Re: [RFC 01/32] Kconfig: introduce and depend on LEGACY_PCI

Mauro Carvalho Chehab
2021-12-28T12:54:49Z

Em Tue, 28 Dec 2021 11:58:55 +0100
Niklas Schnelle <schnelle@linux.ibm.com> escreveu:

> On Tue, 2021-12-28 at 10:15 +0100, Mauro Carvalho Chehab wrote:
> > Em Tue, 28 Dec 2021 09:21:23 +0100
> > Greg Kroah-Hartman <gregkh@linuxfoundation.org> escreveu:
> >
> > > On Mon, Dec 27, 2021 at 05:42:46PM +0100, Niklas Schnelle wrote:
> > > > --- a/drivers/pci/Kconfig
> > > > +++ b/drivers/pci/Kconfig
> > > > @@ -23,6 +23,17 @@ menuconfig PCI
> > > >
> > > > if PCI
> > > >
> > > > +config LEGACY_PCI
> > > > + bool "Enable support for legacy PCI devices"
> > > > + depends on HAVE_PCI
> > > > + help
> > > > + This option enables support for legacy PCI devices. This includes
> > > > + PCI devices attached directly or via a bridge on a PCI Express bus.
> > > > + It also includes compatibility features on PCI Express devices which
> > > > + make use of legacy I/O spaces.
> >
> > This Kconfig doesn't seem what it is needed there, as this should be an
> > arch-dependent feature, and not something that the poor user should be
> > aware if a given architecture supports it or not. Also, the above will keep
> > causing warnings or errors with randconfigs.
> >
> > Also, the "depends on HAVE_CPI" is bogus, as PCI already depends on
> > HAVE_PCI:
>
> Ah yes you're right.
>
> >
> > menuconfig PCI
> > bool "PCI support"
> > depends on HAVE_PCI
> > help
> > This option enables support for the PCI local bus, including
> > support for PCI-X and the foundations for PCI Express support.
> > Say 'Y' here unless you know what you are doing.
> >
> > So, instead, I would expect that a new HAVE_xxx option would be
> > added at arch/*/Kconfig, like:
> >
> > config X86
> > ...
> > select HAVE_PCI_DIRECT_IO
> >
> > It would also make sense to document it at Documentation/features/.
>
> I'll look into that, thanks.
>
> >
> > > All you really care about is the "legacy" I/O spaces here, this isn't
> > > tied to PCI specifically at all, right?
> > >
> > > So why not just have a OLD_STYLE_IO config option or something like
> > > that, to show that it's the i/o functions we care about here, not PCI at
> > > all?
> > >
> > > And maybe not call it "old" or "legacy" as time constantly goes forward,
> > > just describe it as it is, "DIRECT_IO"?
> >
> > Agreed. HAVE_PCI_DIRECT_IO (or something similar) seems a more appropriate
> > name for it.
> >
> > Thanks,
> > Mauro
>
> Hmm, I might be missing something here but that sounds a lot like the
> HAS_IOPORT option added in patch 02.
>
> We add both LEGACY_PCI and HAS_IOPORT to differentiate between two
> cases. HAS_IOPORT is for PC-style devices that are not on a PCI card
> while LEGACY_PCI is for PCI drivers that require port I/O.

I didn't look at the other patches on this series, but why it is needed
to deal with them on a separate way? Won't "PCI" and "HAS_IOPORT" be enough?

I mean, are there any architecture where HAVE_PCI=y and HAS_IOPORT=y
where LEGACY_PCI shall be "n"?

> This
> includes pre-PCIe devices as well as PCIe devices which require
> features like I/O spaces. The "legacy" naming is comes from the PCIe
> spec which in section 2.1.1.2 says "PCI Express supports I/O Space for
> compatibility with legacy devices which require their use. Future
> revisions of this specification may deprecate the use of I/O Space."

I would still avoid calling it LEGACY_PCI, as this sounds too generic.

I didn't read the PCI/PCIe specs, but I suspect that are a lot more
features that were/will be deprecated on PCI specs as time goes by.

So, I would, instead, use something like PCI_LEGACY_IO_SPACE or
HAVE_PCI_LEGACY_IO_SPACE, in order to let it clear what "legacy"
means.

> These two separate config options allow us to compile without support
> for these legacy PCI devices even on a system where inb()/outb() and
> friends are required for some PC style devices and for example ACPI.

Hmm... why this patch make SND_BT87X dependent on LEGACY_PCI?

> @@ -172,6 +177,7 @@ config SND_AZT3328
>
> config SND_BT87X
> tristate "Bt87x Audio Capture"
> + depends on LEGACY_PCI
> select SND_PCM
> help
> If you want to record audio from TV cards based on

I couldn't find any usage of inb/outb & friends on it:

$ grep -E '(inb|outb|inw|outw|inl|outl)\b' ./sound/pci/bt87x.c

It uses, instead, readl/writel:

static inline u32 snd_bt87x_readl(struct snd_bt87x *chip, u32 reg)
{
return readl(chip->mmio + reg);
}

static inline void snd_bt87x_writel(struct snd_bt87x *chip, u32 reg, u32 value)
{
writel(value, chip->mmio + reg);
}

I failed to see what makes it different from VIDEO_BT848 and
DVB_BT8XX drivers. They all support exactly the same chipset:
Brooktree/Conexant BT8xx. On those devices, depending on the exact
model, up to three separate interfaces are provided, each one with
its own Kconfig var:

- audio I/O (SND_BT87X);
- video I/O (VIDEO_BT848);
- MPEG-TS I/O (DVB_BT8XX).

Thanks,
Mauro