Re: Opinion on ordering of writel vs. stores to RAM

[Segher Boessenkool <segher@kernel.crashing.org>]

> >  - writel/readl become totally ordered (including vs. memory).
> > Basically x86-like. Expensive (very expensive even on some
> > architectures) but also very safe.
>
> This approach will minimize driver changes, and would imply the  
> removal
> of some existing mmiowb() and wmb() macros.

Like I tried to explain already, in my competing approach, no drivers
would need changes either.  And you could remove those macro's (or
their more-verbosely-saying-what-their-doing, preferably bus-specific
as well) as well -- but you'll face the wrath of those who care about
performance of those drivers on non-x86 platforms.

> This is what mmiowb() is supposed to be, though only for writes.  I.e.
> two writes from different CPUs may arrive out of order if you don't  
> use
> mmiowb() carefully.  Do you also need a mmiorb() macro or just a
> stronger mmiob()?

I'd name this barrier pci_cpu_to_cpu_barrier() -- what it is supposed
to do is order I/O accesses from the same device driver to the same
device, from different CPUs.  The same driver is never concurrently
running on more than one CPU right now, which is a fine model.

I include "pci_" in the name, so that we can distinguish between
different bus types, which after all have different ordering rules.
PCI is a very common bus type of course, which explains why there
is mmiowb() and no ..rb() -- this is simply not needed on PCI
(PCI MMIO reads are _always_ slow -- non-posted accesses, in PCI
terminology).

> mmiowb() could be written as io_to_io_write_barrier() if we wanted  
> to be
> extra verbose.  AIUI it's the same thing as smb_wmb() but for MMIO
> space.

Except that "main-memory" ("coherent domain") accesses are always
atomic as far as this ordering is concerned -- starting a transaction
and having its result are not separately observable.  For I/O this
is different -- the whole point of mmiowb() was that although without
it the device drivers _start_ the transactions in the right order just
fine, the order the I/O adapters see them might be different (because
there are multiple paths from different CPUs to the same I/O adapter, or
whatnot).

Hence my proposal of calling it pci_cpu_to_cpu_barrier() -- what it
orders is accesses from separate CPUs.  Oh, and it's bus-specific,
of course.


Segher

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