netdev - RE: [PATCH net-next 8/8] net: phy: use '__packed' instead of '__attribute__((__packed_

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Message-ID: <30f32e888f9e40a5b78549609df936d0@AcuMS.aculab.com>
Date:   Wed, 16 Jun 2021 08:47:59 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'liweihang' <liweihang@...wei.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "andrew@...n.ch" <andrew@...n.ch>,
        "hkallweit1@...il.com" <hkallweit1@...il.com>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Linuxarm <linuxarm@...wei.com>,
        liangwenpeng <liangwenpeng@...wei.com>,
        "quentin.schulz@...tlin.com" <quentin.schulz@...tlin.com>,
        "antoine.tenart@...tlin.com" <antoine.tenart@...tlin.com>
Subject: RE: [PATCH net-next 8/8] net: phy: use '__packed' instead of
 '__attribute__((__packed__))'

From: liweihang
> Sent: 16 June 2021 07:17
> 
> On 2021/6/14 22:28, David Laight wrote:
> > From: Weihang Li
> >> Sent: 11 June 2021 07:37
> >>
> >> Prefer __packed over __attribute__((__packed__)).
> >>
> >> Signed-off-by: Wenpeng Liang <liangwenpeng@...wei.com>
> >> Signed-off-by: Weihang Li <liweihang@...wei.com>
> >> ---
> >>  drivers/net/phy/mscc/mscc_ptp.h | 4 ++--
> >>  1 file changed, 2 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/drivers/net/phy/mscc/mscc_ptp.h b/drivers/net/phy/mscc/mscc_ptp.h
> >> index da34653..01f78b4 100644
> > ...
> >>  /* Represents an entry in the timestamping FIFO */
> >>  struct vsc85xx_ts_fifo {
> >>  	u32 ns;
> >>  	u64 secs:48;
> >>  	u8 sig[16];
> >> -} __attribute__((__packed__));
> >> +} __packed;
> >
> > Hmmmm I'd take some convincing that 'u64 secs:48' is anything
> > other than 'implementation defined'.
> > So using it to map a hardware structure seems wrong.
> >
> > If this does map a hardware structure it ought to have
> > 'endianness' annotations.
> > If it doesn't then why the bitfield and why packed?
> >
> > 	David
> 
> Hi David,
> 
> Thank you for your attention. You are right, I found the contents of structure
> vsc85xx_ts_fifo is got from hardware. But I'm not sure if any issues or warnings
> will be introduced into this driver after just changing 'u64 secs:48' to '__be64
> secs:48'.

I've just checked what this structure looks like - see https://godbolt.org/z/h4EqbMoso

Without any 'packed' annotations  'u64 secs:48' is aligned to an 8 byte
boundary, but is only 6 bytes wide (I don't use bitfields)
so the offset of 'sig' is 6 more than 'secs'.

But the size of the whole structure looks wrong.
I'd expect a hardware fifo so be a power of 2 big.
This one is 26 bytes (as above) or 28 bytes if the 'packed'
is only applied to 'secs' (which removed the 4 byte pad before
it while still allowing aligned 4-byte accesses to the structure.

	David

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