linux-kernel - Re: [PATCH v2 2/6] pci/hotplug/pnv_php: Work around switches with broken

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Message-ID: <20250618201722.GA1220739@bhelgaas>
Date: Wed, 18 Jun 2025 15:17:22 -0500
From: Bjorn Helgaas <helgaas@...nel.org>
To: Timothy Pearson <tpearson@...torengineering.com>
Cc: linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-pci <linux-pci@...r.kernel.org>,
	Madhavan Srinivasan <maddy@...ux.ibm.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	christophe leroy <christophe.leroy@...roup.eu>,
	Naveen N Rao <naveen@...nel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Shawn Anastasio <sanastasio@...torengineering.com>,
	Lukas Wunner <lukas@...ner.de>
Subject: Re: [PATCH v2 2/6] pci/hotplug/pnv_php: Work around switches with
 broken

On Wed, Jun 18, 2025 at 02:50:04PM -0500, Timothy Pearson wrote:
> ----- Original Message -----
> > From: "Bjorn Helgaas" <helgaas@...nel.org>
> > To: "Timothy Pearson" <tpearson@...torengineering.com>
> > Cc: "linuxppc-dev" <linuxppc-dev@...ts.ozlabs.org>, "linux-kernel" <linux-kernel@...r.kernel.org>, "linux-pci"
> > <linux-pci@...r.kernel.org>, "Madhavan Srinivasan" <maddy@...ux.ibm.com>, "Michael Ellerman" <mpe@...erman.id.au>,
> > "christophe leroy" <christophe.leroy@...roup.eu>, "Naveen N Rao" <naveen@...nel.org>, "Bjorn Helgaas"
> > <bhelgaas@...gle.com>, "Shawn Anastasio" <sanastasio@...torengineering.com>, "Lukas Wunner" <lukas@...ner.de>
> > Sent: Wednesday, June 18, 2025 2:44:00 PM
> > Subject: Re: [PATCH v2 2/6] pci/hotplug/pnv_php: Work around switches with broken
> 
> > [+cc Lukas, pciehp expert]
> > 
> > On Wed, Jun 18, 2025 at 11:56:54AM -0500, Timothy Pearson wrote:
> >>  presence detection
> > 
> > (subject/commit wrapping seems to be on all of these patches)
> > 
> >> The Microsemi Switchtec PM8533 PFX 48xG3 [11f8:8533] PCIe switch system
> >> was observed to incorrectly assert the Presence Detect Set bit in its
> >> capabilities when tested on a Raptor Computing Systems Blackbird system,
> >> resulting in the hot insert path never attempting a rescan of the bus
> >> and any downstream devices not being re-detected.
> > 
> > Seems like this switch supports standard PCIe hotplug?  Quite a bit of
> > this driver looks similar to things in pciehp.  Is there some reason
> > we can't use pciehp directly?  Maybe pciehp could work if there were
> > hooks for the PPC-specific bits?
> 
> While that is a good long term goal that Raptor is willing to work
> toward, it is non-trivial and will require buy-in from other
> stakeholders (e.g. IBM).  If practical, I'd like to get this series
> merged first, to fix the broken hotplug on our hardware that is
> deployed worldwide, then in parallel see what can be done to merge
> PowerNV support into pciehp.  Would that work?

Yeah, it wouldn't make sense to switch horses at this stage.

I guess I was triggered by this patch, which seems to be a workaround
for a defect in a device that is probably also used on non-PPC
systems, and pciehp would need a similar workaround.  But I guess you
go on to say that pciehp already does something similar, so it guess
it's already covered.

Bjorn