Re: MMIO and gcc re-ordering (Was: [PATCH] [POWERPC] Improve (in|out)_beXX()asm code)

From: Scott Wood
Date: Fri May 23 2008 - 17:13:38 EST

Benjamin Herrenschmidt wrote:
On Fri, 2008-05-23 at 08:36 -0400, Benjamin Herrenschmidt wrote:
- mandate some kind of dma_sync_for_device/cpu on consistent memory.
Almost no driver do that currently tho. They only do that for non
consistent memory mapped with dma_map_*.

- mandate the use of wmb,rmb,mb barriers for use between memory
accesses and MMIOs for ordering them. (ie. fix drivers that don't do
it). Advantage for powerpc is that I can remove (after some auditing of
course) the added heavy barriers in the MMIO accessors themselves.

Note that the above is my preferred approach, and a lot of drivers
happen to already do this.

As Trent pointed out, if you change to eieio in the accessors, that'd require drivers to also use mmiowb() before spin_unlock(), which fewer drivers currently do.

- stick a full memory clobber in all MMIO (and PIO) accessors on all

I like this, combined with introducing raw variants of the non-PCI accessors (in_be32 and such). It's slower and safe by default (i.e. no auditing drivers), but performance-critical paths can be optimized to use raw accessors combined with explicit barriers.

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