linux-kernel - Re: [RFC v2 01/39] Kconfig: introduce HAS_IOPORT option and select it as necessary

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Message-ID: <CAMuHMdV1HpUNKdZDqd0e5BKfr-FqGrwGJJ_xTKw5Z55bdEJa+Q@mail.gmail.com>
Date:   Fri, 6 May 2022 17:02:06 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     "Maciej W. Rozycki" <macro@...am.me.uk>,
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        "David S. Miller" <davem@...emloft.net>
Subject: Re: [RFC v2 01/39] Kconfig: introduce HAS_IOPORT option and select it
 as necessary

Hi David

On Fri, May 6, 2022 at 4:05 PM David Laight <David.Laight@...lab.com> wrote:
> From: Geert Uytterhoeven
> > Sent: 06 May 2022 14:09
> > > The same is really true for other bus type - including ISA and EISA.
> > > (Ignoring the horrid of probing ISI bus devices - hopefully they
> > > are in the ACPI tables??_
> > > If a driver is probed on a ISA bus there ought to be functions
> > > equivalent to pci_ioremap() (for both memory and IO addresses)
> > > that return tokens appropriate for the specific bus.
> > >
> > > That is all a different load of churn.
> >
> > A loooong time ago,  it was suggested to add register accessor
> > functions to struct device, so e.g. readl(dev, offset) would call
> > into these accessors, which would implement the bus-specific behavior.
> > No more worries about readl(), __raw_readl(), ioread32b(), or whatever
> > quirk is needed, at the (small on nowadays' machines) expense of
> > some indirection...
>
> I was just thinking that the access functions might need a 'device'.
> Although you also need the BAR (or equivalent).
> So readl(dev, bar_token, offset) or readl(dev, bar_token + offset).

Note that we do have such a system: regmap.

> Clearly the 'dev' parameter could be compiled out for non-DEBUG
> build on x86 - leaving the current(ish) object code.

Assumed all devices are PCI devices.
E.g. USB devices would still need the indirection.

> You don't want an indirect call (this year), but maybe real
> function call and a few tests won't make that much difference.
> They might affect PCIe writes, but PCIe reads are so slow you
> need to avoid them whenever possible.
> I've not timed reads into something like an ethernet chip,
> but into our fpga they are probably 1000 clocks+.
>
> OTOH I wouldn't want any overhead on the PIO fifo reads
> on one of our small ppc devices.
> We push a lot of data though that fifo and anything extra
> would kill performance.

Right.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds