Re: HELP: Is writeq an atomic operation??

H. Peter Anvin
2008-05-03T00:43:30Z

Roland Dreier wrote:
> > Is a 64bit write to MMIO registers an atomic operation when using the
> > writeq API?
> >
> > My concern is when I send 64bit data via writeq, will it be sent out as
> > two 32 bit writes? If so, is it possible that another CPU be sending
> > the data at the same time. Meaning can I write the 1st 32bit data from
> > CPU-A, meanwhile CPU-B is writing his 32bit data at the same time, and
> > CPU-A didn't complete the full 64bit in one shot. If this could occur,
> > is there an API that I can use to make sure the entire data sent in one
> > atomic operation?
>
> I don't have an authoritative answer, but I can say that I coded
> drivers/infiniband/hw/mthca and .../mlx4 assuming that writeq() is
> atomic in the sense that you say, and no one has reported any problems.
>

If you're not under lock you're screwed on a 32-bit platform.

-hpa