linux-kernel - Re: [PATCH v4 4/5] PCI: host-common: Add link down handling for host bridges

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Message-ID: <20250602211344.GA444082@bhelgaas>
Date: Mon, 2 Jun 2025 16:13:44 -0500
From: Bjorn Helgaas <helgaas@...nel.org>
To: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
Cc: Mahesh J Salgaonkar <mahesh@...ux.ibm.com>,
	Oliver O'Halloran <oohall@...il.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Lorenzo Pieralisi <lpieralisi@...nel.org>,
	Krzysztof Wilczyński <kw@...ux.com>,
	Rob Herring <robh@...nel.org>, Zhou Wang <wangzhou1@...ilicon.com>,
	Will Deacon <will@...nel.org>, Robert Richter <rric@...nel.org>,
	Alyssa Rosenzweig <alyssa@...enzweig.io>,
	Marc Zyngier <maz@...nel.org>,
	Conor Dooley <conor.dooley@...rochip.com>,
	Daire McNamara <daire.mcnamara@...rochip.com>, dingwei@...vell.com,
	cassel@...nel.org, Lukas Wunner <lukas@...ner.de>,
	Krishna Chaitanya Chundru <krishna.chundru@....qualcomm.com>,
	linuxppc-dev@...ts.ozlabs.org, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	linux-riscv@...ts.infradead.org
Subject: Re: [PATCH v4 4/5] PCI: host-common: Add link down handling for host
 bridges

On Fri, May 30, 2025 at 09:39:28PM +0530, Manivannan Sadhasivam wrote:
> On Fri, May 30, 2025 at 06:34:04AM -0500, Bjorn Helgaas wrote:
> > On Fri, May 30, 2025 at 09:16:59AM +0530, Manivannan Sadhasivam wrote:
> > > On Wed, May 28, 2025 at 05:35:00PM -0500, Bjorn Helgaas wrote:
> > > > On Thu, May 08, 2025 at 12:40:33PM +0530, Manivannan Sadhasivam wrote:
> > > > > The PCI link, when down, needs to be recovered to bring it back. But that
> > > > > cannot be done in a generic way as link recovery procedure is specific to
> > > > > host bridges. So add a new API pci_host_handle_link_down() that could be
> > > > > called by the host bridge drivers when the link goes down.
> > > > > 
> > > > > The API will iterate through all the slots and calls the pcie_do_recovery()
> > > > > function with 'pci_channel_io_frozen' as the state. This will result in the
> > > > > execution of the AER Fatal error handling code. Since the link down
> > > > > recovery is pretty much the same as AER Fatal error handling,
> > > > > pcie_do_recovery() helper is reused here. First the AER error_detected
> > > > > callback will be triggered for the bridge and the downstream devices. Then,
> > > > > pci_host_reset_slot() will be called for the slot, which will reset the
> > > > > slot using 'reset_slot' callback to recover the link. Once that's done,
> > > > > resume message will be broadcasted to the bridge and the downstream devices
> > > > > indicating successful link recovery.
> > > > 
> > > > Link down is an event for a single Root Port.  Why would we iterate
> > > > through all the Root Ports if the link went down for one of them?
> > > 
> > > Because on the reference platform (Qcom), link down notification is
> > > not per-port, but per controller. So that's why we are iterating
> > > through all ports.  The callback is supposed to identify the ports
> > > that triggered the link down event and recover them.
> > 
> > Maybe I'm missing something.  Which callback identifies the port(s)
> > that triggered the link down event?
> 
> I was referring to the host_bridge::reset_root_port() callback that resets the
> root ports.
> 
> >  I see that
> > pci_host_handle_link_down() is called by
> > rockchip_pcie_rc_sys_irq_thread() and qcom_pcie_global_irq_thread(),
> > but I don't see the logic that identifies a particular Root Port.
> > 
> > Per-controller notification of per-port events is a controller
> > deficiency, not something inherent to PCIe.  I don't think we should
> > build common infrastructure that resets all the Root Ports just
> > because one of them had an issue.
> 
> Hmm, fair enough.
> 
> > I think pci_host_handle_link_down() should take a Root Port, not a
> > host bridge, and the controller driver should figure out which port
> > needs to be recovered, or the controller driver can have its own loop
> > to recover all of them if it can't figure out which one needs it.
> 
> This should also work. Feel free to drop the relevant commits for
> v6.16, I can resubmit them (including dw-rockchip after -rc1).

OK, I kept "PCI: host-common: Make the driver as a common library for
host controller drivers" (renamed to "PCI: host-common: Convert to
library for host controller drivers") on pci/controller/dw-rockchip
for v6.16 and deferred the rest until later.