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

[John Garry]

On 03/03/2020 16:40, Arnd Bergmann wrote:
> 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.

Ok.

Apart from that, this code is somewhat hidden. I mean, most people would 
consider generic IO port accessors come from asm-generic/io.h only, 
which is not the case here. Maybe this can be better integrated into 
asm-generic/io.h, the only hint today being the logic_pio.h include half 
way through the file.

> 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.

Jiaxun Yang did mention that MIPS may want to move away from its own IO 
space management.

Thanks,
John