Re: [PATCH 35/43] drm/i915: Move __raw_i915_read8() & co. into i915_drv.h

Jesse Barnes
2015-09-21T16:26:02Z

On 09/18/2015 12:26 PM, Ville Syrjälä wrote:
> On Fri, Sep 18, 2015 at 07:44:34PM +0100, Chris Wilson wrote:
> Yeah I guess that was a crappy example. Trying to think of a better one,
> I figure execlist status could be read from memory, but apparently
> that's just mmio. I guess the context update + ELSP write is something
> we do from the irq handler, but there are plenty of barriers between
> those two it seems. Maybe there's no good example here.
>
>>
>> Anyway, I thought we had strongly ordered reads on x64/x32?
>
> The cpu won't reoder reads vs. reads, or writes vs. writes for that
> matter if you ignore the nt stuff and whatnot. But AFAIU the whole
> point of _relaxed() on x86 is that it allows the compiler to reoder
> memory vs. mmio any which way it wants.

If you mean the top level readX_relaxed() functions, those were more
about re-ordering on a large I/O fabric than compiler re-ordering. I'm
not sure how the compiler handles things these days; I thought maybe the
volatile accesses would imply a re-order barrier as well, since they
could have side effects the compiler can't anticipate (i.e. re-ordering
even read after read could change behavior). But it would be good to
double check... and according to memory-barriers.txt we should be safe
there, from the readX_relaxed() section:

"Note that relaxed accesses to the same peripheral are guaranteed to be
ordered with respect to each other."

Jesse