linux-kernel - Re: [PATCH 1/3] PCI: Add support in pci_walk

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Message-ID: <87r1dat6v9.wl-maz@kernel.org>
Date:   Mon, 27 Sep 2021 15:45:14 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Kishon Vijay Abraham I <kishon@...com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        <linux-kernel@...r.kernel.org>, <linux-pci@...r.kernel.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        <lokeshvutla@...com>
Subject: Re: [PATCH 1/3] PCI: Add support in pci_walk_bus() to invoke callback matching RID

On Wed, 22 Sep 2021 02:26:09 +0100,
Kishon Vijay Abraham I <kishon@...com> wrote:
> 
> >>>> -void pci_walk_bus(struct pci_bus *top, int (*cb)(struct pci_dev *, void *),
> >>>> -		  void *userdata)
> >>>> +void __pci_walk_bus(struct pci_bus *top, int (*cb)(struct pci_dev *, void *),
> >>>> +		    void *userdata, u32 rid, u32 mask)
> >>>>  {
> >>>>  	struct pci_dev *dev;
> >>>>  	struct pci_bus *bus;
> >>>> @@ -399,13 +401,16 @@ void pci_walk_bus(struct pci_bus *top, int (*cb)(struct pci_dev *, void *),
> >>>>  		} else
> >>>>  			next = dev->bus_list.next;
> >>>>  
> >>>> +		if (mask != 0xffff && ((pci_dev_id(dev) & mask) != rid))
> >>>
> >>> Why the check for the mask? I also wonder whether the mask should apply
> >>> to the rid as well:
> >>
> >> If the mask is set for all 16bits, then there is not going to be two PCIe
> >> devices which gets the same ITS device ID right? So no need for calculating
> >> total number of vectors?
> > 
> > Are we really arguing about the cost of a compare+branch vs some
> > readability? Or is there an actual correctness issue here?
> 
> It is for correctness. So existing pci_walk_bus() doesn't invoke cb based on
> rid. So when we convert to __pci_walk_bus(), existing callers of pci_walk_bus()
> might not invoke cb for some devices without the check.
> > 
> >>>
> >>> 		if ((pci_dev_id(dev) & mask) != (rid & mask))
> > 
> > Because I think the above expression is a lot more readable (and
> > likely more correct) than what you are suggesting.
> 
> That would result in existing pci_walk_bus() behave differently from what was
> before this patch no?
> 
> I'm having something like this below
> 	+#define pci_walk_bus(top, cb, userdata) \
> 	+	 __pci_walk_bus((top), (cb), (userdata), 0x0, 0xffff)
> 
> So if we add only "if ((pci_dev_id(dev) & mask) != (rid & mask))",
> the callback will not be invoked for any devices (other than one
> with rid = 0)

But that *is* the bug, isn't it? If you want to parse all the devices,
a mask of 0 is what you need. The mask defines the bits that you want
to match against the RID you passed as a parameter. If you don't want
to check any bit, don't pass any!

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.