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

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Date:	Thu, 2 Apr 2015 17:32:14 -0500
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	"linux-kernel@...r.kernel.org" <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-pci@...r.kernel.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	linux-api@...r.kernel.org, Greg KH <gregkh@...uxfoundation.org>,
	Martin Mareš <mj@....cz>
Subject: Re: [PATCH v2] pci: export class IDs from pci_ids.h

[+cc Martin]

On Thu, Apr 2, 2015 at 4:28 PM, Michael S. Tsirkin <mst@...hat.com> 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?
>
> People don't want to add dependencies on them.

Why not?  I'm not a user-space programmer, so it's not obvious to me
what the problems with adding a dependency are.  If a package provides
functionality you want, it seems like a *good* thing to use it and
depend on it rather than reimplementing the functionality yourself.

The /usr/include/pci/header.h supplied by libpci-dev (on Ubuntu) looks
like it has most or all of the constants you want.

Bjorn
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