Re: [PATCH] Add io_sync stubs to generic IO memcpy/memset

[Julian Vetter]

On 1/27/25 16:48, Arnd Bergmann wrote:
> On Mon, Jan 27, 2025, at 15:11, Julian Vetter wrote:
> > On 1/27/25 11:27, Arnd Bergmann wrote:
> > > On Mon, Jan 27, 2025, at 11:04, Julian Vetter wrote:
> >
> > Thank you for your quick reply. You're right, I was just going with the
> > naming used in the powerpc arch which has an io_sync define. I'm now
> > wondering if we can't simply use the read{l,q}/write{l,q} functions
> > (instead of the __raw_xxx version), there are already calls to __io_br
> > before and__io_ar after each read (and write). But this might have
> > performance implications on some architectures, depending what it
> > resolves to.
> >
> > Otherwise I propose renaming the __pre_io_sync and __post_io_sync into a
> > single __io_mbr which is called before and after each loop. Looking at
> > PowerPC and SuperH, both of them could be consolidated into the generic
> > IO memcpy code when adding this. What do you think?
>
> Having barriers between the accesses would be very expensive, and
> prevent the write-combining and prefetching that can otherwise happen
> (depending on mapping flags).

Yes, ok. I see.

> I suspect that the powerpc variant got this wrong for historic
> reasons, but that's hard to tell now. The ppc32 variant didn't
> have barriers at all originally, it was just memcpy/memset
> before it got combined with ppc64 into arch/powerpc.

hmmm... ok. I'm not sure what do make of this. But maybe I can just send 
a patch to the PowerPC mailinglist, without those "sync" calls and see 
what they have to say.

> > The existing ones, especially __io_br unfortunately don't resolve to the
> > right define on these architectures. The __io_ar and __io_br resolve to
> > the right mb() on SuperH and PowerPC as well, but this would again have
> > implications on other architectures.
>
> The barriers in the sh functions seem arbitrary, and I would
> expect them to be wrong.
Ok, same for SuperH, I will send a patch to the mailinglist without the 
'mb()' before and after and see what they say. If they don't like it, I 
will come back to this.

Julian

>        ARnd