Re: [PATCH v2 00/18] Cross-architecture definitions of relaxed MMIO accessors

[Benjamin Herrenschmidt]

On Thu, 2014-05-22 at 17:47 +0100, Will Deacon wrote:
> A corollary to this is that mmiowb() probably needs rethinking. As it currently
> stands, an mmiowb() is required to order MMIO writes to a device from multiple
> CPUs, even if that device is protected by a lock. However, this isn't often used
> in practice, leading to PowerPC implementing both mmiowb() *and* synchronising
> I/O in spin_unlock.
> 
> I would propose making the non-relaxed I/O accessors ordered with respect to
> LOCK/UNLOCK, leaving mmiowb() to be used with the relaxed accessors, if
> required, but would welcome thoughts/suggestions on this topic.

I agree on the proposed semantics, though for us that does mean we still need
that per-cpu flag tracking non-relaxed MMIO stores and corresponding added barrier
in unlock. Eventually, if the use of the relaxed accessors becomes pervasive
enough I suppose I can just make the ordered ones unconditionally do 2 barriers.

Cheers,
Ben.


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/