netdev - Re: [PATCH net-next 3/3] docs: networking: timestamping: add a set of frequently asked questions

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Message-ID: <120430f5-df77-2d56-9801-8e69bd94935a@intel.com>
Date:   Tue, 21 Jul 2020 10:16:32 -0700
From:   Jacob Keller <jacob.e.keller@...el.com>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     kuba@...nel.org, davem@...emloft.net, netdev@...r.kernel.org,
        richardcochran@...il.com, sorganov@...il.com,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH net-next 3/3] docs: networking: timestamping: add a set of
 frequently asked questions



On 7/20/2020 3:13 PM, Vladimir Oltean wrote:>
> Yes, indeed, a lot of them are exclusively checking
> "skb_shinfo(skb)->tx_flags & SKBTX_IN_PROGRESS", without any further
> verification that they have hardware timestamping enabled in the first
> place, a lot more than I remembered. Some of the occurrences are
> actually new.
> 
> I think at least part of the reason why this keeps going on is that
> there aren't any hard and fast rules that say you shouldn't do it. When
> there isn't even a convincing percentage of DSA/PHY drivers that do set
> SKBTX_HW_TSTAMP, the chances are pretty low that you'll get a stacked
> PHC driver that sets the flag, plus a MAC driver that checks for it
> incorrectly. So people tend to ignore this case. Even though, if stacked
> DSA drivers started supporting software TX timestamping (which is not
> unlikely, given the fact that this would also give you compatibility
> with PHY timestamping), I'm sure things would change, because more
> people would become aware of the issue once mv88e6xxx starts getting
> affected.
> 
> What I've been trying to do is at least try to get people (especially
> people who have a lot of XP with 1588 drivers) to agree on a common set
> of guidelines that are explicitly written down. I think that's step #1.
> 
> -Vladimir
> 

Right. I think the guideline should be:

This flag indicates to the stack whether or not a hardware Tx timestamp
has been started. It's primary purpose is to prevent sending software
timestamps if a hw timestamp is provided.

1) set the flag whenever you start a tx timestamp

2) do not assume you are the only driver that will set the flag for a
given skb. Use a separate mechanism to decide if that skb is supposed to
have a timestamp.