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

[David Laight <David.Laight@ACULAB.COM>]

From: Ping-Ke Shih
> Sent: 29 December 2022 09:25
> 
> > -----Original Message-----
> > From: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
> > Sent: Wednesday, December 28, 2022 9:36 PM
> > To: linux-wireless@vger.kernel.org
> > Cc: tony0620emma@gmail.com; kvalo@kernel.org; Ping-Ke Shih <pkshih@realtek.com>;
> tehuang@realtek.com;
> > s.hauer@pengutronix.de; netdev@vger.kernel.org; linux-kernel@vger.kernel.org; Martin Blumenstingl
> > <martin.blumenstingl@googlemail.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@googlemail.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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