Re: About commit "io: change inX() to have their own IO barrier overrides"

[Arnd Bergmann]

On Tue, Mar 3, 2020 at 2:18 PM John Garry <john.garry@xxxxxxxxxx> wrote:
>
> + linux-arch
>
> For background, see
> https://lore.kernel.org/lkml/2e80d7bc-32a0-cc40-00a9-8a383a1966c2@xxxxxxxxxx/
>
> >>
> >> So today only ARM64 uses it for this relevant code, above. But maybe
> >> others in future will want to use it - any arch without native IO port
> >> access is a candidate.
> >
> > I'm looking at Arnd here for help.
> >
> >>
> >>>
> >>> As long as the expectations are set, I see no reason why it shouldn't
> >>> but, I'll let Arnd comment on it too.
> >>
> >> ok, so it looks reasonable consider replicating your change for ***, above.
>
> To be clear, I would make this change in lib/logic_pio.c since
> __io_pbr() can be overridden per-arch:
>
>   #define BUILD_LOGIC_IO(bw, type)
>   type logic_in##bw(unsigned long addr)
>   {
>        type ret = (type)~0;
>        if (addr < MMIO_UPPER_LIMIT) {
> -          ret = read##bw(PCI_IOBASE + addr);
> +          __io_pbr();
> +          ret = __raw_read##bw(PCI_IOBASE + addr);
> +          __io_pbr();

__io_par();

>        } else if (addr >= MMIO_UPPER_LIMIT && addr < IO_SPACE_LIMIT) {
>            struct logic_pio_hwaddr *entry = find_io_range(addr);
>
> ...
>
> (forgetting leX_to_cpu for the moment)

Yes, I suppose this is required to get consistent behavior on arm64,
which overrides __io_par() but not __io_ar(), with the current code
the barrier after read is weaker when LOGIC_PIO is enabled than it
is otherwise.

For other architectures, I suppose we would need another indirection
level, as those can also override the default inb() itself to do something
other than readb(PCI_IOBASE + addr), and that is not handled
here either. We can do that if we need LOGIC_PIO on a second
architecture.

       Arnd