netdev - Re: [net-next 4/8] net/mlx5e: Add device out of buffer counter

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Date:   Fri, 23 Aug 2019 11:16:01 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Saeed Mahameed <saeedm@...lanox.com>
Cc:     "davem@...emloft.net" <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Moshe Shemesh <moshe@...lanox.com>
Subject: Re: [net-next 4/8] net/mlx5e: Add device out of buffer counter

On Fri, 23 Aug 2019 06:00:45 +0000, Saeed Mahameed wrote:
> On Thu, 2019-08-22 at 18:33 -0700, Jakub Kicinski wrote:
> > On Thu, 22 Aug 2019 23:35:52 +0000, Saeed Mahameed wrote:  
> > > From: Moshe Shemesh <moshe@...lanox.com>
> > > 
> > > Added the following packets drop counter:
> > > Device out of buffer - counts packets which were dropped due to
> > > full
> > > device internal receive queue.
> > > This counter will be shown on ethtool as a new counter called
> > > dev_out_of_buffer.
> > > The counter is read from FW by command QUERY_VNIC_ENV.
> > > 
> > > Signed-off-by: Moshe Shemesh <moshe@...lanox.com>
> > > Signed-off-by: Saeed Mahameed <saeedm@...lanox.com>  
> > 
> > Sounds like rx_fifo_errors, no? Doesn't rx_fifo_errors count RX
> > overruns?  
> 
> No, that is port buffer you are looking for and we got that fully
> covered in mlx5. this is different.
> 
> This new counter is deep into the HW data path pipeline and it covers
> very rare and complex scenarios that got only recently introduced with
> swichdev mode and "some" lately added tunnels offloads that are routed
> between VFs/PFs.
> 
> Normally the HW is lossless once the packet passes port buffers into
> the data plane pipeline, let's call that "fast lane", BUT for sriov
> configurations with switchdev mode enabled and some special hand
> crafted tc tunnel offloads that requires hairpin between VFs/PFs, the
> hw might decide to send some traffic to a "service lane" which is still
> fast path but unlike the "fast lane" it handles traffic through "HW
> internal" receive and send queues (just like we do with hairpin) that
> might drop packets. the whole thing is transparent to driver and it is
> HW implementation specific.

I see thanks for the explanation and sorry for the delayed response.
Would it perhaps make sense to indicate the hairpin in the name?
dev_out_of_buffer is quite a generic name, and there seems to be no
doc, nor does the commit message explains it as well as you have..