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

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Date:   Tue, 8 Dec 2020 05:02:20 -0800
From:   Richard Cochran <richardcochran@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Saeed Mahameed <saeed@...nel.org>,
        Eran Ben Elisha <eranbe@...dia.com>,
        "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 Mon, Dec 07, 2020 at 12:42:33PM -0800, Jakub Kicinski wrote:

> The behavior is not entirely dissimilar to the time stamps on
> multi-layered devices (e.g. DSA switches). The time stamp can either 
> be generated when the packet enters the device (current mlx5 behavior)
> or when it actually egresses thru the MAC (what this set adds).

To be useful, the time stamps must be taken on the external ports.
Generating the time stamp at the DMA reception in the device doesn't
even make sense, unless the delay through the device is constant.

> My main concern is the user friendliness. I think there is no question
> that user running ptp4l would want this mlx5 knob to be enabled.

Right.

> Would
> we rather see a patch to ptp4l that turns per driver knob or should we
> shoot for some form of an API that tells the kernel that we're
> expecting ns level time accuracy? 

This is a hardware-specific "feature".  One of the guiding principles
of the linuxptp user space stack is not to become a catalog of
workarounds for random hardware.  IMO the kernel's API should not
encourage "special" hardware either.  After all, we have lots and lots
of PTP hardware supported, all of them already working with the
existing API just fine.

My preference is for a global knob for users of this hardware, either

- a compile time Kconfig option on the driver, or
- some kind of sysctl/debugfs knob

Thanks,
Richard