linux-kernel - Re: [GIT PULL 1/2] asm-generic: rework PCI I/O space access

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Date:   Fri, 2 Jul 2021 12:42:57 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     linux-arch <linux-arch@...r.kernel.org>,
        linux-pci <linux-pci@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Niklas Schnelle <schnelle@...ux.ibm.com>
Subject: Re: [GIT PULL 1/2] asm-generic: rework PCI I/O space access

On Fri, Jul 2, 2021 at 6:48 AM Arnd Bergmann <arnd@...nel.org> wrote:
>
> A rework for PCI I/O space access from Niklas Schnelle:

I pulled this, but then I ended up unpulling.

I don't absolutely _hate_ the concept, but I really find this to be
very unpalatable:

  #if !defined(inb) && !defined(_inb)
  #define _inb _inb
  static inline u8 _inb(unsigned long addr)
  {
  #ifdef PCI_IOBASE
        u8 val;

        __io_pbr();
        val = __raw_readb(PCI_IOBASE + addr);
        __io_par(val);
        return val;
  #else
        WARN_ONCE(1, "No I/O port support\n");
        return ~0;
  #endif
  }
  #endif

because honestly, the notion of a run-time warning for a compile-time
"this cannot work" is just wrong.

If the platform doesn't have inb/outb, and you compile some driver
that uses them, you don't want a run-time warning. Particularly since
in many cases nobody will ever run it, and the main use case was to do
compile-testing across a wide number of platforms.

So if the platform doesn't have inb/outb, they simply should not be
declared, and there should be a *compile-time* error. That is
literally a lot more useful, and it avoids this extra code.

Extra code that not only doesn't add value, but that actually
*subtracts* value is not code I really want to pull.

                     Linus