Re: [RFC] I/O Access Abstractions

[David Howells]

Jes Sorensen <[e-mail]> wrote:
> >>>>> "David" == David Woodhouse <[e-mail]> writes:
> 
> David> Having per-resource I/O methods would help us to remove some of
> David> the cruft which is accumulating in various non-x86 code. Note
> David> that the below is the _core_ routines for _one_ board - I'm not
> David> even including the extra indirection through the machine vector
> David> here....
> 
> Have you considered the method used by the 8390 Ethernet driver?
> For each device, add a pointer to the registers and a register shift.

And also flags to specify which address space the I/O ports reside in, and
which how to adjust the host-PCI bridge to bring the appropriate bit of the
PCI address space into view through the CPU address space window. Don't laugh
- it happens.

> I really don't like hacing virtual access functions that makes memory
> mapped I/O look the same as I/O operations.

Why not? It makes drivers a simpler and more flexible if they can treat
different types of resource in the same way. serial.c is a really good example
of this:

| static _INLINE_ unsigned int serial_in(struct async_struct *info, int offset)
| {
|         switch (info->io_type) {
| #ifdef CONFIG_HUB6
|         case SERIAL_IO_HUB6:
|                 outb(info->hub6 - 1 + offset, info->port);
|                 return inb(info->port+1);
| #endif
|         case SERIAL_IO_MEM:
|                 return readb((unsigned long) info->iomem_base +
|                              (offset<<info->iomem_reg_shift));
| #ifdef CONFIG_SERIAL_GSC
|         case SERIAL_IO_GSC:
|                 return gsc_readb(info->iomem_base + offset);
| #endif
|         default:
|                 return inb(info->port + offset);
|         }
| }

The switch has to be performed at runtime - so you get at least one
conditional branch in your code, probably more. My proposal would replace the
conditional branch with a subroutine (which lacks the pipline stall).

Also a number of drivers (eg: ne2k-pci) have to be bound to compile time to
either I/O space or memory space. This would allow you to do it at runtime
without incurring conditional branching.

> For memory mapped I/O you want to be able to smart optimizations to reduce
> the access on the PCI bus (or similar).

I wouldn't have thought you'd want to reduce the number of accesses to the PCI
bus. If, for example, you did to writes to a memory-mapped I/O register, you
don't want the first to be optimised away.

David