netdev - Re: [PATCH 2/3] x86_64: Define 128-bit memory-mapped I/O operations

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Date:	Wed, 22 Aug 2012 08:49:41 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	David Laight <David.Laight@...LAB.COM>
CC:	Ben Hutchings <bhutchings@...arflare.com>,
	Benjamin LaHaise <bcrl@...ck.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Miller <davem@...emloft.net>, tglx@...utronix.de,
	mingo@...hat.com, netdev@...r.kernel.org,
	linux-net-drivers@...arflare.com, x86@...nel.org
Subject: Re: [PATCH 2/3] x86_64: Define 128-bit memory-mapped I/O operations

On 08/22/2012 08:27 AM, David Laight wrote:
>> Your architecture sounds similar to one I once worked on (Orion
>> Microsystems CNIC/OPA-2).  That architecture had a descriptor ring in
>> device memory, and a single trigger bit would move the head pointer.
>>
>> We used write combining to write out a set of descriptors, and then
>> used
>> a non-write-combining write to do the final write which bumps the head
>> pointer.  The UC write flushes the write combiners ahead of it, so it
>> ends up with two transactions (one for the WC data and one for the UC
>> trigger) but it could frequently push quite a few descriptors in that
>> operation.
>
> The code actually looks more like a normal ethernet ring interface
> with an 'owner' bit in each entry.
> So it is important to write the owner bit last.
>
> It might be possibly to set multiple ring entries in two TLPs
> by first writing all of them (maybe with write combining)
> but without changing the ownership of the first entry.
> Then doing a second transfer to update the owner bit it
> the first entry.
> The order of the writes in the first transfer would then not
> matter.
>
> FWIW can you even guarantee to do an atomic 64bit PCIe transfer
> on many systems (without resorting to a dma unit).
>

On many systems, perhaps, but I suspect that 32 bits is the maximum you 
can truly guarantee.

	-hpa


-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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