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

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Message-Id: <200805021612.34281.jbarnes@virtuousgeek.org>
Date:	Fri, 2 May 2008 16:12:34 -0700
From:	Jesse Barnes <jbarnes@...tuousgeek.org>
To:	"Moore, Eric" <Eric.Moore@....com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: HELP:  Is writeq an atomic operation??

On Friday, May 02, 2008 3:40 pm Moore, Eric 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?
>
>
> 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)

I think this is normal; PCIe defines transactions in terms of dwords, to a 64 
bit write would indeed be a transaction packet with a length of two (it can 
go up to 4k).  AFAIK though transactions are processed as a whole, so even a 
4k write (as long as it's generated as a single transaction) won't result in 
the device seeing e.g. 2x2k writes.  I'd have to double check the routing 
rules to be 100% sure though, maybe in some cases the fabric is allowed to 
break up transactions (?).

Jesse
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