From: Kalle Valo
> Sent: 10 January 2023 12:03
...
> > Most hardware definitions align everything.
> >
> > What you may want to do is add compile-time asserts for the
> > sizes of the structures.
> >
> > Remember that if you have 16/32 bit fields in packed structures
> > on some architectures the compile has to generate code that does
> > byte loads and shifts.
> >
> > The 'misaligned' property is lost when you take the address - so
> > you can easily generate a fault.
> >
> > Adding __packed to a struct is a sledgehammer you really shouldn't need.
>
> Avoiding use of __packed is news to me, but is this really a safe rule?
> Most of the wireless engineers are no compiler experts (myself included)
> so I'm worried. For example, in ath10k and ath11k I try to use __packed
> for all structs which are accessing hardware or firmware just to make
> sure that the compiler is not changing anything.
What may wish to do is get the compiler to generate an error if
it would add any padding - but that isn't what __packed is for
or what it does.
The compiler will only ever add padding to ensure that fields
are correctly aligned (usually a multiple of their size).
There can also be padding at the end of a structure so that arrays
are aligned.
There are some unusual ABI that align all structures on 4 byte
boundaries - but i don't think Linux has any of them.
In any case this rarely matters.
All structures that hardware/firmware access are very likely
to have everything on its natural alignment unless you have a very
old structure hat might have a 16bit aligned 32bit value that
was assumed to be two words.
Now if you have:
struct {
char a[4];
int b;
} __packed foo;
whenever you access foo.b the compiler might have to generate
4 separate byte memory accesses and a load of shift/and/or
instructions in order to avoid a misaligned address trap.
So you don't want to use __packed unless the field is actually
expected to be misaligned.
For most hardware/firmware structures this isn't true.
David
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