradually increase the timeout each
>>> time a timeout occurs. This way, if the first packet is lost, the second
>>> packet is transmitted quite quickly and hence the overall delay is low.
>>> However, if there's still no response, we don't keep spewing out packets
>>> at an insane speed.
>>>
>>> It's arguably more correct to try and find out why the first packet is
>>> lost. However, it seems to disappear inside my Ethenet chip; the TX chip
>>> indicates no error during TX (not that it has much in the way of
>>> reporting...), yet wireshark on the RX side doesn't see any packet.
>>> FWIW, I'm using an ASIX USB Ethernet adapter. Perhaps "link up" is
>>> reported too early or based on the wrong condition in HW, and we should
>>> add some fixed extra delay into the driver. However, this would slow down
>>> every link up event even if it ends up not being needed in some cases.
>>> Having BOOTP retry quickly applies the fix/WAR to every possible
>>> Ethernet device, and is quite simple to implement, so seems a better
>>> solution.
>>>
>>> Signed-off-by: Stephen Warren <swarren at nvidia.com>
>>> ---
>>>   net/bootp.c | 38 +++++++++++++++++++++++++++++++-------
>>>   net/bootp.h |  3 +--
>>>   net/net.c   |  4 ++--
>>>   3 files changed, 34 insertions(+), 11 deletions(-)
>>
>> Sorry for jumping in a little late, but I only encountered this after
>> rebasing on latest origin/master today.
>>
>> With this patch applied, it takes about 8 seconds until U-Boot manages
>> to get a DHCP address (it needs to broadcast 14 times) whereas with the
>> patch reverted I get a reply almost instantly (with only a single
>> broadcast).
>>
>> I'm testing this on a Jetson TK1 (with local patches for PCIe and
>> ethernet support).
>
> Further tests show that if I increase the initial bootp_timeout to
> anywhere beyond 750 ms things don't regress for me. If I go down to 500
> ms it will send out a second broadcast (presumably because the DHCP
> server in my network hasn't replied yet) but still get a reasonably
> quick reply (~ 1 second).
>
> So it would seem that this parameter depends to a large degree on the
> network infrastructure and needs to be carefully tweaked to work well in
> most circumstances.
>
> Could you try if 500, 750 and 1000 ms as initial bootp_timeout value
> give you acceptable results?

That will still introduce extremely large delays into the boot process, 
which is exactly what this patch was trying to avoid. 500..1000ms are 
the same order of magnitude as the initial delay without this patch applied.

Re-transmitting a DHCP request shouldn't prevent a response to the 
previous request from being processed - AFAIK each request is 
idempotent. Can you debug what is causing the 8s delay? Are earlier 
responses received and rejected for some reason, or is your DHCP server 
getting confused by the multiple requests and not responding, or ...?

(and as an aside, how on earth is your DHCP server taking >500ms to 
respond, yet still actually responding?)