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 16:37:49 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Linux PCI <linux-pci@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        Keith Busch <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 3:14 PM Bjorn Helgaas <helgaas@...nel.org> wrote:
>
> 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;

It was the branch I was trying to get rid of by recommending !!, how about:

return (status & PCI_EXP_DPC_STATUS_TRIGGER) == PCI_EXP_DPC_STATUS_TRIGGER;

...just so I know the style preference for future drivers/pci/ work?
That at least matches the pattern in the mm helpers that test pte
bits.

>
> 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?

Right, and when the trigger is clear then dpc_reset_active keeps
holding off the hotplug driver until the dpc reset completes in
polling for the link up event.