Re: [dpdk-dev] [PATCH] net/mlx5: fix link state update

[Matan Azrad <matan@mellanox.com>]

Hi
 From: Benoit Ganne 
> Sent: Monday, March 30, 2020 3:03 PM
> To: Matan Azrad <matan@mellanox.com>; dev@dpdk.org
> Subject: RE: [dpdk-dev] [PATCH] net/mlx5: fix link state update
> 
> Hi Matan,
> 
> >>>> mlx5 PMD refuses to update link state if link speed is defined but
> >>>> status is down or if link speed is undefined but status is up, even
> >>>> if the ioctl() succeeded.
> >>>> This prevents application to detect link up/down event, especially
> >>>> when the link speed is not correctly detected.
> >>> Do you use the wait option? Or no wait?
> >> We are using the no wait option.
> > I suggest to call again if failed for N retries time.
> 
> Unfortunately it will not solve our problem: if link speed is undefined but the
> link is up then the test '!dev_link.link_speed && dev_link.link_status' at
> https://eur03.safelinks.protection.outlook.com/?url=http%3A%2F%2Fgit.dp
> dk.org%2Fdpdk%2Ftree%2Fdrivers%2Fnet%2Fmlx5%2Fmlx5_ethdev.c%23n8
> 99&amp;data=02%7C01%7Cmatan%40mellanox.com%7C8a0fb9f9ba94422a4a
> 4208d7d4a2539c%7Ca652971c7d2e4d9ba6a4d149256f461b%7C0%7C0%7C637
> 211665962283891&amp;sdata=%2FExOJyhm0Bi4kkp434P0U4kRpQs7U0knuTa
> kvsWFO5Q%3D&amp;reserved=0 will always be true and the function will
> always return EAGAIN.
> This actually happens in Azure with CX4-Lx VFs.
> 
> >> What I meant was to let the app decide whether it should retry or
> >> not, based on the data it gets.
> >> Right now, the PMD *prevents* the app to get link state if the link
> >> speed is undefined even if the app does not care about link speed.
> 
> > In mlx5 this is not the case, we have no one updated and second not -
> > there are going together:
> > You can see that we have 2 different system calls: 1 to get up\down
> > and second to get link speed.
> > If link speed doesn't appropriate to the link state it may say that
> > something was changed between the calls and the link status we got
> > from the first call is not correct anymore.
> > In this case, we should call both calls again, that’s what we are
> > doing in "nowait" option.
> > If the user doesn't want "nowait" option, (means PMD is not allowed to
> > take more time for response) he should call again when the callback
> > failed in the time and retries manner the user prefers.
> 
> Ok, now I understand the logic behind the current behavior: the 2 syscalls
> being not atomics, you try to detect inconsistencies that way.
> But if the link speed is undefined, then the state will never be correctly
> updated.

Why link speed is undefined?
Old kernel?
Kernel mlx5 driver issue?

Do you know?

> I still believe it is unnecessarily heavy-handed: in most networking application
> I have seen (and I have 2 examples of current shipping networking products),
> a missing link speed is not critical whereas link being reported as down means
> no traffic flowing.
> 
> Best
> ben