linux-kernel - Re: [PATCH 3/4] arm64: Juno: Add support for the PCIe host bridge on Juno R1

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Date:	Fri, 9 Oct 2015 17:32:52 +0100
From:	Mark Rutland <mark.rutland@....com>
To:	Liviu Dudau <Liviu.Dudau@....com>
Cc:	Arnd Bergmann <arnd@...db.de>, Jon Medhurst <tixy@...aro.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	linux-pci <linux-pci@...r.kernel.org>,
	Linus Walleij <linus.walleij@...aro.org>,
	Will Deacon <will.deacon@....com>,
	LKML <linux-kernel@...r.kernel.org>,
	device-tree <devicetree@...r.kernel.org>,
	Rob Herring <robh+dt@...nel.org>,
	Kumar Gala <galak@...eaurora.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Robin Murphy <robin.murphy@....com>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 3/4] arm64: Juno: Add support for the PCIe host bridge on
 Juno R1

On Fri, Oct 09, 2015 at 05:09:10PM +0100, Liviu Dudau wrote:
> On Fri, Oct 09, 2015 at 05:49:18PM +0200, Arnd Bergmann wrote:
> > On Friday 09 October 2015 16:44:08 Mark Rutland wrote:
> > > On Fri, Oct 09, 2015 at 03:11:07PM +0100, Liviu Dudau wrote:
> > > > On Fri, Oct 09, 2015 at 08:54:33AM -0500, Rob Herring wrote:
> > > > Or maybe I can claim the use of the string on account on being the first on arm64 
> > > > 
> > > > I can add a vendor prefix if you want, but pci-host-generic is going to ignore it 
> > > > *because* it is trying to be a generic driver.
> > > 
> > > The point here is to have the string ready if we need it later, so it's
> > > fine that it's not used currently.
> > > 
> > > Rob's suggestion is that the compatible list should look something like:
> > > 
> > > compatible = "arm,juno-r1-pcie", "plda,xpressrich3", "pci-host-ecam-generic";
> > > 
> > > We can match on "pci-host-ecam-generic" for now (and hopefully forever),
> > > but if for some reason we need to special-case this host controller (or
> > > Juno's integration thereof), we can do that based on the compatible
> > > string.
> > 
> > Sounds good to me, it certainly can't hurt.
> > 
> > 	Arnd
> 
> Hmm, I'm sorry, but this time I'm going to disagree.
> 
> I understand the principle that the DTS is a description of the
> hardware and it should not have any built in knowledge of how a driver
> works but describe the physical properties of the device (where such
> description makes sense, in this case it does).
> 
> However, when ARM has created the Juno platform it has also created a
> standard called SBSA and has claimed that Juno is compliant with that
> standard. My current position (and it used to be MarkR's as well when
> we have argued internally the pros and cons of having a bespoke driver
> for PLDA's XpressRICH3) is that SBSA compliant behaviour *is* the
> expected behaviour and if the device doesn't conform it needs to be
> fixed in firmware.

We are not arguing for a bespoke driver, and in fact we hope to never
have to use the addition strings.

However, experience shows that we might not be lucky, and if we
encounter some bizarre quirk in future we might need to handle that by
knowing what the hardware is.

> Otherwise, I could've posted months ago the other public driver[1]
> that I've wrote that doesn't depend on firmware and could have been
> done with this long time ago.

We're not arguing for kernel support code. What we're arguing for is
data in the DTB that ideally turns out to be irrelevant.

I can't see why you object to that.

Mark.
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