linux-kernel - Re: [PATCH 1/1] tty: serial: handle HAS

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Message-ID: <alpine.DEB.2.21.2410011707550.45128@angie.orcam.me.uk>
Date: Tue, 1 Oct 2024 17:41:23 +0100 (BST)
From: "Maciej W. Rozycki" <macro@...am.me.uk>
To: Niklas Schnelle <schnelle@...ux.ibm.com>
cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
    Jiri Slaby <jirislaby@...nel.org>, 
    Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>, 
    linux-serial@...r.kernel.org, Arnd Bergmann <arnd@...nel.org>, 
    Heiko Carstens <hca@...ux.ibm.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] tty: serial: handle HAS_IOPORT dependencies

Hi Niklas,

> With 2 more HAS_IOPORT patches having gone into v6.12-rc1 I'm looking
> at what's left and we're down to 4 prerequisite patches[0] before being
> able to compile-time disable inb()/outb()/…. This one being by far the
> largest of these. Looking at your suggestion it seems that to compile
> 8250_pci.c without HAS_IOPORT I'll have to add #ifdef CONFIG_HAS_IOPORT
> around the MOXI section as that uses I/O ports unconditionally. The
> rest seems fine and I guess would theoretically work on a system with
> !HAS_IOPORT. I'll send a v2 with that included. 

 I've skimmed over and I agree, though I'd place some of the #ifdefs 
differently, e.g. above #define QPCR_TEST_FOR1.  Overall I think it would 
make sense to reorder code and group stuff that depends on port I/O such 
as to minimise the number of #ifdefs.

 Ideally we could come with a slightly user-friendlier change that would 
report the inability to handle port I/O devices as they are discovered 
rather than just silently ignoring them.

> Note however that even though your POWER9 system does not have I/O port
> support in hardware we still have HAS_IOPORT enabled for arch/powerpc
> if PCI is enabed so even with this patch as is your POWER9 system
> should not be affected.

 I think we need to get this right regardless.  And also while I run a 
generic distribution kernel on my POWER9 as a production system, I'd love 
to see an option to build a tailored configuration that would indeed 
remove support for port I/O from the kernel side for systems like mine 
that have no provision for port I/O in hardware, knowing that such a 
kernel will only ever run on such hardware and discarding compiled code 
that won't ever be used.

  Maciej