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

[Arnd Bergmann]

On Thu, Jun 2, 2022 at 1:21 PM Tetsuo Handa
<penguin-kernel@xxxxxxxxxxxxxxxxxxx> wrote:
> On 2022/06/02 16:38, Arnd Bergmann wrote:
> >> But let's cc the tomoyo and chelsio people.
> >
> > I think both of them work because the structures are always
> > embedded inside of larger structures that have at least word
> > alignment. This is the thing I was looking for, and the
> > __packed attribute was added in error, most likely copied
> > from somewhere else.
>
> The __packed in "struct tomoyo_shared_acl_head" is to embed next
> naturally-aligned member of a larger struct into the bytes that
> would have been wasted if __packed is not specified. For example,
>
> struct tomoyo_shared_acl_head {
>         struct list_head list;
>         atomic_t users;
> } __packed;
>
> struct tomoyo_condition {
>         struct tomoyo_shared_acl_head head;
>         u32 size; /* Memory size allocated for this entry. */
>         (...snipped...)
> };
>
> saves 4 bytes on 64 bits build.
>
> If the next naturally-aligned member of a larger struct is larger than
> the bytes that was saved by __packed, the saved bytes will be unused.

Ok, got it. I think as gcc should still be able to always figure out the
alignment when accessing the atomic, without ever falling back
to byte access on an atomic_get() or atomic_set().

To be on the safe side, I would still either move the __packed attribute
to the 'list' member, or make the structure '__aligned(4)'.

       Arnd