bitfield structures

[Christian König]

Well, making a long story short: 
http://yarchive.net/comp/linux/bitfields.html

Regards,
Christian.

Am 16.10.2014 um 16:14 schrieb Alex Deucher:
> As part of the amdgpu transition, we are moving to using database
> generated register and packet headers.  We have a number of options
> for formatting, some of which involve bitfields (don't worry there
> will also be shift/mask style headers as well which is mainly what we
> use in the code).  I think these formats are cleaner for a number of
> cases, however, as far as I know, C does not define the ordering of
> bits within bitfields.  That said, every compiler I've used seems to
> do what you would expect.  It makes coding a lot cleaner as less
> error-prone in certain cases.  Here are a couple of example of what
> I'm talking about:
>
> A register example:
>
> union GRPH_SWAP_CNTL {
>          struct {
> #if BIG_ENDIAN
>                  unsigned int                                 : 20;
>                  unsigned int             GRPH_ALPHA_CROSSBAR : 2;
>                  unsigned int              GRPH_BLUE_CROSSBAR : 2;
>                  unsigned int             GRPH_GREEN_CROSSBAR : 2;
>                  unsigned int               GRPH_RED_CROSSBAR : 2;
>                  unsigned int                                 : 2;
>                  unsigned int                GRPH_ENDIAN_SWAP : 2;
> #else
>                  unsigned int                GRPH_ENDIAN_SWAP : 2;
>                  unsigned int                                 : 2;
>                  unsigned int               GRPH_RED_CROSSBAR : 2;
>                  unsigned int             GRPH_GREEN_CROSSBAR : 2;
>                  unsigned int              GRPH_BLUE_CROSSBAR : 2;
>                  unsigned int             GRPH_ALPHA_CROSSBAR : 2;
>                  unsigned int                                 : 20;
> #endif
>          } bitfields, bits;
>          unsigned int u32All;
>          signed int i32All;
>          float f32All;
> };
>
> A packet example:
>
> typedef union PM4_TYPE_3_HEADER
> {
>      struct
>      {
> #if BIG_ENDIAN
>          unsigned int type      : 2; ///< packet identifier. It should
> be 3 for type 3 packets
>          unsigned int count     : 14;///< number of DWORDs - 1 in the
> information body.
>          unsigned int opcode    : 8; ///< IT opcode
>          unsigned int reserved1 : 6; ///< reserved
>          unsigned int shaderType: 1; ///< 0: Graphics, 1: Compute Shader
>          unsigned int predicate : 1; ///< predicated version of packet when set
> #else
>          unsigned int predicate : 1; ///< predicated version of packet when set
>          unsigned int shaderType: 1; ///< 0: Graphics, 1: Compute Shader
>          unsigned int reserved1 : 6; ///< reserved
>          unsigned int opcode    : 8; ///< IT opcode
>          unsigned int count     : 14;///< number of DWORDs - 1 in the
> information body.
>          unsigned int type      : 2; ///< packet identifier. It should
> be 3 for type 3 packets
> #endif
>      };
>      unsigned int u32All;
> } PM4_TYPE_3_HEADER;
>
> //--------------------MEM_SEMAPHORE--------------------
>
> enum MEM_SEMAPHORE_signal_type_enum {
> signal_type_mem_semaphore_SIGNAL_TYPE_INCREMENT_0 = 0,
> signal_type_mem_semaphore_SIGNAL_TYPE_WRITE_1 = 1 };
> enum MEM_SEMAPHORE_client_code_enum { client_code_mem_semaphore_CP_0 =
> 0, client_code_mem_semaphore_CB_1 = 1, client_code_mem_semaphore_DB_2
> = 2, client_code_mem_semaphore_RESERVED_3 = 3 };
> enum MEM_SEMAPHORE_sem_sel_enum {
> sem_sel_mem_semaphore_SIGNAL_SEMAPHORE_6 = 6,
> sem_sel_mem_semaphore_WAIT_SEMAPHORE_7 = 7 };
>
> typedef struct _PM4_MEM_SEMAPHORE
> {
>      union
>      {
>          PM4_TYPE_3_HEADER   header;            ///header
>          unsigned int        ordinal1;
>      };
>
>      union
>      {
>          struct
>      {
> #if BIG_ENDIAN
>              unsigned int address_lo:29;
>              unsigned int reserved1:3;
> #else
>              unsigned int reserved1:3;
>              unsigned int address_lo:29;
> #endif
>          } bitfields2;
>          unsigned int ordinal2;
>      };
>
>      union
>      {
>          struct
>      {
> #if BIG_ENDIAN
>              MEM_SEMAPHORE_sem_sel_enum sem_sel:3;
>              unsigned int reserved4:3;
>              MEM_SEMAPHORE_client_code_enum client_code:2;
>              unsigned int reserved3:3;
>              MEM_SEMAPHORE_signal_type_enum signal_type:1;
>              unsigned int reserved2:3;
>              unsigned int use_mailbox:1;
>              unsigned int address_hi:16;
> #else
>              unsigned int address_hi:16;
>              unsigned int use_mailbox:1;
>              unsigned int reserved2:3;
>              MEM_SEMAPHORE_signal_type_enum signal_type:1;
>              unsigned int reserved3:3;
>              MEM_SEMAPHORE_client_code_enum client_code:2;
>              unsigned int reserved4:3;
>              MEM_SEMAPHORE_sem_sel_enum sem_sel:3;
> #endif
>          } bitfields3;
>          unsigned int ordinal3;
>      };
>
> }  PM4MEM_SEMAPHORE, *PPM4MEM_SEMAPHORE;
>
> Are there any strong objections to these sorts of structures?
>
> Thanks,
>
> Alex
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