Re: [PATCH] net: Select PTP_1588_CLOCK in PTP-specific drivers

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Clay,

Thanks for your patch!

On Tue, Apr 28, 2020 at 11:14 AM Clay McClure <clay@daemons.net> wrote:
> Commit d1cbfd771ce8 ("ptp_clock: Allow for it to be optional") changed
> all PTP-capable Ethernet drivers from `select PTP_1588_CLOCK` to `imply
> PTP_1588_CLOCK`, "in order to break the hard dependency between the PTP
> clock subsystem and ethernet drivers capable of being clock providers."
> As a result it is possible to build PTP-capable Ethernet drivers without
> the PTP subsystem by deselecting PTP_1588_CLOCK. Drivers are required to
> handle the missing dependency gracefully.
>
> Some PTP-capable Ethernet drivers (e.g., TI_CPSW) factor their PTP code
> out into separate drivers (e.g., TI_CPTS_MOD). The above commit also
> changed these PTP-specific drivers to `imply PTP_1588_CLOCK`, making it
> possible to build them without the PTP subsystem. But as Grygorii
> Strashko noted in [1]:
>
> On Wed, Apr 22, 2020 at 02:16:11PM +0300, Grygorii Strashko wrote:
>
> > Another question is that CPTS completely nonfunctional in this case and
> > it was never expected that somebody will even try to use/run such
> > configuration (except for random build purposes).
>
> In my view, enabling a PTP-specific driver without the PTP subsystem is
> a configuration error made possible by the above commit. Kconfig should
> not allow users to create a configuration with missing dependencies that
> results in "completely nonfunctional" drivers.
>
> I audited all network drivers that call ptp_clock_register() and found
> six that look like PTP-specific drivers that are likely nonfunctional
> without PTP_1588_CLOCK:
>
>     NET_DSA_MV88E6XXX_PTP
>     NET_DSA_SJA1105_PTP
>     MACB_USE_HWSTAMP
>     CAVIUM_PTP
>     TI_CPTS_MOD
>     PTP_1588_CLOCK_IXP46X
>
> Note how they all reference PTP or timestamping in their name; this is a
> clue that they depend on PTP_1588_CLOCK.
>
> Change these drivers back [2] to `select PTP_1588_CLOCK`. Note that this
> requires also selecting POSIX_TIMERS, a transitive dependency of
> PTP_1588_CLOCK.

If these drivers have a hard dependency on PTP_1588_CLOCK, IMHO they
should depend on PTP_1588_CLOCK, not select PTP_1588_CLOCK.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds