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Date:   Fri, 5 Nov 2021 19:25:39 +0200
From:   Vladimir Oltean <olteanv@gmail.com>
To:     Jakub Kicinski <kuba@kernel.org>
Cc:     Richard Cochran <richardcochran@gmail.com>,
        Martin Kaistra <martin.kaistra@linutronix.de>,
        Florian Fainelli <f.fainelli@gmail.com>,
        Andrew Lunn <andrew@lunn.ch>,
        Vivien Didelot <vivien.didelot@gmail.com>,
        Kurt Kanzenbach <kurt@linutronix.de>,
        "David S. Miller" <davem@davemloft.net>,
        John Stultz <john.stultz@linaro.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Stephen Boyd <sboyd@kernel.org>,
        Russell King <linux@armlinux.org.uk>,
        Marc Kleine-Budde <mkl@pengutronix.de>,
        linux-kernel@vger.kernel.org, netdev@vger.kernel.org
Subject: Re: [PATCH 7/7] net: dsa: b53: Expose PTP timestamping ioctls to
 userspace
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On Fri, Nov 05, 2021 at 08:09:39AM -0700, Jakub Kicinski wrote:
> On Fri, 5 Nov 2021 16:28:33 +0200 Vladimir Oltean wrote:
> > On Fri, Nov 05, 2021 at 07:13:19AM -0700, Richard Cochran wrote:
> > > On Fri, Nov 05, 2021 at 02:38:01PM +0100, Martin Kaistra wrote:  
> > > > Ok, then I will remove HWTSTAMP_FILTER_PTP_V2_(EVENT|SYNC|DELAY_REQ) from
> > > > this list, what about HWTSTAMP_FILTER_ALL?  
> > > 
> > > AKK means time stamp every received frame, so your driver should
> > > return an error in this case as well.  
> > 
> > What is the expected convention exactly? There are other drivers that
> > downgrade the user application's request to what they support, and at
> > least ptp4l does not error out, it just prints a warning.
> 
> Which is sad because that's one of the best documented parts of our API:
> 
>   Desired behavior is passed into the kernel and to a specific device by
>   calling ioctl(SIOCSHWTSTAMP) with a pointer to a struct ifreq whose
>   ifr_data points to a struct hwtstamp_config. The tx_type and
>   rx_filter are hints to the driver what it is expected to do. If
>   the requested fine-grained filtering for incoming packets is not
>   supported, the driver may time stamp more than just the requested types
>   of packets.
> 
>   Drivers are free to use a more permissive configuration than the requested
>   configuration. It is expected that drivers should only implement directly the
>   most generic mode that can be supported. For example if the hardware can
>   support HWTSTAMP_FILTER_V2_EVENT, then it should generally always upscale
>   HWTSTAMP_FILTER_V2_L2_SYNC_MESSAGE, and so forth, as HWTSTAMP_FILTER_V2_EVENT
>   is more generic (and more useful to applications).
> 
>   A driver which supports hardware time stamping shall update the struct
>   with the actual, possibly more permissive configuration. If the
>   requested packets cannot be time stamped, then nothing should be
>   changed and ERANGE shall be returned (in contrast to EINVAL, which
>   indicates that SIOCSHWTSTAMP is not supported at all).
> 
> https://www.kernel.org/doc/html/latest/networking/timestamping.html#hardware-timestamping-configuration-siocshwtstamp-and-siocghwtstamp

Yeah, sorry, I've been all over that documentation file for the past few
days, but I missed that section. "that's one of the best documented
parts of our API" is a nice euphemism for all the SO_TIMESTAMPING flags :)