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From: Arnd Bergmann <arnd@kernel.org>
To: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Hector Martin <marcan@marcan.st>,
	linux-arm Mailing List <linux-arm-kernel@lists.infradead.org>,
	Marc Zyngier <maz@kernel.org>, Rob Herring <robh@kernel.org>,
	Olof Johansson <olof@lixom.net>,
	Krzysztof Kozlowski <krzk@kernel.org>,
	Mark Kettenis <mark.kettenis@xs4all.nl>,
	Tony Lindgren <tony@atomide.com>,
	Mohamed Mediouni <mohamed.mediouni@caramail.com>,
	Stan Skowronek <stan@corellium.com>,
	Alexander Graf <graf@amazon.com>, Will Deacon <will@kernel.org>,
	Linus Walleij <linus.walleij@linaro.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Jonathan Corbet <corbet@lwn.net>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Christoph Hellwig <hch@infradead.org>,
	"David S. Miller" <davem@davemloft.net>,
	devicetree <devicetree@vger.kernel.org>,
	"open list:SERIAL DRIVERS" <linux-serial@vger.kernel.org>,
	Linux Documentation List <linux-doc@vger.kernel.org>,
	Linux Samsung SOC <linux-samsung-soc@vger.kernel.org>,
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	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [RFT PATCH v3 12/27] of/address: Add infrastructure to declare MMIO as non-posted
Date: Fri, 5 Mar 2021 17:43:27 +0100	[thread overview]
Message-ID: <CAK8P3a1X4DyWdeBM1Vx+QMXU7+VhJrLHFLVzwAE4a4mb_xuqMQ@mail.gmail.com> (raw)
In-Reply-To: <CAHp75Vd6adVM94G1vCrQcZoegQFWHbK14YRRuBTQZwrM5CV2jQ@mail.gmail.com>

On Fri, Mar 5, 2021 at 5:08 PM Andy Shevchenko
<andy.shevchenko@gmail.com> wrote:
>
> On Fri, Mar 5, 2021 at 5:55 PM Hector Martin <marcan@marcan.st> wrote:
> > On 06/03/2021 00.13, Andy Shevchenko wrote:
>
> ...
>
> > >> -       return ioremap(res.start, resource_size(&res));
> > >> +       if (res.flags & IORESOURCE_MEM_NONPOSTED)
> > >> +               return ioremap_np(res.start, resource_size(&res));
> > >> +       else
> > >> +               return ioremap(res.start, resource_size(&res));
> > >
> > > This doesn't sound right. Why _np is so exceptional? Why don't we have
> > > other flavours (it also rings a bell to my previous comment that the
> > > flag in ioresource is not in the right place)?
> >
> > This is different from other variants, because until now *drivers* have
> > made the choice of what ioremap mode to use based on device requirements
> > (which means ioremap() 99% of the time, and then framebuffers and other
> > memory-ish things such use something else). Now we have a *SoC fabric*
> > that is calling the shots on what ioremap mode we have to use - and
> > *every* non-PCIe driver needs to use ioremap_np() on these SoCs, or they
> > break. So it seems a lot cleaner to make the choice for drivers here to
> > upgrade ioremap() to ioremap_np() for SoCs that need it.
>
> Yes, that is a good idea. Once we discussed x86 and _uc cases and
> actually on x86 it makes a lot of sense to have ioremap() ==
> ioremap_uc(). Can't be this a similar case here?

The difference is that ioremap() should be ioremap_uc() on /all/
architectures, it's just that x86 and ia64 for a long time were the
exception and defined ioremap() as 'do whatever the mtrr says'.

> Arnd, what do you think of actually providing an ioremap() as some
> kind of "best for the architecture the code is running on"?

Linus has been pretty clear about wanting all the default functions
to have similar behavior across architectures and be at least as
strict as x86. In case of ioremap() that usually means that writes
are posted, because they are that way on PCI buses on x86.

