Re: [RFC 02/32] Kconfig: introduce HAS_IOPORT option and select it as necessary

From: Niklas Schnelle <schnelle@linux.ibm.com>
To: Michael Schmitz <schmitzmic@gmail.com>, Arnd Bergmann <arnd@kernel.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>,
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	Greg Ungerer <gerg@linux-m68k.org>
Subject: Re: [RFC 02/32] Kconfig: introduce HAS_IOPORT option and select it as necessary
Date: Fri, 31 Dec 2021 12:28:23 +0100	[thread overview]
Message-ID: <072b9080be4a408052bf2c2cc1a9be0089cce5cc.camel@linux.ibm.com> (raw)
In-Reply-To: <0211719b-8402-9865-8e5d-5c0a35715816@gmail.com>

On Thu, 2021-12-30 at 16:44 +1300, Michael Schmitz wrote:
> Hi Arnd,
> 
> Am 30.12.2021 um 14:48 schrieb Arnd Bergmann:
> > On Tue, Dec 28, 2021 at 11:15 PM Michael Schmitz <schmitzmic@gmail.com> wrote:
> > > Am 29.12.2021 um 16:41 schrieb Arnd Bergmann:
> > > > On Tue, Dec 28, 2021 at 8:20 PM Michael Schmitz <schmitzmic@gmail.com> wrote:
---8<---
> 
> > What some other architectures do is to rely on inb()/outb() to have a
> > zero-based offset, and use an io_offset in PCI buses to ensure that a
> > low port number on the bus gets translated into a pointer value for the
> > virtual mapping in the kernel, which is then represented as an unsigned
> > int.
> 
> M54xx does just that for Coldfire:
> 
> arch/m68k/include/asm/io_no.h:
> #define PCI_IO_PA	0xf8000000		/* Host physical address */
> 
> (used to set PCI BAR mappings, so matches your definition above).
> 
> All other (MMU) m68k users of inb()/outb() apply an io_offset in the 
> platform specific address translation:
> 
> 
---8<---
> So as long as support for any of the m68k PCI or ISA bridges is selected 
> in the kernel config, the appropriate IO space mapping is applied. If no 
> support for PCI or ISA bridges is selected, we already fall back to zero 
> offset mapping (but as far as I can tell, it shouldn't be possible to 
> build a kernel without bridge support but drivers that require it).
> 
> > As this is indistinguishable from architectures that just don't have
> > a base address for I/O ports (we unfortunately picked 0 as the default
> > PCI_IOBASE value), my suggestion was to start marking architectures
> > that may have this problem as using HAS_IOPORT in order to keep
> > the existing behavior unchanged. If m68k does not suffer from this,
> > making HAS_IOPORT conditional on those config options that actually
> > need it would of course be best.
> 
> Following your description, HAS_IOPORT would be required for neither of 
> PCI, ISA or ATARI_ROM_ISA ??
> 

No, HAS_IOPORT being set just means that inb() etc. exist and are
functional be it as special instructions like on x86 or via an I/O
address offset. As I understand it if you do have PCI, ISA or
ATARI_ROM_ISA they are functional. If none of them are set and your
zero offset mapping means these accessors can't actually be used you
could make the declerations ifdeffed on CONFIG_HAS_IOPORT to detect the
cases where somone managed to build drivers that require them and that
would result in a compile time error instead of silently, or with a
NULL pointer warning, compiling code that won't work.