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

[Ping-Ke Shih]

On Sat, 2022-12-31 at 16:57 +0000, David Laight wrote:
> From: Ping-Ke Shih
> > Sent: 29 December 2022 09:25
> > 
> > > -----Original Message-----
> > > From: Martin Blumenstingl <martin.blumenstingl@xxxxxxxxxxxxxx>
> > > Sent: Wednesday, December 28, 2022 9:36 PM
> > > To: linux-wireless@xxxxxxxxxxxxxxx
> > > Cc: tony0620emma@xxxxxxxxx; kvalo@xxxxxxxxxx; Ping-Ke Shih <pkshih@xxxxxxxxxxx>;
> > tehuang@xxxxxxxxxxx;
> > > s.hauer@xxxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; Martin
> > > Blumenstingl
> > > <martin.blumenstingl@xxxxxxxxxxxxxx>
> > > 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@xxxxxxxxxxxxxx>
> > > ---
> > >  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.

Uh. I typo the sentence. Originally, I would like to type
"...bit fields are NOT suitable...".

In this driver, efuse is read into a u8 array, and cast to this struct
pointer to access the field. 

> 
> 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.

Yes, this kind of struct has endian problem. Fortunately, we don't
actually access values via bit fields.

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

If we actually want to access these values, we will define masks
and use {u8,_le16,le32}_get_bits() or bare '&' bit operation to access 
them.

> 
> 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.
> 

Understood. We only add __packed to the struct which is used to
access predefined format, like efuse content defined by vendor.

Ping-Ke