linux-kernel - Re: [RFC v2 10/39] gpio: add HAS

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Date:   Mon, 2 May 2022 14:21:06 +0100 (BST)
From:   "Maciej W. Rozycki" <macro@...am.me.uk>
To:     William Breathitt Gray <william.gray@...aro.org>
cc:     Niklas Schnelle <schnelle@...ux.ibm.com>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-pci@...r.kernel.org, Arnd Bergmann <arnd@...nel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Bartosz Golaszewski <brgl@...ev.pl>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>
Subject: Re: [RFC v2 10/39] gpio: add HAS_IOPORT dependencies

On Fri, 29 Apr 2022, William Breathitt Gray wrote:

> > Good question. As far as I can see most (all?) of these have "select
> > ISA_BUS_API" which is "def_bool ISA". Now "config ISA" seems to
> > currently be repeated in architectures and doesn't have an explicit
> > HAS_IOPORT dependency (it maybe should have one). But it does only make
> > sense on architectures with HAS_IOPORT set.
> 
> There is such a thing as ISA DMA, but you'll still need to initialize
> the device via the IO Port bus first, so perhaps setting HAS_IOPORT for
> "config ISA" is the right thing to do: all ISA devices are expected to
> communicate in some way via ioport.

 Strictly speaking you can make an ISA device that only does MMIO (and I 
believe in the early PC days there used to be ISA memory expansion cards 
along with the EMS standard) which is also why the host memory area in the 
15-16MiB range, the top 1MiB addressable on 16-bit ISA, can be excluded 
from decoding to DRAM and accesses made there forwarded to ISA in I 
believe all chipsets that provide actual ISA bus circuitry (rather than 
just a degenerate form like LPC).  That's an exception rather than the 
rule though, nearly all ISA devices do decode in the port I/O space.  
After all I/O is what the port I/O address space has been invented for.

 FWIW,

  Maciej