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

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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



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