linux-kernel - Re: mainline build failure due to f1e4c916f97f ("drm/edid: add EDID block count and size helpers")

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Date:   Tue, 31 May 2022 09:41:24 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Julia Lawall <Julia.Lawall@...ia.fr>,
        Jani Nikula <jani.nikula@...el.com>,
        Sudip Mukherjee <sudipm.mukherjee@...il.com>,
        Russell King <linux@...linux.org.uk>,
        Viresh Kumar <vireshk@...nel.org>,
        Shiraz Hashim <shiraz.linux.kernel@...il.com>,
        Ville Syrjälä <ville.syrjala@...ux.intel.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        SoC Team <soc@...nel.org>
Subject: Re: mainline build failure due to f1e4c916f97f ("drm/edid: add EDID
 block count and size helpers")

On Tue, May 31, 2022 at 1:04 AM Arnd Bergmann <arnd@...db.de> wrote:
>
> As an experiment: what kind of results would we get when looking
> for packed structures and unions that contain any of these:

Yeah, any atomics or locks should always be aligned, and won't even
work (or might be *very* slow) on multiple architectures. Even x86 -
which does very well on unaligned data - reacts badly to sufficiently
unaligned atomics (ie cacheline crossing).

I don't think we have that. Not only because it would already cause
breakage, but simply because the kinds of structures that people pack
aren't generally the kind that contain these kinds of things.

That said, you might have a struct that is packed, but that
intentionally aligns parts of itself, so it *could* be valid.

But it would probably not be a bad idea to check that packed
structures/unions don't have atomic types or locks in them. I _think_
we're all good, but who knows..

            Linus