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

[Arnd Bergmann]

On Mon, May 30, 2022 at 4:08 PM Jani Nikula <jani.nikula at intel.com> wrote:
> On Mon, 30 May 2022, Arnd Bergmann <arnd at arndb.de> wrote:
> > struct my_driver_priv {
> >        struct device dev;
> >        u8 causes_misalignment;
> >        spinlock_t lock;
> >        atomic_t counter;
> > } __packed; /* this annotation is harmful because it breaks the atomics */
>
> I wonder if this is something that could be caught with coccinelle. Or
> sparse. Are there any cases where this combo is necessary? (I can't
> think of any, but it's a low bar. ;)
>
> Cc: Julia.

I think one would first have to make a list of data types that are not
meant to be in a packed structure. It could be a good start to
search for any packed aggregates with a pointer, atomic_t or spinlock_t
in them, but there are of course many more types that you won't
find in hardware structures.

> > or if the annotation does not change the layout like
> >
> > struct my_dma_descriptor {
> >      __le64 address;
> >      __le64 length;
> > } __packed; /* does not change layout but makes access slow on some
> > architectures */
>
> Why is this the case, though? I'd imagine the compiler could figure this
> out.

When you annotate the entire structure as __packed without an
extra __aligned() annotation, the compiler has to assume that the
structure itself is unaligned as well. On many of the older architectures,
this will result in accessing the values one byte at a time. Marking
the structure as "__packed __aligned(8)" instead would be harmless.

When I have a structure with a few misaligned members, I generally
prefer to only annotate the members that are not naturally aligned,
but this approach is not very common.

         Arnd