[dpdk-dev] [PATCH] net/mlx5: fix link status initialization

[Nélio Laranjeiro]

On Sun, Apr 08, 2018 at 01:09:27PM +0000, Shahaf Shuler wrote:
> Thursday, April 5, 2018 9:51 AM, Nélio Laranjeiro:
> > Subject: Re: [PATCH] net/mlx5: fix link status initialization
> > 
> > On Thu, Apr 05, 2018 at 05:35:57AM +0000, Shahaf Shuler wrote:
> > > Wednesday, April 4, 2018 3:11 PM, Nélio Laranjeiro:
> > > > Subject: Re: [PATCH] net/mlx5: fix link status initialization
> > > >
> > > > On Wed, Apr 04, 2018 at 09:58:33AM +0000, Shahaf Shuler wrote:
> > > > > Wednesday, April 4, 2018 10:30 AM, Nélio Laranjeiro:
> > > > > > Subject: Re: [PATCH] net/mlx5: fix link status initialization
> > > > > >
> > > > > > On Tue, Apr 03, 2018 at 07:48:17AM +0300, Shahaf Shuler wrote:
> 
> [..]
> 
> > > >
> > > > According to your analysis, this is only necessary when the LCS is
> > > > configured in the device.  Why not adding this call to
> > > > mlx5_dev_interrupt_handler_install() which is responsible to install
> > > > the LCS callback.
> > >
> > > I think it is good practice whether or not LSC is set.
> > > The link status should be initialized to the correct value after the probe.
> > 
> > There is no guarantee the link will be accurate, at probe time the link may be
> > up so internal information has a status up with a speed with this patch.
> > The application probes a second port, in the mean time the link of the first
> > port goes down, the interrupt is still not installed and the internal status
> > becomes wrong (still up whereas the port is down).
> > 
> > Finally at start, the device installs the handler, but the link is still down
> > whereas internally it is up, the application will call
> > rte_eth_link_get_nowait() which will directly copy the wrong internal status
> > to the application.
> 
> This is not correct.
> Using Verbs, the async_fd on which link status interrupts are reported is created on probe. 
> Even if the interrupt handler is not installed, interrupts still
> trigger on this fd. They will be processed when the interrupt handler
> will be installed as part of the port start. 
> So in fact you have the whole trace on the link status changes waiting
> to be processed upon port start. 

Right, but in such case, this patch still does not solves the issue.
Until the dev_start() is called, the link may be inconsistent with the
real status.

example: 

 pci_probe --> link is up.
 leaving pci probe, the link goes downs --> internally the PMD has a link up.

Until the dev_start() is called any call to rte_eth_link_get_nowait()
will copy the internal PMD status which is not accurate.

>From this point, the issue seems to fully come from
rte_eth_link_get_nowait() which should not make any copy until the port
is not started, until then the link may be inconsistent between the
driver and the device.

Regards,

-- 
Nélio Laranjeiro
6WIND