netdev - Re: [net-next V2 08/15] net/mlx5e: Add TX PTP port object support

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Date:   Mon, 07 Dec 2020 00:37:45 -0800
From:   Saeed Mahameed <saeed@...nel.org>
To:     Richard Cochran <richardcochran@...il.com>,
        Eran Ben Elisha <eranbe@...dia.com>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        Tariq Toukan <tariqt@...dia.com>
Subject: Re: [net-next V2 08/15] net/mlx5e: Add TX PTP port object support

On Sun, 2020-12-06 at 09:08 -0800, Richard Cochran wrote:
> On Sun, Dec 06, 2020 at 03:37:47PM +0200, Eran Ben Elisha wrote:
> > Adding new enum to the ioctl means we have add
> > (HWTSTAMP_TX_ON_TIME_CRITICAL_ONLY for example) all the way -
> > drivers,
> > kernel ptp, user space ptp, ethtool.
> > 

Not exactly,
1) the flag name should be HWTSTAMP_TX_PTP_EVENTS, similar to what we
already have in RX, which will mean: 
HW stamp all PTP events, don't care about the rest.

2) no need to add it to drivers from the get go, only drivers who are
interested may implement it, and i am sure there are tons who would
like to have this flag if their hw timestamping implementation is slow
! other drivers will just keep doing what they are doing, timestamp all
traffic even if user requested this flag, again exactly like many other
drivers do for RX flags (hwtstamp_rx_filters).

> > My concerns are:
> > 1. Timestamp applications (like ptp4l or similar) will have to add
> > support
> > for configuring the driver to use HWTSTAMP_TX_ON_TIME_CRITICAL_ONLY
> > if
> > supported via ioctl prior to packets transmit. From application
> > point of
> > view, the dual-modes (HWTSTAMP_TX_ON_TIME_CRITICAL_ONLY ,
> > HWTSTAMP_TX_ON)
> > support is redundant, as it offers nothing new.
> 
> Well said.
> 

disagree, it is not a dual mode, just allow the user to have better
granularity for what hw stamps, exactly like what we have in rx.

we are not adding any new mechanism.

> > 2. Other vendors will have to support it as well, when not sure
> > what is the
> > expectation from them if they cannot improve accuracy between them.
> 
> If there were multiple different devices out there with this kind of
> implementation (different levels of accuracy with increasing run time
> performance cost), then we could consider such a flag.  However, to
> my
> knowledge, this feature is unique to your device.
> 

I agree, but i never meant to have a flag that indicate two different
levels of accuracy, that would be a very wild mistake for sure! 

The new flag will be about selecting granularity of what gets a hw
stamp and what doesn't, aligning with the RX filter API.

> > This feature is just an internal enhancement, and as such it should
> > be added
> > only as a vendor private configuration flag. We are not offering
> > here about
> > any standard for others to follow.
> 
> +1
> 

Our driver feature is and internal enhancement yes, but the suggested
flag is very far from indicating any internal enhancement, is actually
an enhancement to the current API, and is a very simple extension with
wide range of improvements to all layers.

Our driver can optimize accuracy when this flag is set, other drivers
might be happy to implement it since they already have a slow hw and
this flag would allow them to run better TCP/UDP performance while
still performing ptp hw stamping, some admins/apps will use it to avoid
stamping all traffic on tx, win win win.