RE: [PATCH 1/4] rtw88: Add packed attribute to the eFuse structs

[David Laight]

From: Martin Blumenstingl
> Sent: 04 January 2023 17:49
> 
> On Wed, Jan 4, 2023 at 5:31 PM David Laight <David.Laight@xxxxxxxxxx> wrote:
> [...]
> > > > What you may want to do is add compile-time asserts for the
> > > > sizes of the structures.
> > > Do I get you right that something like:
> > >   BUILD_BUG_ON(sizeof(rtw8821c_efuse) != 256);
> > > is what you have in mind?
> >
> > That looks like the one...
> I tried this (see the attached patch - it's just meant to show what I
> did, it's not meant to be applied upstream).
> With the attached patch but no other patches this makes the rtw88
> driver compile fine on 6.2-rc2.
> 
> Adding __packed to struct rtw8723d_efuse changes the size of that
> struct for me (I'm compiling for AArch64 / ARM64).
> With the packed attribute it has 267 bytes, without 268 bytes.
> 
> Do you have any ideas as to why that is?

Tail padding - you won't get an odd length for a structure
that contains a 16bit item.

OTOH I doubt you care about the size of that structure, just
the offset of the union and the sizes of the union members.

	David

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