Re: [RFC PATCH fwctl 3/5] pds_fwctl: initial driver framework

[Jason Gunthorpe <jgg@nvidia.com>]

On Thu, Feb 13, 2025 at 03:06:14PM -0800, Nelson, Shannon wrote:

> > > +/**
> > > + * struct pds_fwctl_cmd - Firmware control command structure
> > > + * @opcode: Opcode
> > > + * @rsvd:   Word boundary padding
> > > + * @ep:     Endpoint identifier.
> > > + * @op:     Operation identifier.
> > > + */
> > > +struct pds_fwctl_cmd {
> > > +     u8     opcode;
> > > +     u8     rsvd[3];
> > > +     __le32 ep;
> > > +     __le32 op;
> > > +} __packed;
> > None of these actually need to be packed given explicit padding to
> > natural alignment of all fields.  Arguably it does no harm though
> > so up to you.
> 
> Old belt-and-suspenders habits...

In that case it is worth knowing that __packed also changes the
assumed alignment of the struct. You can access a __packed struct at
any byte.

On x86 this is mostly meaningless, but on other arches it can effect
code generation as the compiler will have to assume that, say, a 64
bit load is not naturally aligned and emit a more expensive sequence
to load it.

Which is why you occasionally see things like:
  __attribute__ ((packed,aligned(8)));

Which says that there is no padding inside the struct, but also that
the compiler can assume a guaranteed starting alignment for the
memory.

Jason