Re: [PATCH] tty: serial: bcm63xx: allow building on ARM64

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Hi Greg,

On Wed, Nov 25, 2020 at 9:53 AM Greg Kroah-Hartman
<gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> On Wed, Nov 25, 2020 at 09:38:50AM +0100, Rafał Miłecki wrote:
> > On 25.11.2020 09:23, Greg Kroah-Hartman wrote:
> > > On Wed, Nov 25, 2020 at 09:13:52AM +0100, Rafał Miłecki wrote:
> > > > From: Rafał Miłecki <rafal@xxxxxxxxxx>
> > > >
> > > > Hardware supported by bcm63xx is also used by BCM4908 SoCs family that
> > > > is ARM64.
> > > >
> > > > Signed-off-by: Rafał Miłecki <rafal@xxxxxxxxxx>
> > > > ---
> > > >   drivers/tty/serial/Kconfig | 3 ++-
> > > >   1 file changed, 2 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/drivers/tty/serial/Kconfig b/drivers/tty/serial/Kconfig
> > > > index 28f22e58639c..6907c5b17a0e 100644
> > > > --- a/drivers/tty/serial/Kconfig
> > > > +++ b/drivers/tty/serial/Kconfig
> > > > @@ -1133,7 +1133,8 @@ config SERIAL_TIMBERDALE
> > > >   config SERIAL_BCM63XX
> > > >           tristate "Broadcom BCM63xx/BCM33xx UART support"
> > > >           select SERIAL_CORE
> > > > - depends on MIPS || ARM || COMPILE_TEST
> > > > + depends on MIPS || ARM || ARM64 || COMPILE_TEST

Why not s/ARM64/ARCH_BCM4908/?

> > >
> > > Why do we have an arch dependancy at all now?
> >
> > From my experience "depends" is often used to limit symbol visibility to
> > applicable platforms only. I don't think Broadcom has any x86, risc, etc.
> > platforms so it's useless there.
> >
> > As for testing driver compilation on unused arch-s I thought that's what
> > COMPILE_TEST is for.
> >
> > Am I wrong there? I'm afraid we don't have clear Documentation on that.
> > Please kindly point me to some info if I'm wrong.
>
> If COMPILE_TEST is working for this driver, then trying to restrict it
> to a specific arch is usually pointless and the arch dependency can be
> removed, keeping patches like this from having to be made over time to
> add it to new arches :)
>
> > > > + default ARCH_BCM4908
> > >
> > > Really?  I thought we were getting rid of these "ARCH_platform_type" of

No we are not.

> > > things.  That's what a defconfig file is for, right?

FWIW, the arm64 defconfig file enables about everything, for all arm64
platforms.

> > I had to miss something, last time I checked Linus called defconfigs a
> > garbage and wanted to get rid of them:
> > https://lwn.net/Articles/391372/
> >
> > There are also no platform defconfigs in arch/arm64/ at all. Should I
> > handle it with arch/arm64/Kconfig.platforms and "select SERIAL_BCM63XX"?
>
> I thought we were trying to get rid of arm64 "platforms" as well.  My
> point being, why is this needed at all?

To prevent asking the user about a driver that is completely useless for
the system(s) the user is compiling a kernel for.

Do you want to let distros compile all arm/arm64-only SoC drivers for x86, too?

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

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



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