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Date:   Sun, 1 Jan 2023 11:54:28 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Ping-Ke Shih' <pkshih@...ltek.com>,
        "martin.blumenstingl@...glemail.com" 
        <martin.blumenstingl@...glemail.com>,
        "linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>
CC:     "kvalo@...nel.org" <kvalo@...nel.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: Ping-Ke Shih
> Sent: 01 January 2023 11:42
> 
> 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@...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.
> 
> 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.

Then define it as such.
The 16bit endianness and bit-order dependant bitfields serve
no purpose. 

> > 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.

But you can't take the address of bitfield members.
Define the data properly.

> >
> > 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.

No - that doesn't mean you need to use __packed.
It does mean that you shouldn't use bitfields.
Look at all the hardware drivers, they use structs to map device
registers and absolutely require the compile not add padding.

	David

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