There are definitely some advantages of making all writes non-posted
by default on Arm because that would be a simpler model, but there
are some important downsides:

- non-posted writes can be much slower than posted ones, depending
  on the specific hardware

- it would change the behavior of all Arm platforms, with no easy
  way to validate it

- setting ioremap() on PCI buses non-posted only makes them
  only slower but not more reliable, because the non-posted flag
  on the bus is discarded by the PCI host bridge.

> Otherwise if the same driver happens to be needed on different
> architectures, oops, ifdeffery or simple conditionals over the code is
> really not the best way to solve it.

The behavior of devm_platform_ioremap_resource() is now to
do the right thing automatically, and I think that is good enough. For
all I can tell, we can use that in all drivers without conditional
compilation.

> > If we don't do something like this here or in otherwise common code,
> > we'd have to have an open-coded "if apple then ioremap_np, else ioremap"
> > in every driver that runs on-die devices on these SoCs, even ones that
> > are otherwise standard and need few or no Apple-specific quirks.
>
> Exactly! But what about architectures where _uc is that one? So, why
> does your patch only take part of _np case?
> (Hint we have x86 Device Tree based platforms)

Usually, the driver knows the requirements, and they are independent
of the platform. If a driver wants _wc or _wt mappings, it will want that
on all machines, and should be able to deal with platforms that implement
that through a stricter mapping.

The two drivers that need to override ioremap() to ioremap_uc()
in order to override the mtrr already have that logic. You are right
that these are a bit like the _np() case in that the device needs
something stricter than the default mapping, but the difference is
that for x86 ioremap_uc() this is needed to work around broken
firmware, while for the apple ioremap_np() case we trust the firmware
to tell us what to do.

> Yep, and why not to make ioremap() == ioremap_nc() on architecture
> that requires it?
> Can it be detected at run time?

I think doing this requires auditing a lot of legacy driver code,
especially drivers/video/fbdev. Once all drivers that need write-combining
mappings explicitly ask for them, the default ioremap can be changed
over to act like ioremap_nc() on the remaining two architectures that
don't do that yet.

There has been a lot of work toward that goal, but the problem
is knowing exactly which drivers need it.

       Arnd

  reply	other threads:[~2021-03-05 16:44 UTC|newest]

