Re: [PATCH v8 3/4] gpio: rpmsg: add generic rpmsg GPIO driver

[Andrew Lunn]

> > > struct msg_hdr {
> > >     u8  id;
> > >     u32 size;
> > >     u8  flags;
> > > };
> > 
> > That is just a bad design. The point of not allowing __packed is that it forces you
> > to design your structures correctly. Maybe AI has no idea of taste, but Maintainer
> > do and would not allow a u32 to be unaligned like this.
> > 
> 
> Let’s keep the discussion technical instead of taste. 😊
> 
> My point with the earlier example was simply to illustrate how layout differences can 
> happen across architectures or compilers. I’m more interested in understanding how 
> you would prefer this specific structure to be defined so that it avoids unaligned fields 
> while still maintaining a stable on‑wire format.

struct msg_hdr {
    u32 size;
    u8  id;
    u8  flags;
};

The compiler will lay this out as you expect, it won't add any padding
between the fields.

You need to be careful with sizeof(struct msg_hdr). On 8 and 16 bit
machines, it is probably 6. On 32, or 64 bit machine it is probably 8.

I would say having a 6 byte message is probably a bad design, and you
should try to make the u32 a u16 if you can. Again, not using the
__packed is making you think about the design, and probably makes the
design better as a result.

    Andrew