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HELP: Is writeq an atomic operation??

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From: Moore, Eric <Eric.Moore@...>
To: <linux-kernel@...>
Subject: HELP: Is writeq an atomic operation??
Date: Friday, May 2, 2008 - 6:40 pm

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?


Here is a trace from pci express analyzer.   I'm sending
0x0800010000000000 to the adress DD1400C0 using writeq.   Notice that in
the TLP header it sent a 32bit Memory write with data length of two.

Trace follows:

Link Tra(597) Downstream 2.5(x1) TLP(1992) Mem MWr(32)(10:00000) TC(0)
TD(0) 
_______| EP(0) Attributes(01) Length(2) RequesterID(000:02:0) Tag(8) 
_______| Address(DD1400C0) 1st BE(1111) Last BE(1111) Data(08000100
00000000) 
_______| VC ID(0) Explicit ACK(Packet #1195) Metrics # Packets(2) 
_______| Time Stamp(0003 . 120 181 840 s) 
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Messages in current thread:
HELP: Is writeq an atomic operation??, Moore, Eric, (Fri May 2, 6:40 pm)
Re: HELP: Is writeq an atomic operation??, H. Peter Anvin, (Fri May 2, 8:41 pm)
Re: HELP: Is writeq an atomic operation??, Jesse Barnes, (Fri May 2, 7:12 pm)
Re: HELP: Is writeq an atomic operation??, Roland Dreier, (Fri May 2, 7:04 pm)
RE: HELP: Is writeq an atomic operation??, Moore, Eric, (Fri May 2, 7:20 pm)
Re: HELP: Is writeq an atomic operation??, Roland Dreier, (Fri May 2, 8:10 pm)
Re: HELP: Is writeq an atomic operation??, Andi Kleen, (Fri May 2, 6:50 pm)
RE: HELP: Is writeq an atomic operation??, Moore, Eric, (Fri May 2, 7:03 pm)
Re: HELP: Is writeq an atomic operation??, Andi Kleen, (Fri May 2, 7:13 pm)
Re: HELP: Is writeq an atomic operation??, Roland Dreier, (Fri May 2, 6:46 pm)
Re: HELP: Is writeq an atomic operation??, Benjamin Herrenschmidt, (Sat May 3, 6:37 pm)
Re: HELP: Is writeq an atomic operation??, Roland Dreier, (Sun May 4, 1:01 pm)
Re: HELP: Is writeq an atomic operation??, H. Peter Anvin, (Fri May 2, 8:42 pm)
Re: HELP: Is writeq an atomic operation??, Alan Cox, (Sat May 3, 10:35 am)
Re: HELP: Is writeq an atomic operation??, H. Peter Anvin, (Sat May 3, 1:40 pm)