Re: HELP: Is writeq an atomic operation??

Roland Dreier
2008-05-02T22:46:31Z

> 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.

But I'm sure no one has stressed the drivers on 64-bit mips or anything
unusual like that.

- R.