linux-kernel - Re: Overhead of io{read,write}{8,16,32,64} on x86

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Message-Id: <f548e9e7-e499-4e26-87d9-c45ce69236a1@app.fastmail.com>
Date:   Wed, 01 Nov 2023 10:08:42 +0100
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Jiaxun Yang" <jiaxun.yang@...goat.com>,
        Linux-Arch <linux-arch@...r.kernel.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        "Baoquan He" <bhe@...hat.com>
Subject: Re: Overhead of io{read,write}{8,16,32,64} on x86

On Tue, Oct 31, 2023, at 22:41, Jiaxun Yang wrote:
> Hi all,
>
> I'm trying to improve Kernel's support of devices that have ioports
> mapped into MMIO, that involves converting existing driver which is
> using {in,out}{l,w,b} to use io{read,write}{8,16,32,64}, so they can
> benefit from ioport_map and pci_iomap.
>
> However, the problem is io{read,write}{8,16,32,64} will incur penalty
> on x86 by introducing extra function calls (they are not inlined) and
> having extra condition judgment on MMIO vs PIO.
>
> x86 folks, do you think this kind of overhead is acceptable? I do think
> most of PCI/ISA drivers will need to be converted.
>
> linux-arch folks, do you think it will be better if we introduce a
> variant of io{read,write}{8,16,32,64} that direct to PIO on x86 but
> remains the same functionality on other architectures?

I think in general there is not much of a problem here since
the inb()/outb() operations themselves are extremely slow already,
in particular the outb() writes are non-posted unlike writeb().

My feeling is that converting to ioread/iowrite is generally a win
for any driver that already needs to support both cases (e.g.
serial-8250) since this can unify the two code paths.

However, for drivers that only support inb()/outb() today, I don't
see a real benefit in converting them from the traditional methods.

Another question is whether we actually want to keep the ISA-only
drivers around. Usually once you look closely, any particular
ISA driver tends to be entirely unused already and can be removed,
aside from a few known devices that are either soldered-down on
motherboards or that have an LPC variant using the same ISA driver.

     Arnd