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

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Message-Id: <1209854242.26383.30.camel@pasglop>
Date:	Sun, 04 May 2008 08:37:22 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
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??


On Fri, 2008-05-02 at 15:46 -0700, 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.
> 
> But I'm sure no one has stressed the drivers on 64-bit mips or anything
> unusual like that.

Surely only on 64 bits archs right ?

Ben.


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