[Cc: +Sasha]
Dear Christopher,
Am 07.11.24 um 00:53 schrieb Hall, Christopher S:
From: Paul Menzel <pmen...@molgen.mpg.de>
Sent: Wednesday, November 06, 2024 3:14 PM
Subject: Re: [Intel-wired-lan] [PATCH iwl-net v3 2/6] igc: Lengthen the
hardware retry time to prevent timeouts
I’d use the more specific summary/title below:
Will do.
igc: Lengthen hardware retry time to 4 μs to prevent timeouts
Am 06.11.24 um 19:47 schrieb Christopher S M Hall:
Lengthen the hardware retry timer to four microseconds.
The i225/i226 hardware retries if it receives an inappropriate response
from the upstream device. If the device retries too quickly, the root
port does not respond.
Any idea why? Is it documented somewhere?
I do not. Theoretically, 1 us should work, but it does not. It could be a root
port problem or an issue with i225/i226 NIC. I am not able to directly observe
the state of either. 4 us has worked in all my testing I am comfortable with
that value. 2 us also works, but given the limited hardware at my disposal
I doubled the value to 4 us to be safe. PTM is not time critical. Typically,
software initiates a transaction between 8 and 32 times per second. There
is no performance impact for PTM or any other function of the card. The
timeout occurs rarely, but if the retry time is too short the PTM state
machine does not recover.
Thank you for clearing this up. If it’s not time critical, why not
revert the original patch and go back to 10 μs.
The referenced commit 6b8aa753a9f9 (igc: Decrease PTM short interval
from 10 us to 1 us) also says, that 1 μs was suggested by the hardware
team. Were you able to talk to them?
The issue can be reproduced with the following:
$ sudo phc2sys -R 1000 -O 0 -i tsn0 -m
Note: 1000 Hz (-R 1000) is unrealistically large, but provides a way to
quickly reproduce the issue.
PHC2SYS exits with:
"ioctl PTP_OFFSET_PRECISE: Connection timed out" when the PTM transaction
fails
Why four microseconds, and not some other value?
See above.
It’d be great, if you extended the commit message.
Fixes: 6b8aa753a9f9 ("igc: Decrease PTM short interval from 10 us to 1 us")
-#define IGC_PTM_SHORT_CYC_DEFAULT 1 /* Default short cycle interval */
+#define IGC_PTM_SHORT_CYC_DEFAULT 4 /* Default short cycle interval */
Maybe also add a comment, that 1 μs should work, but does not.
Kind regards,
Paul
