linux-kernel - Re: [PATCH v2 1/2] PCI: Initialize RCB from pci_configure

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Message-ID: <20260122103655.GA1239220@bhelgaas>
Date: Thu, 22 Jan 2026 04:36:55 -0600
From: Bjorn Helgaas <helgaas@...nel.org>
To: Håkon Bugge <haakon.bugge@...cle.com>
Cc: Bjorn Helgaas <bhelgaas@...gle.com>,
	Johannes Thumshirn <morbidrsa@...il.com>, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
	Alex Williamson <alex@...zbot.org>,
	Niklas Schnelle <schnelle@...ux.ibm.com>
Subject: Re: [PATCH v2 1/2] PCI: Initialize RCB from pci_configure_device

[+cc Alex, Niklas]

On Wed, Jan 21, 2026 at 04:40:10PM -0600, Bjorn Helgaas wrote:
> On Wed, Jan 21, 2026 at 12:35:40PM +0100, Håkon Bugge wrote:
> > Commit e42010d8207f ("PCI: Set Read Completion Boundary to 128 iff
> > Root Port supports it (_HPX)") fixed a bogus _HPX type 2 record, which
> > instructed program_hpx_type2() to set the RCB in an endpoint,
> > although it's RC did not have the RCB bit set.
> > 
> > e42010d8207f fixed that by qualifying the setting of the RCB in the
> > endpoint with the RC supporting an 128 byte RCB.
> > 
> > In retrospect, the program_hpx_type2() should only modify the AER
> > bits, and stay away from fiddling with the Link Control Register.
> > 
> > Hence, we explicitly program the RCB from pci_configure_device(). RCB
> > is RO in Root Ports, and in VFs, the bit is RsvdP, so for these two
> > cases we skip programming it. Then, if the Root Port has RCB set and
> > it is not set in the EP, we set it.
> > 
> > Fixes: Commit e42010d8207f ("PCI: Set Read Completion Boundary to 128 iff Root Port supports it (_HPX)")
> > Signed-off-by: Håkon Bugge <haakon.bugge@...cle.com>
> > 
> > ---
> > 
> > Note, that the current duplication of pcie_root_rcb_set() will be
> > removed in the next commit.
> > ---
> >  drivers/pci/probe.c | 36 ++++++++++++++++++++++++++++++++++++
> >  1 file changed, 36 insertions(+)
> > 
> > diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> > index 41183aed8f5d9..347af29868124 100644
> > --- a/drivers/pci/probe.c
> > +++ b/drivers/pci/probe.c
> > @@ -2410,6 +2410,41 @@ static void pci_configure_serr(struct pci_dev *dev)
> >  	}
> >  }
> >  
> > +static bool pcie_root_rcb_set(struct pci_dev *dev)
> > +{
> > +	struct pci_dev *rp = pcie_find_root_port(dev);
> > +	u16 lnkctl;
> > +
> > +	if (!rp)
> > +		return false;
> > +
> > +	pcie_capability_read_word(rp, PCI_EXP_LNKCTL, &lnkctl);
> > +
> > +	return !!(lnkctl & PCI_EXP_LNKCTL_RCB);
> > +}
> > +
> > +static void pci_configure_rcb(struct pci_dev *dev)
> > +{
> > +	/*
> > +	 * Obviously, we need a Link Control register. The RCB is RO
> > +	 * in Root Ports, so no need to attempt to set it for
> > +	 * them. For VFs, the RCB is RsvdP, so, no need to set it.
> > +	 * Then, if the Root Port has RCB set, then we set for the EP
> > +	 * unless already set.
> > +	 */
> > +	if (pcie_cap_has_lnkctl(dev) &&
> > +	    (pci_pcie_type(dev) != PCI_EXP_TYPE_ROOT_PORT) &&
> > +	    !dev->is_virtfn && pcie_root_rcb_set(dev)) {
> > +		u16 lnkctl;
> > +
> > +		pcie_capability_read_word(dev, PCI_EXP_LNKCTL, &lnkctl);
> > +		if (lnkctl & PCI_EXP_LNKCTL_RCB)
> > +			return;
> > +
> > +		pcie_capability_write_word(dev, PCI_EXP_LNKCTL, lnkctl | PCI_EXP_LNKCTL_RCB);
> > +	}
> 
> RCB isn't meaningful for switches, so we'll read their LNKCTL
> unnecessarily.  I propose something like this, which also clears RCB
> if it's set when it shouldn't be (I think this would indicate a
> firmware defect):
> 
>         /*
>          * Per PCIe r7.0, sec 7.5.3.7, RCB is only meaningful in Root Ports
>          * (where it is read-only), Endpoints, and Bridges.  It may only be
>          * set for Endpoints and Bridges if it is set in the Root Port.
>          */
>         if (!pci_is_pcie(dev) ||
>             pci_pcie_type(dev) == PCI_EXP_TYPE_ROOT_PORT ||
>             pci_pcie_type(dev) == PCI_EXP_TYPE_UPSTREAM ||
>             pci_pcie_type(dev) == PCI_EXP_TYPE_DOWNSTREAM ||
>             dev->is_virtfn)
>                 return;
> 
>         rcb = pcie_root_rcb_set(dev);
> 
>         pcie_capability_read_word(dev, PCI_EXP_LNKCTL, &lnkctl);
>         if (rcb) {
>                 if (lnkctl & PCI_EXP_LNKCTL_RCB)
>                         return;
> 
>                 lnkctl |= PCI_EXP_LNKCTL_RCB;
>         } else {
>                 if (!(lnkctl & PCI_EXP_LNKCTL_RCB))
>                         return;
> 
>                 pci_info(FW_INFO "clearing RCB (RCB not set in Root Port)\n");
>                 lnkctl &= ~PCI_EXP_LNKCTL_RCB;

On second thought, I think this is too aggressive.  I think VM guests
will often see endpoints but not the Root Port.  In that case,
pcie_root_rcb_set() would return false because it couldn't find the
RP, but the RP might actually have RCB set.  Then we would clear the
endpoint RCB unnecessarily, which should be safe but would reduce
performance and will result in annoying misleading warnings.

Could either ignore this case (as in your original patch) or bring
pcie_root_rcb_set() inline here and return early if we can't find the
RP.

>         }
> 
>         pcie_capability_write_word(dev, PCI_EXP_LNKCTL, lnkctl);
> 
> > +}
> > +
> >  static void pci_configure_device(struct pci_dev *dev)
> >  {
> >  	pci_configure_mps(dev);
> > @@ -2419,6 +2454,7 @@ static void pci_configure_device(struct pci_dev *dev)
> >  	pci_configure_aspm_l1ss(dev);
> >  	pci_configure_eetlp_prefix(dev);
> >  	pci_configure_serr(dev);
> > +	pci_configure_rcb(dev);
> >  
> >  	pci_acpi_program_hp_params(dev);
> >  }
> > -- 
> > 2.43.5
> >