linux-kernel - Re: [Linaro-acpi] [PATCH v5 18/18] Documentation: ACPI for ARM64

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Message-ID: <40609667.NRBYGys7aL@wuerfel>
Date:	Tue, 06 Jan 2015 20:30:40 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	"linaro-acpi@...ts.linaro.org" <linaro-acpi@...ts.linaro.org>,
	Will Deacon <Will.Deacon@....com>,
	Lv Zheng <lv.zheng@...el.com>, Rob Herring <robh@...nel.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Robert Moore <robert.moore@...el.com>,
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
	Liviu Dudau <Liviu.Dudau@....com>,
	Robert Richter <rric@...nel.org>,
	Jason Cooper <jason@...edaemon.net>,
	Marc Zyngier <Marc.Zyngier@....com>,
	"jcm@...hat.com" <jcm@...hat.com>, Mark Brown <broonie@...nel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Randy Dunlap <rdunlap@...radead.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Olof Johansson <olof@...om.net>
Subject: Re: [Linaro-acpi] [PATCH v5 18/18] Documentation: ACPI for ARM64

On Tuesday 06 January 2015 14:11:33 Catalin Marinas wrote:
> On Tue, Jan 06, 2015 at 01:59:27PM +0000, Arnd Bergmann wrote:
> > On Tuesday 06 January 2015 11:20:01 Catalin Marinas wrote:
> > > On Mon, Jan 05, 2015 at 08:16:30PM +0000, Arnd Bergmann wrote:
> > > > On Monday 05 January 2015 13:13:02 Catalin Marinas wrote:
> > > > > > since passing no DT tables to OS but
> > > > > > acpi=force is missing is a corner case, we can do a follow up patch to
> > > > > > fix that, does it make sense?
> > > > > 
> > > > > Not entirely. Why would no dtb and no acpi=force be a corner case? I
> > > > > thought this should be the default when only ACPI tables are passed, no
> > > > > need for an additional acpi=force argument.
> > > > 
> > > > We don't really support the case of only ACPI tables for now. The expectation
> > > > is that you always have working DT support, at least for the next few years
> > > > as ACPI features are ramping up, and without acpi=force it should not try
> > > > to use ACPI at all.
> > > 
> > > So if both DT and ACPI are present, just use DT unless acpi=force is
> > > passed. So far I think we agree but what I want to avoid is always
> > > mandating acpi=force even when the DT tables are missing (in the long
> > > run).
> > > 
> > > Now, what's preventing a vendor firmware from providing only ACPI
> > > tables? Do we enforce it in some way (arm-acpi.txt, kernel warning etc.)
> > > that both DT and ACPI are supported, or at least that dts files are
> > > merged in the kernel first?
> > 
> > We have no way of enforcing what a board vendor ships, so if they want
> > to have ACPI-only machines for MS Windows, they just won't work by
> > default on Linux.
> 
> What do you mean by "won't work by default on Linux"? Assuming no
> additional drivers are needed (i.e. a few devices mentioned in SBSA and
> the rest on a PCIe bus, using existing drivers without further
> modifications), do you still want mainline to fail to boot on such
> ACPI-only systems?

I mean it won't work without acpi=force on the command line.

> > Once ACPI support is mature enough, we can also have a whitelist or a
> > different default for using it automatically when no DT is present.
> 
> Having a white-list requires some for of SoC identification. Does ACPI
> provide such thing (like "model" or "compatible" strings in the top DT
> node)?

This is independent of the SoC: All SoCs we support have DT drivers, so
machines like this would be board whose vendor doesn't care about Linux,
but happens to use a SoC that we support. What we would need here is
a whitelist of boards.

> > For drivers merged upstream, I would insist that every driver merged
> > for an ARM64 platform has a documented DT binding that is used in the
> > driver.
> 
> That's fine by me. I just hope that for hardware aimed at ACPI we won't
> need many non-PCIe drivers.

Yes, that is a reasonable assumption. Looking at the AMD patches that
were posted already, there are however a few devices that are not on
the PCI bus:

- pl011 uart
- pl022 spi
- pl061 gpio
- pl330 dma
- designware i2c
- custom 10gbit ethernet
- probably some I forgot

Most of these will likely just not be needed on servers (pl330) or
handled by AML (spi, i2c, gpio) transparently without needing a
driver, but we do need to support the ethernet port and the uart.
I expect that other SoCs come with different ethernet implementations
that are also not discoverable through PCI.

	Arnd
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