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

From: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
To: "Pali Rohár" <pali@kernel.org>
Cc: "Tomasz Maciej Nowak" <tmn505@gmail.com>,
	linux-pci@vger.kernel.org, Xogium <contact@xogium.me>,
	linux-kernel@vger.kernel.org, "Marek Behún" <marek.behun@nic.cz>,
	"Remi Pommarel" <repk@triplefau.lt>,
	"Thomas Petazzoni" <thomas.petazzoni@bootlin.com>,
	"Bjorn Helgaas" <bhelgaas@google.com>,
	linux-arm-kernel@lists.infradead.org,
	"Andrew Murray" <amurray@thegoodpenguin.co.uk>
Subject: Re: [PATCH v3] PCI: aardvark: Don't touch PCIe registers if no card connected
Date: Mon, 13 Jul 2020 12:23:25 +0100	[thread overview]
Message-ID: <20200713112325.GA25865@e121166-lin.cambridge.arm.com> (raw)
In-Reply-To: <20200713082747.e3q3ml3wpbszn4j7@pali>

On Mon, Jul 13, 2020 at 10:27:47AM +0200, Pali Rohár wrote:
> 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]

This resource can be limited to the root bus number only before calling
pci_host_probe() (ie see pci_parse_request_of_pci_ranges() and code in
pci_scan_bridge_extend() that programs primary/secondary/subordinate
busses) but I think that only papers over the issue, it does not fix it.

I will go over the thread again but I suspect I can merge the patch even
though I still believe there is work to be done to understand the issue
we are facing.

Lorenzo

>   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

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