RE: [RFC v2 10/39] gpio: add HAS_IOPORT dependencies

[Maciej W. Rozycki]

On Tue, 3 May 2022, David Laight wrote:

> > > 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.
> > 
> > Adding that dependency seems like the right solution to me.
> 
> I think it all depends on what HAS_IOPORT is meant to mean and
> how portable kernel binaries need to be.
> 
> x86 is (probably) the only architecture that actually has 'in'
> and 'out' instructions - but that doesn't mean that some other
> cpu (and I mean cpu+pcb not architecture) have the ability to
> generate 'IO' bus cycles on a specific physical bus.

 I am fairly sure IA-64 has some form of IN/OUT machine instructions too.

> While the obvious case is a physical address window that generates
> PCI(e) IO cycles from normal memory cycles it isn't the only one.
> 
> I've used sparc cpu systems that have pcmcia card slots.
> These are pretty much ISA and the drivers might expect to
> access port 0x300 (etc) - certainly that would be right on x86.
> 
> In this case is isn't so much that the ISA_BUS depends on support
> for in/out but that presence of the ISA bus provides the required
> in/out support.

 Well, one can implement a pluggable PCI/e expansion card with a PCI-ISA 
bridge on it and a backplane to plug ISA cards into.  Without support for 
issuing I/O cycles to PCI from the host however you won't be able to make 
use of the ISA backplane except maybe for some ancient ISA memory cards.  
So logically I think CONFIG_ISA should depend on CONFIG_HAS_IOPORT and 
CONFIG_HAS_IOPORT ought to be selected by platform configurations.

 ISTR there was a company that manufactured a USB-ISA option (providing an 
external ISA backplane).  We never supported it, but in principle if we 
wanted to, then it would be the USB-ISA device's driver config option that 
CONFIG_ISA would additionally depend on as an alternative.  That wouldn't 
enable CONFIG_HAS_IOPORT though because the presence of this particular 
USB-ISA device would not itself permit the use of I/O cycles with any 
PCI/e buses a machine might independently have, so devices for PCI/e 
options that require port I/O shouldn't be made available at the same 
time.

 I think that company might have actually manufactured a similar PCI-ISA 
option as well, but that I suppose did rely on support for I/O cycles on 
PCI.  Early 2000s BTW.

  Maciej