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    SubjectRe: [PATCH 1/4] rtw88: Add packed attribute to the eFuse structs
    Date
    On Sun, 2023-01-01 at 11:54 +0000, David Laight wrote:
    > 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@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.
    > >
    > > 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.

    Yes, it should not use bit filed. Instead, use a __le16 for all fields, such as

    struct rtw8821ce_efuse {
    ...
    __le16 caps;
    ...
    }


    #define CAPS_RES0 GENMASK(1, 0)
    #define CAPS_LTR_EN BIT(2)
    #define CAPS_RES1 GENMASK(4, 3)
    #define CAPS_OBFF GENMASK(6, 5)
    ...


    Assume the pointer of efuse content is 'const u8 *efuse_raw;'

    const struct rtw8821ce_efuse *efuse = (const struct rtw8821ce_efuse *)efuse_raw;

    Then, get ltr_en

    ltr_en = le16_get_bits(efuse->caps, CAPS_LTR_EN);


    >
    > > > 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.
    >

    I think the original struct has two problem -- endian and __packed.

    I mentioned endian part above.

    Taking below as example to explain why I need __packed,

    struct rtw8821ce_efuse {
    ...
    u8 data1; // offset 0x100
    __le16 data2; // offset 0x101-0x102
    ...
    } __packed;

    Without __packed, compiler could has pad between data1 and data2,
    and then get wrong result.

    In this patch, struct isn't to map registers. Instead it is used to
    access efuse content of a u8 array existing in memory.

    Ping-Ke


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