linux-kernel - Re: [PATCH v2] pci: export class IDs from pci

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Message-ID: <20150405130256-mutt-send-email-mst@redhat.com>
Date:	Sun, 5 Apr 2015 13:20:12 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Greg KH <gregkh@...uxfoundation.org>
Cc:	Bjorn Helgaas <bhelgaas@...gle.com>, linux-kernel@...r.kernel.org,
	Jonathan Corbet <corbet@....net>,
	"David S. Miller" <davem@...emloft.net>,
	Hans Verkuil <hans.verkuil@...co.com>,
	Mauro Carvalho Chehab <mchehab@....samsung.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	stephen hemminger <stephen@...workplumber.org>,
	Masahiro Yamada <yamada.m@...panasonic.com>,
	linux-pci@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-api@...r.kernel.org
Subject: Re: [PATCH v2] pci: export class IDs from pci_ids.h

On Thu, Apr 02, 2015 at 11:59:21PM +0200, Greg KH wrote:
> On Thu, Apr 02, 2015 at 11:28:19PM +0200, Michael S. Tsirkin wrote:
> > On Thu, Apr 02, 2015 at 03:53:47PM -0500, Bjorn Helgaas wrote:
> > > On Mon, Mar 30, 2015 at 01:33:28PM +0200, Michael S. Tsirkin wrote:
> > > > The basic class ID macros in pci_ids.h are pretty useful for userspace
> > > > using the pci sysfs interface, and they aren't fundamentally different
> > > > from the constants in pci_regs.h - both are defined in the
> > > > pci spec.
> > > > 
> > > > At the moment userspace is forced to duplicate these macros
> > > > (e.g. QEMU does this, so do seabios, gpxe, and others), it is better to
> > > > expose them in /usr/include/linux/pci_ids.h so everyone can just include
> > > > this header.
> > > 
> > > I agree that it would be nice for applications to get these definitions
> > > from a single place, but I'm not sure that include/uapi/linux/pci_ids.h
> > > needs to be that place.
> > > 
> > > These constants are just copies of what's in the spec, and I don't think
> > > you're suggesting that the constants are necessary to use a kernel API.
> > > 
> > > I know the kernel does provide access to values via sysfs "class" files,
> > > but the kernel is just passing the values through from the hardware.
> > > That's analogous to reading the class with setpci, and I don't think it
> > > leads to a requirement that the kernel export all the information about how
> > > to interpret the class values.
> > > 
> > > I haven't looked at libpci or libudev, but it sounds like you think those
> > > are not good solutions.  Is that because they don't currently have this
> > > information?  People don't want to add dependencies on them?
> > > 
> > > Bjorn
> > 
> > People don't want to add dependencies on them.
> 
> So you want us to create yet-a-third-location for these values?

I propose making this part of base Linux system headers, yes.

BTW it's not a third location actually. It would be a third location in a
library, but there are many other packages that duplicate this code.
Since we have this code around anyway, if we just export it, then
there's hope that with time all this duplicate code will go away, then
when e.g. a new value is added to spec, people will add it to linux and
everyone benefits.  As I work on QEMU, I know QEMU will do it,
likely bios and gpxe too, I expect libpci/libudev to be no different.

> Both
> libpci and libudev are on all systems.  And it's not a run-time
> dependancy you are talking about here, but a build-time one, which is
> quite different.

Yes: build-time dependencies annoy developers, run-time ones annoy users :)
In a sense build-time dependencies are more annoying as it's harder to
resolve them.

> Having a build-time dependancy on packages that ship
> with all Linux distributions seems acceptable to me.
> 
> thanks,
> 
> greg k-h
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