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

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Message-ID: <7f75a99604394c47bd646c6a024cb27a@AcuMS.aculab.com>
Date:   Tue, 10 Jan 2023 12:34:31 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Kalle Valo' <kvalo@...nel.org>
CC:     'Martin Blumenstingl' <martin.blumenstingl@...glemail.com>,
        Ping-Ke Shih <pkshih@...ltek.com>,
        "linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.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: 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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