Thread overview: 136+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-04 21:38 [RFT PATCH v3 00/27] Apple M1 SoC platform bring-up Hector Martin
2021-03-04 21:38 ` [RFT PATCH v3 01/27] arm64: Cope with CPUs stuck in VHE mode Hector Martin
2021-03-24 18:05   ` Will Deacon
2021-03-24 20:00     ` Marc Zyngier
2021-03-26  7:54       ` Hector Martin
2021-03-04 21:38 ` [RFT PATCH v3 02/27] dt-bindings: vendor-prefixes: Add apple prefix Hector Martin
2021-03-08 20:26   ` Rob Herring
2021-03-04 21:38 ` [RFT PATCH v3 03/27] dt-bindings: arm: apple: Add bindings for Apple ARM platforms Hector Martin
2021-03-05 10:16   ` Linus Walleij
2021-03-08 20:27   ` Rob Herring
2021-03-04 21:38 ` [RFT PATCH v3 04/27] dt-bindings: arm: cpus: Add apple,firestorm & icestorm compatibles Hector Martin
2021-03-08 20:27   ` Rob Herring
2021-03-04 21:38 ` [RFT PATCH v3 05/27] arm64: cputype: Add CPU implementor & types for the Apple M1 cores Hector Martin
2021-03-24 18:13   ` Will Deacon
2021-03-04 21:38 ` [RFT PATCH v3 06/27] dt-bindings: timer: arm,arch_timer: Add interrupt-names support Hector Martin
2021-03-05 10:18   ` Linus Walleij
2021-03-08 11:12   ` Marc Zyngier
2021-03-08 17:14   ` Tony Lindgren
2021-03-08 20:38   ` Rob Herring
2021-03-08 22:42     ` Marc Zyngier
2021-03-09 16:11       ` Rob Herring
2021-03-09 20:28         ` Hector Martin
2021-03-04 21:38 ` [RFT PATCH v3 07/27] arm64: arch_timer: implement support for interrupt-names Hector Martin
2021-03-05 10:19   ` Linus Walleij
2021-03-08 11:13   ` Marc Zyngier
2021-03-04 21:38 ` [RFT PATCH v3 08/27] asm-generic/io.h: Add a non-posted variant of ioremap() Hector Martin
2021-03-05 14:45   ` Andy Shevchenko
2021-03-05 15:19     ` Hector Martin
2021-03-08 11:20   ` Marc Zyngier
2021-03-24 18:12   ` Will Deacon
2021-03-24 19:09     ` Arnd Bergmann
2021-03-25 14:07       ` Hector Martin
2021-03-25 14:49         ` Will Deacon
2021-03-04 21:38 ` [RFT PATCH v3 09/27] docs: driver-api: device-io: Document I/O access functions Hector Martin
2021-03-05 10:22   ` Linus Walleij
2021-03-04 21:38 ` [RFT PATCH v3 10/27] docs: driver-api: device-io: Document ioremap() variants & access funcs Hector Martin
2021-03-05 10:25   ` Linus Walleij
2021-03-05 15:09     ` Andy Shevchenko
2021-03-05 15:51       ` Arnd Bergmann
2021-03-09 20:29         ` Hector Martin
2021-03-04 21:38 ` [RFT PATCH v3 11/27] arm64: Implement ioremap_np() to map MMIO as nGnRnE Hector Martin
2021-03-08 11:22   ` Marc Zyngier
2021-03-24 18:18   ` Will Deacon
2021-03-04 21:38 ` [RFT PATCH v3 12/27] of/address: Add infrastructure to declare MMIO as non-posted Hector Martin
2021-03-05 10:28   ` Linus Walleij
2021-03-05 15:13   ` Andy Shevchenko
2021-03-05 15:55     ` Hector Martin
2021-03-05 16:08       ` Andy Shevchenko
2021-03-05 16:43         ` Arnd Bergmann [this message]
2021-03-05 17:19           ` Hector Martin
2021-03-05 16:05     ` Rob Herring
2021-03-05 17:39   ` Rob Herring
2021-03-05 18:18     ` Hector Martin
2021-03-05 21:17       ` Arnd Bergmann
2021-03-08 15:56         ` Rob Herring
2021-03-08 20:29           ` Arnd Bergmann
2021-03-08 21:13             ` Rob Herring
2021-03-08 21:56               ` Arnd Bergmann
2021-03-09 15:48                 ` Rob Herring
2021-03-09 20:23                   ` Hector Martin
2021-03-09 22:06                     ` Rob Herring
2021-03-10  8:26                       ` Hector Martin
2021-03-10 17:01                         ` Rob Herring
2021-03-11  9:12                           ` Arnd Bergmann
2021-03-11 12:11                             ` Hector Martin
2021-03-11 13:35                               ` Arnd Bergmann
2021-03-11 16:07                             ` Rob Herring
2021-03-11 16:48                               ` Arnd Bergmann
2021-03-11 18:10                                 ` Rob Herring
