linux-kernel - Re: [PATCH v2 3/4] PCI: Add link down handling for host bridges

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Message-ID: <lsehjhqicvit32jcsjkfqemgypnpim6zbxwapzdrncm3hwrp44@bvwg2acyyvle>
Date: Thu, 17 Apr 2025 13:24:41 +0530
From: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To: Krishna Chaitanya Chundru <krishna.chundru@....qualcomm.com>
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>, dingwei@...vell.com, cassel@...nel.org, 
	Lukas Wunner <lukas@...ner.de>, linuxppc-dev@...ts.ozlabs.org, linux-pci@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH v2 3/4] PCI: Add link down handling for host bridges

On Wed, Apr 16, 2025 at 11:21:49PM +0530, Krishna Chaitanya Chundru wrote:
> 
> 
> On 4/16/2025 9:59 PM, Manivannan Sadhasivam via B4 Relay wrote:
> > From: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
> > 
> > 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,
> > pcie_do_slot_reset() will be called for each slots, which will reset the
> > slots 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.
> > 
> > In case if the AER support is not enabled in the kernel, only
> > pci_bus_error_reset() will be called for each slots as there is no way we
> > could inform the drivers about link recovery.
> > 
> The PCIe endpoint drivers are registering with err_handlers and they
> will be invoked only from pcie_do_recovery, but there are getting built
> by default irrespective of AER is enabled or not.
> 

AER is *one* of the functionalities of an endpoint. And the endpoint could
mostly work without AER reporting (except for AER fatal/non-fatal where recovery
need to be performed by the host). So it wouldn't make sense to add AER
dependency for them.

> Does it make sense to built err.c irrespective of AER is enabled or not
> to use common logic without the need of having dependency on AER.
> 

Well, yes and no. Right now, only DPC reuses the err handlers except AER. But
DPC driver itself is functional dependent on AER. So I don't think it is really
required to build err.c independent of AER. But I will try to rework the code in
the future for fixing things like 'AER' prefix added to logs and such.

> Also since err.c is tied with AER, DPC also had a hard requirement
> to enable AER which is not needed technically.
> 

DPC driver is functional dependent on AER.

- Mani

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