linux-kernel - Re: [PATCH v3] PCI: aardvark: Don't touch PCIe registers if no card connected

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Date:   Mon, 13 Jul 2020 10:27:47 +0200
From:   Pali Rohár <pali@...nel.org>
To:     Lorenzo Pieralisi <lorenzo.pieralisi@....com>
Cc:     Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        Andrew Murray <amurray@...goodpenguin.co.uk>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Marek Behún <marek.behun@....cz>,
        Remi Pommarel <repk@...plefau.lt>,
        Tomasz Maciej Nowak <tmn505@...il.com>,
        Xogium <contact@...ium.me>, linux-pci@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] PCI: aardvark: Don't touch PCIe registers if no card
 connected

On Friday 10 July 2020 10:18:00 Lorenzo Pieralisi wrote:
> On Thu, Jul 09, 2020 at 05:09:59PM +0200, Pali Rohár wrote:
> > > I understand that but the bridge bus resource can be trimmed to just
> > > contain the root bus because that's the only one where there is a
> > > chance you can enumerate a device.
> > 
> > It is possible to register only root bridge without endpoint?
> 
> It is possible to register the root bridge with a trimmed IORESOURCE_BUS
> so that you don't enumerate anything other than the root port.

Hello Lorenzo! I really do not know how to achieve it. From code it
looks like that pci/probe.c scans child buses unconditionally.

pci-aardvark.c calls pci_host_probe() which calls functions
pci_scan_root_bus_bridge() which calls pci_scan_child_bus() which calls
pci_scan_child_bus_extend() which calls pci_scan_bridge_extend() (bridge
needs to be reconfigured) which then try to probe child bus via
pci_scan_child_bus_extend() because bridge is not card bus.

In function pci_scan_bridge_extend() I do not see a way how to skip
probing for child buses which would avoid enumerating aardvark root
bridge when PCIe device is not connected.

dmesg output contains:

  advk-pcie d0070000.pcie: link never came up
  advk-pcie d0070000.pcie: PCI host bridge to bus 0000:00
  pci_bus 0000:00: root bus resource [bus 00-ff]
  pci_bus 0000:00: root bus resource [mem 0xe8000000-0xe8ffffff]
  pci_bus 0000:00: root bus resource [io  0x0000-0xffff] (bus address [0xe9000000-0xe900ffff])
  pci_bus 0000:00: scanning bus
  pci 0000:00:00.0: [1b4b:0100] type 01 class 0x060400
  pci 0000:00:00.0: reg 0x38: [mem 0x00000000-0x000007ff pref]
  pci_bus 0000:00: fixups for bus
  pci 0000:00:00.0: scanning [bus 00-00] behind bridge, pass 0
  pci 0000:00:00.0: bridge configuration invalid ([bus 00-00]), reconfiguring
  pci 0000:00:00.0: scanning [bus 00-00] behind bridge, pass 1
  pci_bus 0000:01: scanning bus
  advk-pcie d0070000.pcie: advk_pcie_valid_device