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

[Jani Nikula]

On Mon, 30 May 2022, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> On Mon, May 30, 2022 at 11:33 AM Jani Nikula <jani.nikula@xxxxxxxxx> wrote:
>>
>> That is, for EDID. Makes you wonder about all the other packed structs
>> with enum members across the kernel.
>
> It is not the 'enum' that is special here, it's the 'union' having
> unpacked members,

Obviously meant union not enum, that was just a -ENOCOFFEE on my part.

> and the same thing happens when you have nested structs: both the inner
> and the outer aggregate need to be packed, either with __packed at the
> end, or on each individual member that is not fully aligned to
> max(sizeof(member), 4)).
>
> I think in general, most __packed annotations we have in the kernel are
> completely pointless because they do not change the structure layout on
> any architecture but instead just make member access slower on

Please explain.

They are used quite a bit for parsing blob data, or
serialization/deserialization, like in the EDID case at hand. Try
removing __attribute__((packed)) from include/drm/drm_edid.h and see the
sizeof(struct edid) on any architecture.

BR,
Jani.

> architectures that lack unaligned load/store instructions. There have
> definitely been other cases though where a __packed annotation is
> not needed on any sane architecture but is needed for OABI ARM.
>
> Overall I'm not that worried because the only machines running OABI
> kernels would be on really old hardware that runs a limited set of
> driver code.
>
> A completely different matter are the extraneous __packed annotations
> that lead to possible problems when accessed through a misaligned
> pointer. We ignore -Waddress-of-packed-member and -Wcast-align
> in the kernel, so these never get caught at build time, but we have
> seen bugs from gcc making incorrect assumptions about alignment
> even on architectures that have unaligned load/store instructions.
>
>       Arnd

-- 
Jani Nikula, Intel Open Source Graphics Center