linux-kernel - Re: [PATCH v3 1/2] PCI: Initialize RCB from pci_configure

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Message-ID: <20260127215844.GA377410@bhelgaas>
Date: Tue, 27 Jan 2026 15:58:44 -0600
From: Bjorn Helgaas <helgaas@...nel.org>
To: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
Cc: Håkon Bugge <haakon.bugge@...cle.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Niklas Schnelle <schnelle@...ux.ibm.com>,
	Alex Williamson <alex@...zbot.org>,
	Johannes Thumshirn <morbidrsa@...il.com>, linux-pci@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>, linux-acpi@...r.kernel.org
Subject: Re: [PATCH v3 1/2] PCI: Initialize RCB from pci_configure_device

On Thu, Jan 22, 2026 at 03:45:58PM +0200, Ilpo Järvinen wrote:
> On Thu, 22 Jan 2026, 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.

> > +	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(dev, FW_INFO "clearing RCB (RCB not set in Root Port)\n");
> > +		lnkctl &= ~PCI_EXP_LNKCTL_RCB;
> > +	}
> > +
> > +	pcie_capability_write_word(dev, PCI_EXP_LNKCTL, lnkctl);
> 
> So this sequence is effectively implementing this simple statement:
> 
> 	pcie_capability_clear_and_set_word(dev, PCI_EXP_LNKCTL,
> 					   PCI_EXP_LNKCTL_RCB,
> 					   rcb ? PCI_EXP_LNKCTL_RCB : 0);
> 
> + the print.
> 
> Is there a good reason why you want to avoid the write by using early 
> returns?

Good question, pcie_capability_clear_and_set_word() is much more
readable.

> I also wonder if those clear & set & clean_and_set interfaces should 
> implement the write avoidance if it's an useful thing (callers should be 
> checked they're not used for RW1C bits if that's implemented though).