Re: [PATCH 19/19] PCI: Set bridge map_irq and swizzle_irq to default functions

From: Bjorn Helgaas <helgaas@kernel.org>
To: Jiaxun Yang <jiaxun.yang@flygoat.com>
Cc: "Rob Herring" <robh@kernel.org>,
	"Lorenzo Pieralisi" <lorenzo.pieralisi@arm.com>,
	"Jonathan Hunter" <jonathanh@nvidia.com>,
	"Thierry Reding" <thierry.reding@gmail.com>,
	"Krzysztof Wilczyński" <kw@linux.com>,
	"Tiezhu Yang" <yangtiezhu@loongson.cn>,
	"Huacai Chen" <chenhuacai@kernel.org>,
	"Ray Jui" <rjui@broadcom.com>,
	"Scott Branden" <sbranden@broadcom.com>,
	bcm-kernel-feedback-list@broadcom.com,
	linux-arm-kernel@lists.infradead.org,
	linux-pci <linux-pci@vger.kernel.org>,
	linux-tegra@vger.kernel.org
Subject: Re: [PATCH 19/19] PCI: Set bridge map_irq and swizzle_irq to default functions
Date: Wed, 12 Jan 2022 09:19:03 -0600	[thread overview]
Message-ID: <20220112151903.GA257550@bhelgaas> (raw)
In-Reply-To: <013cc25a-d460-49eb-8cc0-8f59a0c5a45e@www.fastmail.com>

On Wed, Jan 12, 2022 at 12:57:44PM +0000, Jiaxun Yang wrote:
> 在2022年1月11日一月 下午9:46，Bjorn Helgaas写道：
> > [-cc many, +cc iproc, loongson, tegra maintainers]
> >
> > On Tue, Jul 21, 2020 at 08:25:14PM -0600, Rob Herring wrote:
> >> The majority of DT based host drivers use the default .map_irq() and
> >> .swizzle_irq() functions, so let's initialize the function pointers to
> >> the default and drop setting them in the host drivers.
> >> 
> >> Drivers like iProc which don't support legacy interrupts need to set
> >> .map_irq() back to NULL.
> >
> > Probably a dumb question...
> >
> > This patch removed all the ->swizzle_irq users in drivers/pci/, which
> > is great -- IIUC swizzling is specified by the PCI-to-PCI Bridge Spec,
> > r1.2, sec 9.1, and should not be device-specific.  I assume the few
> > remaining arch/ users (arm and alpha) are either bugs or workarounds
> > for broken devices.
> >
> > My question is why we still have a few users of ->map_irq: loongson,
> > tegra, iproc.  Shouldn't this mapping be described somehow via DT?
> 
> For Loongson we are describing IRQ map in DT for newer platforms.
> But for legacy platforms (AMD RS780E North Bridge) with i8259 irqchip,
> we need to read PCI IRQ registers to get mapping information.
> 
> It is not known until boot time, so we have to use map_irq callback.

I see these:

  Documentation/devicetree/bindings/pci/loongson.yaml
  arch/mips/boot/dts/loongson/rs780e-pch.dtsi

which makes me think there are Loongson systems with DT.  Are there
some Loongson systems with DT and some legacy ones without?

The only driver I see is drivers/pci/controller/pci-loongson.c.  Is
that used for all Loongson system?  It unconditionally uses ->map_irq
= loongson_map_irq().

loongson_map_irq() reads PCI_INTERRUPT_LINE; I think that depends on
firmware having previously programmed it, right?

Bjorn