2021-03-12 10:20                                   ` Arnd Bergmann
2021-03-09 11:14               ` Linus Walleij
2021-03-09 12:41                 ` Arnd Bergmann
2021-03-09 15:40                   ` Linus Walleij
2021-03-04 21:38 ` [RFT PATCH v3 13/27] arm64: Add Apple vendor-specific system registers Hector Martin
2021-03-24 18:38   ` Will Deacon
2021-03-24 18:59     ` Mark Rutland
2021-03-24 19:04       ` Will Deacon
2021-03-26  6:23         ` Hector Martin
2021-03-04 21:38 ` [RFT PATCH v3 14/27] arm64: move ICH_ sysreg bits from arm-gic-v3.h to sysreg.h Hector Martin
2021-03-08 11:39   ` Marc Zyngier
2021-03-24 18:23   ` Will Deacon
2021-03-04 21:38 ` [RFT PATCH v3 15/27] dt-bindings: interrupt-controller: Add DT bindings for apple-aic Hector Martin
2021-03-08 21:16   ` Rob Herring
2021-03-04 21:38 ` [RFT PATCH v3 16/27] irqchip/apple-aic: Add support for the Apple Interrupt Controller Hector Martin
2021-03-05 15:05   ` Andy Shevchenko
2021-03-08 11:50     ` Marc Zyngier
2021-03-08 12:02       ` Andy Shevchenko
2021-03-26 13:40     ` Hector Martin
2021-03-08 13:31   ` Marc Zyngier
2021-03-26  7:57     ` Hector Martin
2021-03-24 19:57   ` Will Deacon
2021-03-26  8:58     ` Hector Martin
2021-03-29 12:04       ` Will Deacon
2021-04-01 13:16         ` Hector Martin
2021-03-04 21:38 ` [RFT PATCH v3 17/27] arm64: Kconfig: Introduce CONFIG_ARCH_APPLE Hector Martin
2021-03-08 15:35   ` Marc Zyngier
2021-03-09 20:30     ` Hector Martin
2021-03-04 21:38 ` [RFT PATCH v3 18/27] tty: serial: samsung_tty: Separate S3C64XX ops structure Hector Martin
2021-03-05 10:30   ` Krzysztof Kozlowski
2021-03-04 21:38 ` [RFT PATCH v3 19/27] tty: serial: samsung_tty: Add ucon_mask parameter Hector Martin
2021-03-05 10:34   ` Krzysztof Kozlowski
2021-03-04 21:38 ` [RFT PATCH v3 20/27] tty: serial: samsung_tty: Add s3c24xx_port_type Hector Martin
2021-03-05 10:49   ` Krzysztof Kozlowski
2021-03-04 21:38 ` [RFT PATCH v3 21/27] tty: serial: samsung_tty: IRQ rework Hector Martin
2021-03-05 10:51   ` Krzysztof Kozlowski
2021-03-05 15:17   ` Andy Shevchenko
2021-03-05 16:16     ` Hector Martin
2021-03-05 16:20       ` Andy Shevchenko
2021-03-05 16:29         ` Hector Martin
2021-03-07 11:34           ` Krzysztof Kozlowski
2021-03-07 16:01             ` Arnd Bergmann
2021-03-07 19:51               ` Krzysztof Kozlowski
2021-03-04 21:38 ` [RFT PATCH v3 22/27] tty: serial: samsung_tty: Use devm_ioremap_resource Hector Martin
2021-03-05 10:54   ` Krzysztof Kozlowski
2021-03-05 15:19     ` Andy Shevchenko
2021-03-04 21:38 ` [RFT PATCH v3 23/27] dt-bindings: serial: samsung: Add apple,s5l-uart compatible Hector Martin
2021-03-08 21:17   ` Rob Herring
2021-03-04 21:38 ` [RFT PATCH v3 24/27] tty: serial: samsung_tty: Add support for Apple UARTs Hector Martin
2021-03-05 10:58   ` Krzysztof Kozlowski
2021-03-05 15:28   ` Andy Shevchenko
2021-03-05 17:04     ` Hector Martin
2021-03-07 11:40       ` Krzysztof Kozlowski
2021-03-04 21:39 ` [RFT PATCH v3 25/27] tty: serial: samsung_tty: Add earlycon " Hector Martin
2021-03-05 10:55   ` Krzysztof Kozlowski
2021-03-10 23:11   ` Linus Walleij
2021-03-04 21:39 ` [RFT PATCH v3 26/27] dt-bindings: display: Add apple,simple-framebuffer Hector Martin
2021-03-08 21:18   ` Rob Herring
2021-03-09 16:37   ` Linus Walleij
2021-03-09 20:35     ` Hector Martin
2021-03-04 21:39 ` [RFT PATCH v3 27/27] arm64: apple: Add initial Apple Mac mini (M1, 2020) devicetree Hector Martin
2021-03-05 11:03   ` Krzysztof Kozlowski
2021-03-05 11:14     ` Hector Martin
2021-03-05 11:45       ` Krzysztof Kozlowski
2021-03-05 15:59       ` Mark Kettenis
2021-03-05 16:50         ` Hector Martin
2021-03-05 10:11 ` [RFT PATCH v3 00/27] Apple M1 SoC platform bring-up Hector Martin

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