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

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Message-ID: <4c4551c787ee4fc9ac40b34707d7365a@AcuMS.aculab.com>
Date:   Thu, 5 Jan 2023 08:34:40 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Martin Blumenstingl' <martin.blumenstingl@...glemail.com>
CC:     Ping-Ke Shih <pkshih@...ltek.com>,
        "linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>,
        "kvalo@...nel.org" <kvalo@...nel.org>,
        "tehuang@...ltek.com" <tehuang@...ltek.com>,
        "s.hauer@...gutronix.de" <s.hauer@...gutronix.de>,
        "tony0620emma@...il.com" <tony0620emma@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 1/4] rtw88: Add packed attribute to the eFuse structs

From: Martin Blumenstingl
> Sent: 04 January 2023 17:49
> 
> On Wed, Jan 4, 2023 at 5:31 PM David Laight <David.Laight@...lab.com> 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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