memory barriers in bus_dmamap_sync() ?

Luigi Rizzo rizzo at
Tue Jan 10 22:24:01 UTC 2012

On Tue, Jan 10, 2012 at 01:52:49PM -0800, Adrian Chadd wrote:
> On 10 January 2012 13:37, Luigi Rizzo <rizzo at> wrote:
> > I was glancing through manpages and implementations of bus_dma(9)
> > and i am a bit unclear on what this API (in particular, bus_dmamap_sync() )
> > does in terms of memory barriers.
> >
> > I see that the x86/amd64 and ia64 code only does the bounce buffers.
> > The mips seems to do some coherency-related calls.
> >
> > How do we guarantee, say, that a recently built packet is
> > to memory before issuing the tx command to the NIC ?
> The drivers should be good examples of doing the right thing. You just
> do pre-map and post-map calls as appropriate.
> Some devices don't bother with this on register accesses and this is a
> bug. (eg, ath/ath_hal.) Others (eg iwn) do explicit flushes where
> needed.

so you are saying that drivers are correct unless they are buggy :)

Anyways... i see that some drivers use wmb() and rmb() and redefine
their own version, usually based on lfence/sfence even on i386

	#define rmb()	__asm volatile("lfence" ::: "memory")
	#define wmb()	__asm volatile("sfence" ::: "memory")

whereas the standard definitions are slightly different, e.g.

    #define      rmb()   __asm __volatile("lock; addl $0,(%%esp)" : : : "memory")
    #define      wmb()   __asm __volatile("lock; addl $0,(%%esp)" : : : "memory")

and our bus_space API in sys/x86/include/bus.h is a bit unclear to
me (other than the fact that having 4 unused arguments don't really
encourage its use...)

    static __inline void
    bus_space_barrier(bus_space_tag_t tag __unused, bus_space_handle_t bsh __unused,
		      bus_size_t offset __unused, bus_size_t len __unused, int flags)
    #ifdef __GNUCLIKE_ASM
	    if (flags & BUS_SPACE_BARRIER_READ)
    #ifdef __amd64__
		    __asm __volatile("lock; addl $0,0(%%rsp)" : : : "memory");
		    __asm __volatile("lock; addl $0,0(%%esp)" : : : "memory");
		    __asm __volatile("" : : : "memory");


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