linux-kernel - Re: [PATCH] arm64: dts: marvell: armada-37xx: Set linux,pci-domain to zero

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Message-ID: <20210415104537.403de52e@thinkpad>
Date:   Thu, 15 Apr 2021 10:45:37 +0200
From:   Marek Behun <marek.behun@....cz>
To:     Pali Rohár <pali@...nel.org>
Cc:     Rob Herring <robh+dt@...nel.org>, Andrew Lunn <andrew@...n.ch>,
        Gregory Clement <gregory.clement@...tlin.com>,
        Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        devicetree@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        PCI <linux-pci@...r.kernel.org>
Subject: Re: [PATCH] arm64: dts: marvell: armada-37xx: Set linux,pci-domain
 to zero

On Thu, 15 Apr 2021 10:36:40 +0200
Pali Rohár <pali@...nel.org> wrote:

> On Tuesday 13 April 2021 13:17:29 Rob Herring wrote:
> > On Mon, Apr 12, 2021 at 7:41 AM Pali Rohár <pali@...nel.org> wrote:  
> > >
> > > Since commit 526a76991b7b ("PCI: aardvark: Implement driver 'remove'
> > > function and allow to build it as module") PCIe controller driver for
> > > Armada 37xx can be dynamically loaded and unloaded at runtime. Also driver
> > > allows dynamic binding and unbinding of PCIe controller device.
> > >
> > > Kernel PCI subsystem assigns by default dynamically allocated PCI domain
> > > number (starting from zero) for this PCIe controller every time when device
> > > is bound. So PCI domain changes after every unbind / bind operation.  
> > 
> > PCI host bridges as a module are relatively new, so seems likely a bug to me.  
> 
> Why a bug? It is there since 5.10 and it is working.
> 
> > > Alternative way for assigning PCI domain number is to use static allocated
> > > numbers defined in Device Tree. This option has requirement that every PCI
> > > controller in system must have defined PCI bus number in Device Tree.  
> > 
> > That seems entirely pointless from a DT point of view with a single PCI bridge.  
> 
> If domain id is not specified in DT then kernel uses counter and assigns
> counter++. So it is not pointless if we want to have stable domain id.

What Rob is trying to say is that
- the bug is that kernel assigns counter++
- device-tree should not be used to fix problems with how kernel does
  things
- if a device has only one PCIe controller, it is pointless to define
  it's pci-domain. If there were multiple controllers, then it would
  make sense, but there is only one