linux-kernel - Re: HELP: Is writeq an atomic operation??

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Date:	Fri, 02 May 2008 15:46:20 -0700
From:	Roland Dreier <rdreier@...co.com>
To:	"Moore\, Eric" <Eric.Moore@....com>
Cc:	<linux-kernel@...r.kernel.org>
Subject: Re: HELP:  Is writeq an atomic operation??

 > 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.
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