netdev - Re: [PATCHv3 net-next 1/2] net_tstamp: add new flag HWTSTAMP_FLAG_BONDED_PHC

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Message-ID: <20211228071528.040fd3e3@kicinski-fedora-PC1C0HJN.hsd1.ca.comcast.net>
Date:   Tue, 28 Dec 2021 07:15:28 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Hangbin Liu <liuhangbin@...il.com>
Cc:     netdev@...r.kernel.org, Jay Vosburgh <j.vosburgh@...il.com>,
        Veaceslav Falico <vfalico@...il.com>,
        Andy Gospodarek <andy@...yhouse.net>,
        "David S . Miller" <davem@...emloft.net>,
        Richard Cochran <richardcochran@...il.com>,
        Heiner Kallweit <hkallweit1@...il.com>
Subject: Re: [PATCHv3 net-next 1/2] net_tstamp: add new flag
 HWTSTAMP_FLAG_BONDED_PHC_INDEX

On Tue, 28 Dec 2021 15:01:13 +0800 Hangbin Liu wrote:
> When implement the user space support for this feature. I realized that
> we can't use the new flag directly as the user space tool needs to have
> backward compatibility. Because run the new tool with this flag enabled
> on old kernel will get -EINVAL error. And we also could not use #ifdef
> directly as HWTSTAMP_FLAG_BONDED_PHC_INDEX is a enum.
> 
> Do you think if we could add a #define in linux/net_tstamp.h like
> 
> #define HWTSTAMP_FLAGS_SUPPORT 1
> 
> So that the user space tool could use it like
> 
> #ifdef HWTSTAMP_FLAGS_SUPPORT
>        cfg->flags = HWTSTAMP_FLAG_BONDED_PHC_INDEX;
> #endif

We could set it on SIOCGHWTSTAMP to let user space know that it's
necessary for a given netdev.