linux-kernel - Re: [PATCH v5 4/4] PCI: keystone: Add support to build as a loadable module

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Message-ID: <aRYk4Xj1SNEFYW-J@shell.armlinux.org.uk>
Date: Thu, 13 Nov 2025 18:35:13 +0000
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Bjorn Helgaas <helgaas@...nel.org>
Cc: Siddharth Vadapalli <s-vadapalli@...com>, lpieralisi@...nel.org,
	kwilczynski@...nel.org, mani@...nel.org, robh@...nel.org,
	bhelgaas@...gle.com, jingoohan1@...il.com,
	christian.bruel@...s.st.com, krishna.chundru@....qualcomm.com,
	qiang.yu@....qualcomm.com, shradha.t@...sung.com,
	thippeswamy.havalige@....com, inochiama@...il.com,
	fan.ni@...sung.com, cassel@...nel.org, kishon@...nel.org,
	18255117159@....com, rongqianfeng@...o.com, jirislaby@...nel.org,
	linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, srk@...com
Subject: Re: [PATCH v5 4/4] PCI: keystone: Add support to build as a loadable
 module

On Thu, Nov 13, 2025 at 12:13:55PM -0600, Bjorn Helgaas wrote:
> >  config PCI_KEYSTONE_HOST
> > -	bool "TI Keystone PCIe controller (host mode)"
> > +	tristate "TI Keystone PCIe controller (host mode)"
> >  	depends on ARCH_KEYSTONE || ARCH_K3 || COMPILE_TEST
> >  	depends on PCI_MSI
> >  	select PCIE_DW_HOST
> > -	select PCI_KEYSTONE
> > +	select PCI_KEYSTONE if ARM
> > +	select PCI_KEYSTONE_TRISTATE if !ARM
> 
> This is kind of a lot of dancing to make keystone built-in on ARM32
> because hook_fault_code() is __init, while making it modular
> everywhere else.
> 
> Is hook_fault_code() __init for some intrinsic reason?  All the
> existing callers are __init, so that's one reason.  But could it be
> made non-__init?

Yes. To discourage use in modules, because there is *no* way to safely
remove a hook.

While one can call hook_fault_code() with a NULL handler, that doesn't
mean that another CPU isn't executing in that function. If that code
gets unmapped while another CPU is executing it (because of a module
being unmapped) then we'll get another fault.

Trying to throw locks at this doesn't help - not without holding locks
over the execution of the called function, which *will* be extremely
detrimental on all fault handling, and probably introduce deadlocks.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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