linux-kernel - Re: [PATCH v1 1/1] PCI: pciehp: Skip DLLSC handling if DPC is triggered

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Date:   Fri, 12 Mar 2021 17:14:16 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
Cc:     bhelgaas@...gle.com, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, ashok.raj@...el.com,
        dan.j.williams@...el.com, keith.busch@...el.com,
        knsathya@...nel.org, Lukas Wunner <lukas@...ner.de>
Subject: Re: [PATCH v1 1/1] PCI: pciehp: Skip DLLSC handling if DPC is
 triggered

On Fri, Mar 12, 2021 at 02:11:03PM -0800, Kuppuswamy, Sathyanarayanan wrote:
> On 3/12/21 1:33 PM, Bjorn Helgaas wrote:
> > On Mon, Mar 08, 2021 at 10:34:10PM -0800, sathyanarayanan.kuppuswamy@...ux.intel.com wrote:
> > > From: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>

> > > +bool is_dpc_reset_active(struct pci_dev *dev)
> > > +{
> > > +	struct pci_host_bridge *host = pci_find_host_bridge(dev->bus);
> > > +	u16 status;
> > > +
> > > +	if (!dev->dpc_cap)
> > > +		return false;
> > > +
> > > +	/*
> > > +	 * If DPC is owned by firmware and EDR is not supported, there is
> > > +	 * no race between hotplug and DPC recovery handler. So return
> > > +	 * false.
> > > +	 */
> > > +	if (!host->native_dpc && !IS_ENABLED(CONFIG_PCIE_EDR))
> > > +		return false;
> > > +
> > > +	if (atomic_read_acquire(&dev->dpc_reset_active))
> > > +		return true;
> > > +
> > > +	pci_read_config_word(dev, dev->dpc_cap + PCI_EXP_DPC_STATUS, &status);
> > > +
> > > +	return !!(status & PCI_EXP_DPC_STATUS_TRIGGER);
> > 
> > I know it's somewhat common in drivers/pci/, but I'm not really a
> > big fan of "!!".
> I can change it to use ternary operator.
> (status & PCI_EXP_DPC_STATUS_TRIGGER) ? true : false;

Ternary isn't terrible, but what's wrong with:

  if (status & PCI_EXP_DPC_STATUS_TRIGGER)
    return true;
  return false;

which matches the style of the rest of the function.

Looking at this again, we return "true" if either dpc_reset_active or
PCI_EXP_DPC_STATUS_TRIGGER.  I haven't worked this all out, but that
pattern feels racy.  I guess the thought is that if
PCI_EXP_DPC_STATUS_TRIGGER is set, dpc_reset_link() will be invoked
soon and we don't want to interfere?