Re: [PATCH 0/6] PCI: dwc: Revert Link Up IRQ support

From: Niklas Cassel <cassel@kernel.org>
To: Lukas Wunner <lukas@wunner.de>
Cc: "Jingoo Han" <jingoohan1@gmail.com>,
	"Manivannan Sadhasivam" <mani@kernel.org>,
	"Lorenzo Pieralisi" <lpieralisi@kernel.org>,
	"Krzysztof Wilczy�?ski" <kwilczynski@kernel.org>,
	"Rob Herring" <robh@kernel.org>,
	"Bjorn Helgaas" <bhelgaas@google.com>,
	"Heiko Stuebner" <heiko@sntech.de>,
	"Shawn Lin" <shawn.lin@rock-chips.com>,
	"FUKAUMI Naoki" <naoki@radxa.com>,
	"Krishna chaitanya chundru" <quic_krichai@quicinc.com>,
	linux-pci@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
	linux-rockchip@lists.infradead.org,
	linux-arm-msm@vger.kernel.org
Subject: Re: [PATCH 0/6] PCI: dwc: Revert Link Up IRQ support
Date: Tue, 11 Nov 2025 13:39:06 +0100	[thread overview]
Message-ID: <aRMuascJFSuE6NBd@ryzen> (raw)
In-Reply-To: <aRMZS3EUYx189Xup@wunner.de>

Hello Lukas,

On Tue, Nov 11, 2025 at 12:08:59PM +0100, Lukas Wunner wrote:
> On Tue, Nov 11, 2025 at 11:51:00AM +0100, Niklas Cassel wrote:
> > Revert all patches related to pcie-designware Root Complex Link Up IRQ
> > support.
> > 
> > While this fake hotplugging was a nice idea, it has shown that this feature
> > does not handle PCIe switches correctly:
> [...]
> > During the initial scan, PCI core doesn't see the switch and since the Root
> > Port is not hot plug capable, the secondary bus number gets assigned as the
> > subordinate bus number. This means, the PCI core assumes that only one bus
> > will appear behind the Root Port since the Root Port is not hot plug
> > capable.
> > 
> > This works perfectly fine for PCIe endpoints connected to the Root Port,
> > since they don't extend the bus. However, if a PCIe switch is connected,
> > then there is a problem when the downstream busses starts showing up and
> > the PCI core doesn't extend the subordinate bus number after initial scan
> > during boot.
> 
> In principle it is possible to set the is_hotplug_bridge flag on the
> bridge to force allocation of more buses.  We've already got a quirk
> to set the flag on a Conventional PCI bridge whose hotplug capability
> cannot be discovered otherwise (see quirk_hotplug_bridge() in
> drivers/pci/quirks.c).

While pdev->is_hotplug_bridge was also mentioned by Mani, he seems to
prefer a revert + eventual migration to pwrctrl framework:
https://lore.kernel.org/linux-pci/4f4wsgf56eublizg63fz6xmdjixesalb2q3rxetphd55jpqqju@zfyzsxfgiyim/
https://lore.kernel.org/linux-pci/2n3wamm3txxc6xbmvf3nnrvaqpgsck3w4a6omxnhex3mqeujib@2tb4svn5d3z6/

Kind regards,
Niklas