netdev - Re: [PATCH] ptp: ocp: don't allow on S390

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Date:   Fri, 20 Aug 2021 11:59:31 +0200
From:   Niklas Schnelle <schnelle@...ux.ibm.com>
To:     Christian Borntraeger <borntraeger@...ibm.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Hendrik Brueckner <brueckner@...ux.vnet.ibm.com>,
        linux-s390 <linux-s390@...r.kernel.org>
Cc:     netdev@...r.kernel.org, Richard Cochran <richardcochran@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] ptp: ocp: don't allow on S390

On Fri, 2021-08-20 at 10:02 +0200, Christian Borntraeger wrote:
> 
> On 20.08.21 00:58, Randy Dunlap wrote:
> > On 8/16/21 2:41 PM, Jonathan Lemon wrote:
> > > On Mon, Aug 16, 2021 at 02:15:51PM -0700, Randy Dunlap wrote:
> > > > On 8/16/21 2:09 PM, Jonathan Lemon wrote:
> > > > > On Fri, Aug 13, 2021 at 01:30:26PM -0700, Randy Dunlap wrote:
> > > > > > There is no 8250 serial on S390. See commit 1598e38c0770.
> > > > > 
> > > > > There's a 8250 serial device on the PCI card.   Its been
> > > > > ages since I've worked on the architecture, but does S390
> > > > > even support PCI?
> > > > 
> > > > Yes, it does.
> 
> We do support PCI, but only a (very) limited amount of cards.
> So there never will be a PCI card with 8250 on s390 and
> I also doubt that we will see the "OpenCompute TimeCard"
> on s390.
> 
> So in essence the original patch is ok but the patch below
> would also be ok for KVM. But it results in a larger kernel
> with code that will never be used. So I guess the original
> patch is the better choice.

It looks to me like the SERIAL_8250 driver can be build as a module so
would then not increase the kernel image size or am I missing
something?

In that case I would vote for the patch below. For PCI on s390 we do
intend to, in principle, support arbitrary PCI devices and have already
seen cases where non-trivial cards that were never before tested on
s390 did "just work" once someone needed them.

While I do agree that both 8250 and the Time Card are unlikely to be
used on s390 never say never and compile testing a variety of driver
code against our PCI primitives is good for quality control.

In the end I'm okay with either.

> 
> > > > > > Is this driver useful even without 8250 serial?
> > > > > 
> > > > > The FB timecard has an FPGA that will internally parse the
> > > > > GNSS strings and correct the clock, so the PTP clock will
> > > > > work even without the serial devices.
> > > > > 
> > > > > However, there are userspace tools which want to read the
> > > > > GNSS signal (for holdolver and leap second indication),
> > > > > which is why they are exposed.
> > > > 
> > > > So what do you recommend here?
> > > 
> > > Looking at 1598e38c0770, it appears the 8250 console is the
> > > problem.  Couldn't S390 be fenced by SERIAL_8250_CONSOLE, instead
> > > of SERIAL_8250, which would make the 8250 driver available?
> > 
> > OK, that sounds somewhat reasonable.
> > 
> > > For now, just disabling the driver on S390 sounds reasonable.
> > > 
> > 
> > S390 people, how does this look to you?
> > 
> > This still avoids having serial 8250 console conflicting
> > with S390's sclp console.
> > (reference commit 1598e38c0770)
> > 
> > 
> > ---
> >   drivers/tty/serial/8250/Kconfig |    2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > --- linux-next-20210819.orig/drivers/tty/serial/8250/Kconfig
> > +++ linux-next-20210819/drivers/tty/serial/8250/Kconfig
> > @@ -6,7 +6,6 @@
> > 
> >   config SERIAL_8250
> >       tristate "8250/16550 and compatible serial support"
> > -    depends on !S390
> >       select SERIAL_CORE
> >       select SERIAL_MCTRL_GPIO if GPIOLIB
> >       help
> > @@ -85,6 +84,7 @@ config SERIAL_8250_FINTEK
> >   config SERIAL_8250_CONSOLE
> >       bool "Console on 8250/16550 and compatible serial port"
> >       depends on SERIAL_8250=y
> > +    depends on !S390
> >       select SERIAL_CORE_CONSOLE
> >       select SERIAL_EARLYCON
> >       help
> > 
> >