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

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Date:   Thu, 29 Dec 2022 11:35:59 +0000
From:   Ping-Ke Shih <pkshih@...ltek.com>
To:     "martin.blumenstingl@...glemail.com" 
        <martin.blumenstingl@...glemail.com>
CC:     "linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>,
        "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

On Thu, 2022-12-29 at 11:37 +0100, Martin Blumenstingl wrote:
> Hi Ping-Ke,
> 
> On Thu, Dec 29, 2022 at 10:25 AM Ping-Ke Shih <pkshih@...ltek.com> wrote:
> [...]
> > > @@ -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.
> My understanding is that it should look like this (replacing all of res0..res3):
>     __le16 some_field_name;                     /* 0xf4 */
> How to call that single __le16 field then?

You are right. Maybe, we can name it 'pcie_cap'.
But, we don't use them for now, so it is harmless to preserve them as is. 


> 
> I also tried using bit-fields for an __le16 (so basically the same as
> my patch but using __le16 instead of u16) but that makes sparse
> complain:
>   error: invalid bitfield specifier for type restricted __le16
> 
> 

We can fix it by: 

      u8 res0:2;                      /* 0xf4 */
      u8 ltr_en:1;
      u8 res1:2;
      u8 obff:2;
-     u8 res2:3;
+     u8 res2_1:1;
+     u8 res2_2:2;
      u8 obff_cap:2;
      u8 res3:4;

I'm not sure why people merge bit fields res2_1:1 and res2_2:2 that
should be in different u8. I have confirmed this with internal data.

--
Ping-Ke