linux-kernel - Re: [GIT PULL] ARM: SoC fixes for v5.10, part 3

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Date:   Mon, 30 Nov 2020 10:15:29 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     Doug Anderson <dianders@...omium.org>,
        Arnd Bergmann <arnd@...nel.org>, SoC Team <soc@...nel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ulf Hansson <ulf.hansson@...aro.org>
Subject: Re: [GIT PULL] ARM: SoC fixes for v5.10, part 3

On Mon, Nov 30, 2020 at 10:05 AM Russell King - ARM Linux admin
<linux@...linux.org.uk> wrote:
>
> If you think that /dev/sda for example is always the machine's internal
> HDD, that is wrong.

Yes. See the whole part about

 "Note that it really is only the internal devices that matter. Once you
  start plugging in an external USB hug and random devices, ordering
  clearly won't be reliable.

  So this is not a "everything must be ordered". But people who think
  that that then means "everythinbg can be random" are wrong"

in my email.

And the key word here is:

> I have a HP Pavilion laptop with its internal HDD with a Windows
> installation. Because I didn't want to destroy that in any way, I
> bought an external USB3 SATA enclosure and SSD, and installed Debian
> Stable on there.

.. but it will still generally be stable with the same hardware
configuration, and not fluctuate randomly from boot to boot when the
hardware is the same.

Is it a guarantee? No. External plugged in hardware can change things.
In fact, when you have things like Thunderbolt involved, since it just
looks like PCI, even the PCI probing order won't be the same with or
without the plugged-in device.

But again: avoid randomness. The difference between non-random and
random is that one is predictable and one is not.

Predictability is good. It's not necessariyl always achievable, but
it's very much something we should strive for VERY HARD.

Predictability and reproducibility help debugging enormously. It means
that things like "git bisect" work a lot better. It makes user reports
much more understandable when two users with identical hardware see
identical issues.

Seriously. Anybody who argues against reproducibility is so far out to
lunch that it's not even funny.

You seem to argue on a complete technicality.

            Linus