netdev - Re: [PATCH 4/6] arm64/io: Provide a WC friendly __iowriteXX

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Message-ID: <20240223130308.GF13330@nvidia.com>
Date: Fri, 23 Feb 2024 09:03:08 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: David Laight <David.Laight@...lab.com>
Cc: 'Niklas Schnelle' <schnelle@...ux.ibm.com>,
	Alexander Gordeev <agordeev@...ux.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christian Borntraeger <borntraeger@...ux.ibm.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
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	Vasily Gorbik <gor@...ux.ibm.com>,
	Heiko Carstens <hca@...ux.ibm.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Justin Stitt <justinstitt@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Leon Romanovsky <leon@...nel.org>,
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	Ingo Molnar <mingo@...hat.com>, Bill Wendling <morbo@...gle.com>,
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	Nick Desaulniers <ndesaulniers@...gle.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Paolo Abeni <pabeni@...hat.com>,
	Salil Mehta <salil.mehta@...wei.com>,
	Jijie Shao <shaojijie@...wei.com>,
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	Mark Rutland <mark.rutland@....com>,
	Michael Guralnik <michaelgur@...lanox.com>,
	"patches@...ts.linux.dev" <patches@...ts.linux.dev>,
	Will Deacon <will@...nel.org>
Subject: Re: [PATCH 4/6] arm64/io: Provide a WC friendly __iowriteXX_copy()

On Fri, Feb 23, 2024 at 12:19:24PM +0000, David Laight wrote:

> Since writes get 'posted' all over the place.
> How many writes do you need to do before write-combining makes a
> difference?

The issue is that the HW can optimize if the entire transaction is
presented in one TLP, if it has to reassemble the transaction it takes
a big slow path hit.

Jason