linux-kernel - Re: [PATCH 1/4] pci: Add PCI_BUS() and PCI_DEVID() interfaces to return bus number and device id

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Date:	Mon, 25 Feb 2013 14:23:27 -0700
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	shuah.khan@...com
Cc:	dhowells@...hat.com, Joerg Roedel <joro@...tes.org>,
	paulmck@...ux.vnet.ibm.com, linasvepstas@...il.com,
	davej@...hat.com, tglx@...utronix.de, mtk.manpages@...il.com,
	iommu@...ts.linux-foundation.org,
	LKML <linux-kernel@...r.kernel.org>, linux-pci@...r.kernel.org,
	shemminger@...tta.com, jiang.liu@...wei.com, wangyijing@...wei.com,
	shuahkhan@...il.com
Subject: Re: [PATCH 1/4] pci: Add PCI_BUS() and PCI_DEVID() interfaces to
 return bus number and device id

On Mon, Feb 25, 2013 at 9:37 AM, Shuah Khan <shuah.khan@...com> wrote:
> On Wed, 2013-02-20 at 18:19 -0700, Bjorn Helgaas wrote:
>> On Mon, Feb 11, 2013 at 4:00 PM, Shuah Khan <shuah.khan@...com> wrote:
>> > pci defines PCI_DEVFN(), PCI_SLOT(), and PCI_FUNC() interfaces, however,
>> > it doesn't have interfaces to return PCI bus and PCI device id. Drivers
>> > (AMD IOMMU, and AER) implement module specific definitions for PCI_BUS()
>> > and AMD_IOMMU driver also has a module specific interface to calculate PCI
>> > device id from bus number and devfn.
>> >
>> > Add PCI_BUS and PCI_DEVID interfaces to return PCI bus number and PCI device
>> > id respectively to avoid the need for duplicate definitions in other modules.
>> > AER driver code and AMD IOMMU driver define PCI_BUS. AMD IOMMU driver defines
>> > an interface to calculate device id from bus number, and devfn pair.
>> >
>> > Signed-off-by: Shuah Khan <shuah.khan@...com>
>> > ---
>> >  include/uapi/linux/pci.h |    4 ++++
>> >  1 file changed, 4 insertions(+)
>> >
>> > diff --git a/include/uapi/linux/pci.h b/include/uapi/linux/pci.h
>> > index 3c292bc0..6b2c8b3 100644
>> > --- a/include/uapi/linux/pci.h
>> > +++ b/include/uapi/linux/pci.h
>> > @@ -30,6 +30,10 @@
>> >  #define PCI_DEVFN(slot, func)  ((((slot) & 0x1f) << 3) | ((func) & 0x07))
>> >  #define PCI_SLOT(devfn)                (((devfn) >> 3) & 0x1f)
>> >  #define PCI_FUNC(devfn)                ((devfn) & 0x07)
>> > +#define PCI_DEVID(bus, devfn)  ((((u16)bus) << 8) | devfn)
>> > +
>> > +/* return bus from PCI devid = ((u16)bus_number) << 8) | devfn */
>> > +#define PCI_BUS(x) (((x) >> 8) & 0xff)
>> >
>> >  /* Ioctls for /proc/bus/pci/X/Y nodes. */
>> >  #define PCIIOC_BASE            ('P' << 24 | 'C' << 16 | 'I' << 8)
>>
>> David, can you point me at a description of include/uapi ... what is
>> there and why, and how we should decide what new things go in
>> include/uapi/linux/pci.h as opposed to include/linux/pci.h?  Maybe
>> there should be something in Documentation/?
>>
>> I'm guessing it's something to do with being exported to userland, but
>> I'm not sure the things in this patch (PCI_DEV_ID, PCI_BUS) are really
>> exportable in the sense of being used for syscalls, etc.
>>
>
> Bjorn,David,
>
> Looks like the following thread answers some of the questions about when
> this uapi export was done on the existing defines.
>
> https://lkml.org/lkml/2011/7/28/198
>
> Sounds like the concern is that the older defines PCI_DEVFN, PCI_SLOT,
> PCI_FUNC,  and PCI_DEVID could be exported, but not the new ones I
> added. I could find any discussion on whether these four older defines
> are exportable or the reasons for the export in the above thread.

I think David's disintegration script took include/linux/pci.h, left
the #ifdef __KERNEL__ parts there, and moved everything else (which
wasn't much) to include/uapi/linux/pci.h.

It's obvious that the PCIIOC_ #defines need to be exported to
user-space for ioctls.  It's not obvious to me why PCI_DEVFN,
PCI_SLOT, and PCI_FUNC need to be exported to user-space.  But I can
imagine user-space using functionality like that, even if it's not
connected to a kernel interface.  I assume the intent of the
disintegration is that only include/uapi would be exposed to
user-space, so keeping those definitions in include/linux/pci.h would
break any user programs that used them.

> So the question is if uapi/linux.pci.h isn't the right place, do you
> have a recommendation on where they belong. The only alternative I can
> think of is include/linux/pci.h. It makes functional and logical sense
> to add the new defines to where the existing ones are defines. At least,
> not knowing the details of the change that moved PCI_DEVFN etc. to
> uapi/pci.h, that is my conclusion.

Using the linux-fullhist tree, I found these:

059d367 Import 2.1.82 -- moved PCI_DEVFN outside #ifdef __KERNEL__
b039547 Import 2.1.76 -- PCI_DEVFN was inside #ifdef __KERNEL__
f6d9739 Import 2.1.68pre1 -- added #ifdef __KERNEL__ (enclosing PCI_DEVFN)
940649f Import 1.3.0 -- added PCI_DEVFN

There's no indication of *why* PCI_DEVFN was exported, of course.

Bottom line, I think it's reasonable to keep PCI_DEVFN, et al., in
uapi/linux/pci.h to keep from breaking user-programs, even though if
we were adding them today we would probably put them in the
kernel-only linux/pci.h.  For the new ones you're adding, I'd propose
putting them in the kernel-only linux/pci.h because we know no user
programs use them.

It's not nice and consistent, but it does follow the simple rule of
"don't expose things to user-space unnecessarily."  We might want to
add a comment to keep somebody from cleaning it up later.

Bjorn
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