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

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Date:	Fri, 02 May 2008 17:42:58 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Roland Dreier <rdreier@...co.com>
CC:	"Moore\, Eric" <Eric.Moore@....com>, linux-kernel@...r.kernel.org
Subject: Re: HELP:  Is writeq an atomic operation??

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
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