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From: Jason Gunthorpe <jgg@nvidia.com>
To: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
	Clint Sbisa <csbisa@amazon.com>, <linux-pci@vger.kernel.org>,
	Bjorn Helgaas <helgaas@kernel.org>,
	<linux-arm-kernel@lists.infradead.org>, <will@kernel.org>,
	<catalin.marinas@arm.com>, Leon Romanovsky <leon@kernel.org>
Subject: Re: [PATCH] arm64: Enable PCI write-combine resources under sysfs
Date: Wed, 16 Sep 2020 09:12:26 -0300	[thread overview]
Message-ID: <20200916121226.GN1573713@nvidia.com> (raw)
In-Reply-To: <701012f288231d0d0733bf1c2c8fdbd9caa074fd.camel@kernel.crashing.org>

On Wed, Sep 16, 2020 at 05:59:24PM +1000, Benjamin Herrenschmidt wrote:
> On Tue, 2020-09-15 at 20:40 -0300, Jason Gunthorpe wrote:
> > Not quite, upstream kernel will never use WC on those
> > devices. DEVICE_GRE is not supported in upstream,
> > arch_can_pci_mmap_wc() is always false and the WC tester will always
> > fail.
> > 
> > > With the patch, those device will now use MT_DEVICE_NC.
> > 
> > Which doesn't do WC at all on some ARM implementations.
> 
> Lovely... this is arm64 btw, still the case ?

Yep

> Also we could make this a variable rather than a constant and choose
> a more appropriate set of flags at boot time....

It is a function, so it could check the CPU ID for the known broken
devices and block them.
 
> > > Why would that be a regression ? 
> > 
> > Using the WC submission flow when it doesn't work costs something like
> > 10% performance vs using the non-WC flow.
> 
> You mean the driver uses a different path to the HW which ahs that
> overhead, not that MMIOs have that overhead right ?

The different path has overhead of doing extra useless MMIOs because
they don't combine

Jason

  reply	other threads:[~2020-09-16 13:27 UTC|newest]

Thread overview: 59+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-31 15:18 [PATCH] arm64: Enable PCI write-combine resources under sysfs Clint Sbisa
2020-08-31 23:24 ` Benjamin Herrenschmidt
2020-09-01 18:37 ` Bjorn Helgaas
2020-09-01 23:22   ` Benjamin Herrenschmidt
2020-09-02  8:57     ` Will Deacon
2020-09-02 11:32 ` Lorenzo Pieralisi
2020-09-02 14:29   ` Clint Sbisa
2020-09-02 16:47     ` Lorenzo Pieralisi
2020-09-02 17:21       ` Catalin Marinas
2020-09-02 17:54         ` Lorenzo Pieralisi
2020-09-02 23:03           ` Benjamin Herrenschmidt
2020-09-02 23:08         ` Benjamin Herrenschmidt
2020-09-02 23:08           ` Benjamin Herrenschmidt
2020-09-02 23:07       ` Benjamin Herrenschmidt
2020-09-03 11:08         ` Lorenzo Pieralisi
2020-09-03 14:36           ` Clint Sbisa
2020-09-03 22:26           ` Benjamin Herrenschmidt
2020-09-07 23:33           ` Benjamin Herrenschmidt
2020-09-10  9:46             ` Lorenzo Pieralisi
2020-09-10 10:54               ` Leon Romanovsky
2020-09-10 12:37               ` Jason Gunthorpe
2020-09-10 15:17                 ` Lorenzo Pieralisi
2020-09-10 17:10                   ` Jason Gunthorpe
2020-09-10 21:46                     ` Benjamin Herrenschmidt
2020-09-10 23:29                       ` Jason Gunthorpe
2020-09-11  0:39                         ` Benjamin Herrenschmidt
2020-09-11 14:21                           ` Jason Gunthorpe
2020-09-11 21:42                           ` Clint Sbisa
2020-09-14 14:17                             ` Jason Gunthorpe
2020-09-14 14:24                               ` Clint Sbisa
2020-09-14 14:38                                 ` Jason Gunthorpe
2020-09-14 21:42                                   ` Benjamin Herrenschmidt
2020-09-14 22:00                                     ` Benjamin Herrenschmidt
2020-09-14 22:32                                       ` Clint Sbisa
2020-09-14 22:57                                       ` Jason Gunthorpe
2020-09-14 23:25                                         ` Benjamin Herrenschmidt
2020-09-15 10:18                                           ` Lorenzo Pieralisi
2020-09-15 11:05                                             ` Jason Gunthorpe
2020-09-15 23:17                                               ` Benjamin Herrenschmidt
2020-09-15 23:40                                                 ` Jason Gunthorpe
2020-09-16  7:59                                                   ` Benjamin Herrenschmidt
2020-09-16 12:12                                                     ` Jason Gunthorpe [this message]
2020-09-16 14:09                                                       ` Lorenzo Pieralisi
2020-09-16 14:14                                                         ` Jason Gunthorpe
2020-09-16 23:59                                                       ` Benjamin Herrenschmidt
2020-09-17 10:28                                                         ` Lorenzo Pieralisi
2020-09-17 11:32                                                           ` Jason Gunthorpe
2020-09-17 14:01                                                             ` Lorenzo Pieralisi
2020-09-17 16:08                                                               ` Will Deacon
2020-09-16 12:48                                                     ` Leon Romanovsky
2020-09-16  8:33                                                   ` Will Deacon
2020-09-16  8:48                                                     ` Catalin Marinas
2020-09-16 14:15                                                       ` Lorenzo Pieralisi
2020-09-16 17:00                                                         ` Catalin Marinas
2020-09-16 21:29                                                           ` Benjamin Herrenschmidt
2020-09-16 12:08                                                     ` Jason Gunthorpe
2020-09-15 23:00                                             ` Benjamin Herrenschmidt
2020-09-15 23:12                                               ` Clint Sbisa
2020-09-14 21:41                               ` Benjamin Herrenschmidt

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