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

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Date:   Tue, 28 Dec 2021 08:17:48 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Richard Cochran <richardcochran@...il.com>
Cc:     Hangbin Liu <liuhangbin@...il.com>, 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>,
        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 08:00:50 -0800 Richard Cochran wrote:
> On Tue, Dec 28, 2021 at 07:15:28AM -0800, Jakub Kicinski wrote:
> > 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.  
> 
> What about adding matching #defines into the enum declaration?
> 
> enum hwtstamp_flags {
> 	HWTSTAMP_FLAG_BONDED_PHC_INDEX = (1<<0),
> #define HWTSTAMP_FLAG_BONDED_PHC_INDEX (1<<0)
> };
> 
> IIRC I have seen this pattern used in the kernel, but ATM I can't find any example :(

That's still just a compile-time fix, if the user space binary 
is distributed in binary form (distro package) there is no knowing
on which kernel versions it will run. I think runtime probing will
be necessary.

If we want the define it should be to the enum name:

What about adding matching #defines into the enum declaration?

enum hwtstamp_flags {
	HWTSTAMP_FLAG_BONDED_PHC_INDEX = (1<<0),
#define HWTSTAMP_FLAG_BONDED_PHC_INDEX HWTSTAMP_FLAG_BONDED_PHC_INDEX
};

Examples in include/uapi/linux/rtnetlink.h