Re: [PATCH net-next 0/3] Document more PTP timestamping known quirks

From: Jacob Keller <jacob.e.keller@intel.com>
To: Vladimir Oltean <olteanv@gmail.com>, Sergey Organov <sorganov@gmail.com>
Cc: kuba@kernel.org, davem@davemloft.net, netdev@vger.kernel.org,
	richardcochran@gmail.com, linux-doc@vger.kernel.org
Subject: Re: [PATCH net-next 0/3] Document more PTP timestamping known quirks
Date: Fri, 17 Jul 2020 16:13:58 -0700	[thread overview]
Message-ID: <ccabed4c-2b4c-05c6-650e-7783acfa50b3@intel.com> (raw)
In-Reply-To: <20200717215719.nhuaak2xu4fwebqp@skbuf>

On 7/17/2020 2:57 PM, Vladimir Oltean wrote:
> On Sat, Jul 18, 2020 at 12:13:42AM +0300, Sergey Organov wrote:
>> Vladimir Oltean <olteanv@gmail.com> writes:
>>
>>> I've tried to collect and summarize the conclusions of these discussions:
>>> https://patchwork.ozlabs.org/project/netdev/patch/20200711120842.2631-1-sorganov@gmail.com/
>>> https://patchwork.ozlabs.org/project/netdev/patch/20200710113611.3398-5-kurt@linutronix.de/
>>> which were a bit surprising to me. Make sure they are present in the
>>> documentation.
>>
>> As one of participants of these discussions, I'm afraid I incline to
>> alternative approach to solving the issues current design has than the one
>> you advocate in these patch series.
>>
>> I believe its upper-level that should enforce common policies like
>> handling hw time stamping at outermost capable device, not random MAC
>> driver out there.
>>
>> I'd argue that it's then upper-level that should check PHY features, and
>> then do not bother MAC with ioctl() requests that MAC should not handle
>> in given configuration. This way, the checks for phy_has_hwtstamp()
>> won't be spread over multiple MAC drivers and will happily sit in the
>> upper-level ioctl() handler.
>>
>> In other words, I mean that it's approach taken in ethtool that I tend
>> to consider being the right one.
>>
>> Thanks,
>> -- Sergey
> 
> Concretely speaking, what are you going to do for
> skb_defer_tx_timestamp() and skb_defer_rx_timestamp()? Not to mention
> subtle bugs like SKBTX_IN_PROGRESS. If you don't address those, it's
> pointless to move the phy_has_hwtstamp() check to net/core/dev_ioctl.c.
> 
> The only way I see to fix the bug is to introduce a new netdev flag,
> NETIF_F_PHY_HWTSTAMP or something like that. Then I'd grep for all
> occurrences of phy_has_hwtstamp() in the kernel (which currently amount
> to a whopping 2 users, 3 with your FEC "fix"), and declare this
> netdevice flag in their list of features. Then, phy_has_hwtstamp() and
> phy_has_tsinfo() and what not can be moved to generic places (or at
> least, I think they can), and those places could proceed to advertise
> and enable PHY timestamping only if the MAC declared itself ready. But,
> it is a bit strange to introduce a netdev flag just to fix a bug, I
> think.
> 

This approach doesn't seem bad to me. We then document that
NETIF_F_PHY_HWTSTAMP should only set of the correct conditions are met.

> Thanks,
> -Vladimir
> 