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

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Message-ID: <87da8c82dec749dc826b5a1b4c4238aa@AcuMS.aculab.com>
Date:   Sat, 31 Dec 2022 16:57:14 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Ping-Ke Shih' <pkshih@...ltek.com>,
        Martin Blumenstingl <martin.blumenstingl@...glemail.com>,
        "linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>
CC:     "tony0620emma@...il.com" <tony0620emma@...il.com>,
        "kvalo@...nel.org" <kvalo@...nel.org>,
        "tehuang@...ltek.com" <tehuang@...ltek.com>,
        "s.hauer@...gutronix.de" <s.hauer@...gutronix.de>,
        "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: Ping-Ke Shih
> Sent: 29 December 2022 09:25
> 
> > -----Original Message-----
> > From: Martin Blumenstingl <martin.blumenstingl@...glemail.com>
> > Sent: Wednesday, December 28, 2022 9:36 PM
> > To: linux-wireless@...r.kernel.org
> > Cc: tony0620emma@...il.com; kvalo@...nel.org; Ping-Ke Shih <pkshih@...ltek.com>;
> tehuang@...ltek.com;
> > s.hauer@...gutronix.de; netdev@...r.kernel.org; linux-kernel@...r.kernel.org; Martin Blumenstingl
> > <martin.blumenstingl@...glemail.com>
> > Subject: [PATCH 1/4] rtw88: Add packed attribute to the eFuse structs
> >
> > The eFuse definitions in the rtw88 are using structs to describe the
> > eFuse contents. Add the packed attribute to all structs used for the
> > eFuse description so the compiler doesn't add gaps or re-order
> > attributes.
> >
> > Also change the type of the res2..res3 eFuse fields to u16 to avoid the
> > following warning, now that their surrounding struct has the packed
> > attribute:
> >   note: offset of packed bit-field 'res2' has changed in GCC 4.4
> >
> > Fixes: e3037485c68e ("rtw88: new Realtek 802.11ac driver")
> > Fixes: ab0a031ecf29 ("rtw88: 8723d: Add read_efuse to recognize efuse info from map")
> > Fixes: 769a29ce2af4 ("rtw88: 8821c: add basic functions")
> > Fixes: 87caeef032fc ("wifi: rtw88: Add rtw8723du chipset support")
> > Fixes: aff5ffd718de ("wifi: rtw88: Add rtw8821cu chipset support")
> > Signed-off-by: Martin Blumenstingl <martin.blumenstingl@...glemail.com>
> > ---
> >  drivers/net/wireless/realtek/rtw88/main.h     |  6 +++---
> >  drivers/net/wireless/realtek/rtw88/rtw8723d.h |  6 +++---
> >  drivers/net/wireless/realtek/rtw88/rtw8821c.h | 20 +++++++++----------
> >  drivers/net/wireless/realtek/rtw88/rtw8822b.h | 20 +++++++++----------
> >  drivers/net/wireless/realtek/rtw88/rtw8822c.h | 20 +++++++++----------
> >  5 files changed, 36 insertions(+), 36 deletions(-)
> >
> 
> [...]
> 
> > @@ -43,13 +43,13 @@ struct rtw8821ce_efuse {
> >  	u8 link_cap[4];
> >  	u8 link_control[2];
> >  	u8 serial_number[8];
> > -	u8 res0:2;			/* 0xf4 */
> > -	u8 ltr_en:1;
> > -	u8 res1:2;
> > -	u8 obff:2;
> > -	u8 res2:3;
> > -	u8 obff_cap:2;
> > -	u8 res3:4;
> > +	u16 res0:2;			/* 0xf4 */
> > +	u16 ltr_en:1;
> > +	u16 res1:2;
> > +	u16 obff:2;
> > +	u16 res2:3;
> > +	u16 obff_cap:2;
> > +	u16 res3:4;
> 
> These should be __le16. Though bit fields are suitable to efuse layout,
> we don't access these fields for now. It would be well.

IIRC the assignment of actual bits to bit-fields is (at best)
architecturally defined - so isn't really suitable for anything
where the bits have to match a portable memory buffer.
The bit allocation isn't tied to the byte endianness.

To get an explicit layout you have to do explicit masking.

You also don't need __packed unless the 'natural' alignment
of fields would need gaps or the actual structure itself might
be misaligned in memory.
While C compilers are allowed to add arbitrary padding the Linux kernel
requires that they don't.
I'm also pretty sure that compilers are not allowed to reorder fields.

Specifying __packed can add considerable run-time (and code size)
overhead on some architectures - it should only be used if actually
needed.

	David

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